C programming part1

Devi Ahilya Vishvavidyalaya

1/12/2007 School of Computer Science hmehta.scs@dauniv.ac.in 1

What is a Computer Program?

A program is a set of step-by-step
instructions to the computer telling it to
carry out the tasks that you want it to do
to produce the results you want.


What is Programming?

• Programming consists of two distinct
steps:

• algorithmic design (the problem solving
stage, analogous to the work of an
architect designing a building)

• coding (the construction phase)


Levels of Programming
Languages
• Machine language
• Assembly Language
• High Level Languages


Machine Language
• Actual binary code that gives basic
instructions to the computer.
• These are usually simple commands
like adding two numbers or moving
data from one memory location to
another.
• Different for each computer processor


Assembly Language
• A way for humans to program computers
directly without memorizing strings of binary
numbers.
• There is a one-to-one correspondence with
machine code.
– For example ADD and MOV are mnemonics for
addition and move operations that can be specified
in single machine language instructions.
Different for each computer processor


High-level language

• Permits humans to write complex
programs without going step-by step.
• High-level languages include Pascal,
FORTRAN, Java, Visual Basic, and
many more.
• One command in a high-level
language may translate to tens of
machine language instructions.


Translation
Computers can only run machine language
programs directly.
Assembly language programs are assembled, or
translated into machine language.
Likewise, programs written in high-level languages,
like Java, must also be translated into machine
language before they can be run. To do this
translation compile a program.


Translation
10100110 01110110
#include <stdio.h> 00100110 00000000
11111010 11111010
int main() 01001110 10100110
{ 11100110 10010110
printf(“Hello World”); 11001110 00101110
10100110 01001110
11111010 01100110
return 0;
01001110 10000110
}
etc...

Source code Executable code

• Compilers and linkers translate a high
level program into executable machine
code

Structured Programming
• STRUCTURED PROGRAMMING ≡ A
technique for organizing and coding computer
programs in which a hierarchy of modules is
used, each having a single entry and a single
exit point, and in which control is passed
downward through the structure withOUT
UNconditional branches to higher levels of
the structure. Three types of control flow are
used: (1) sequential, (2) selection, and (3)
iteration.


Programming language C

• C is a general purpose programming language.

• C is a middle level language.

• C is a structured language.


Programming language C
Why C is called “a middle level language”?

C contains the features of high level language
portability — it is easy to adapt software written
for one type of computer to another type. the
functionality low level language.

- operators such as &, |,>,< etc. simulate to low
level instruction codes.
- Direct manipulation of bits, bytes and addresses.


Writing C Programs
• A programmer uses a text editor to create or
modify files containing C code.
• Code is also known as source code.
• A file containing source code is called a source
file.
• After a C source file has been created, the
programmer must invoke the C compiler
before the program can be executed (run).


Invoking the tcc Compiler
At the prompt, type

tcc pgm.c

where pgm.c is the C program source
file.

There is a better way use of IDE instead of
command.


The Result : pgm.obj, pgm.exe
• If there are no errors in pgm.c, this command
produces an executable file, which is one that
can be executed (run).
• The tcc compiler puts exe extension of the
executable file. Also the obj file contains the
machine level code.
• To execute the program, at the prompt, type
pgm.exe
• Although we call this process “compiling a
program,” what actually happens is more
complicated.

3 Stages of Compilation
Stage 1: Preprocessing
– Performed by a program called the preprocessor
– Modifies the source code (in RAM) according to
preprocessor directives (preprocessor
commands) embedded in the source code
– Strips comments and white space from the code
– The source code as stored on disk is not modified.


3 Stages of Compilation (con’t)
Stage 2: Compilation
• Performed by a program called the compiler
• Translates the preprocessor-modified source
code into object code (machine code)
• Checks for syntax errors and warnings
• Saves the object code to a disk file, if instructed
to do so (we will not do this).
• If any compiler errors are received, no object code
file will be generated.
• An object code file will be generated if only
warnings, not errors, are received.

3 Stages of Compilation (con’t)
Stage 3: Linking
• Combines the program object code with other
object code to produce the executable file.
• The other object code can come from the Run-
Time Library, other libraries, or object files that
you have created.
• Saves the executable code to a disk file. On
the Linux system, that file is called a.out.
• If any linker errors are received, no executable file
will be generated.


Program Development
Editor

Source File pgm.c

Preprocessor

Modified Source Code in RAM

Compiler
Program Object Code File pgm.obj
Other Object Code Files (if any)

Linker

Executable File pgm.exe

A Simple C Program
/* Filename: hello.c
Author: Brian Kernighan & Dennis Ritchie
Date written: ?/?/1978
Description: This program prints the greeting
“Hello, World!”
*/
#include <stdio.h>
int main ( void )
{
printf ( “Hello, World!n” ) ;
return 0 ;
}

Anatomy of a C Program
program header comment

preprocessor directives (if any)

int main ( )
{
statement(s)
return 0 ;
}

Program Header Comment
• A comment is descriptive text used to help a
reader of the program understand its
content.
• All comments must begin with the characters
/* and end with the characters */
• These are called comment delimiters
• The program header comment always
comes first.


Preprocessor Directives
• Lines that begin with a # in column 1 are
called preprocessor directives
(commands).
• Example: the #include <stdio.h> directive
causes the preprocessor to include a copy of
the standard input/output header file stdio.h at
this point in the code.
• This header file was included because it
contains information about the printf ( )
function that is used in this program.

stdio.h
• When we write our programs, there are
libraries of functions to help us so that we
do not have to write the same code over
and over again.
• Some of the functions are very complex
and long. Not having to write them
ourselves make it easier and faster to
write programs.
• Using the functions will also make it easier
to learn to program!

int main ( void )
• Every program must have a function called
main. This is where program execution begins.
• main() is placed in the source code file as the
first function for readability.
• The reserved word “int” indicates that main()
returns an integer value.
• The parentheses following the reserved word
“main” indicate that it is a function.
• The reserved word “void” means nothing is
there.

The Function Body
• A left brace (curly bracket) -- { -- begins the
body of every function. A corresponding
right brace -- } -- ends the function body.

• The style is to place these braces on
separate lines in column 1 and to indent the
entire function body 3 to 5 spaces.


printf (“Hello, World!n”) ;
• This line is a C statement.
• It is a call to the function printf ( ) with a
single argument (parameter), namely the
string “Hello, World!n”.
• Even though a string may contain many
characters, the string itself should be
thought of as a single quantity.
• Notice that this line ends with a semicolon.
All statements in C end with a semicolon.

return 0 ;
• Because function main() returns an integer value,
there must be a statement that indicates what this
value is.
• The statement
return 0 ;
indicates that main() returns a value of zero to
the operating system.
• A value of 0 indicates that the program successfully
terminated execution.
• Do not worry about this concept now. Just
remember to use the statement.

Another C Program
/*****************************************
** File: proj1.c
** Author: ___________
** Date: 9/15/01
** E-mail: _________________
**
** This program prompts the user for two integer values then displays
** their product.
**
***********************************************/


Another C Program (con’t)
#include <stdio.h>
int main( void )
{
int value1, value2, product ;
printf(“Enter two integer values: “) ;
scanf(“%d%d”, &value1, &value2) ;
product = value1 * value2 ;
printf(“Product = %dn”, product) ;
return 0 ;
}

Tokens
• The smallest element in the C language is
the token.
• It may be a single character or a sequence
of characters to form a single item.


Tokens are:

• Tokens can be:
– Numeric constants
– Character constants
– String constants
– Keywords
– Names (identifiers)
– Punctuation
– Operators


Numeric Constants

• Numeric constants are an uninterrupted
sequence of digits (and may contain a
period). They never contain a comma.
• Examples:
– 123
– 98.6
– 1000000


Character Constants

• One character from a defined character
set.
• Surrounded on the single quotation mark.
• Examples:
– ‘A’
– ‘a’
– ‘$’
– ‘4’


String Constants

• A sequence characters surrounded by
double quotation marks.
• Considered a single item.
• Examples:
– “DAVV”
– “I like ice cream.”
– “123”
– “DHOOM-2”
– “car”

Keywords

• Sometimes called reserved words.
• Are defined as a part of the C language.
• Can not be used for anything else!
• Examples:
– int
– while
– for


Names
• Sometimes called identifiers.
• Can be of anything length, but on the first 31 are
significant (too long is as bad as too short).
• Are case sensitive:
– abc is different from ABC
• Must begin with a letter and the rest can be
letters, digits, and underscores.
• There can be one exception to beginning letter
that variable name can start with underscore( _ )
but it is used by C library.


Punctuation

• Semicolons, colons, commas,
apostrophes, quotation marks, braces,
brackets, and parentheses.
• ; : , ‘ “ [ ] { } ( )


Operators

• There are operators for:
– assignments
– mathematical operations
– relational operations
– Boolean operations
– bitwise operations
– shifting values
– calling functions
– subscripting
– obtaining the size of an object
– obtaining the address of an object
– referencing an object through its address


What Are Variables in C?
• Variables in C have the same meaning as
variables in algebra. That is, they represent
some unknown, or variable, value.

x=a+b
z + 2 = 3(y - 5)
• Remember that variables in algebra are
represented by a single alphabetic
character.

Naming Variables
• Variables in C may be given representations
containing multiple characters. But there are
rules for these representations.
• Variable names (identifiers) in C
– May only consist of letters, digits, and
underscores
– May be as long as you like, but only the first 31
characters are significant
– May not begin with a digit
– May not be a C reserved word (keyword)

Reserved Words (Keywords) in
C

• auto break int long
• case char register return
• const continue short signed
• default do sizeof static
• double else struct switch
• enum extern typedef union
• float for unsigned void
• goto if volatile while


Naming Conventions
• C programmers generally agree on the
following conventions for naming variables.
– Begin variable names with lowercase letters
– Use meaningful identifiers
– Separate “words” within identifiers with
underscores or mixed upper and lower case.
– Examples: surfaceArea surface_Area
surface_area
– Be consistent!


Naming Conventions (con’t)
• Use all uppercase for symbolic constants
(used in #define preprocessor directives).
• Note: symbolic constants are not variables,
but make the program easier to read.
• Examples:
#define PI 3.14159
#define AGE 52


Case Sensitivity
• C is case sensitive
– It matters whether an identifier, such as a
variable name, is uppercase or lowercase.
– Example:
area
Area
AREA
ArEa
are all seen as different variables by the
compiler.

Which Are Legal Identifiers?
AREA area_under_the_curve
3D num45
Last-Chance #values
x_yt3 pi
num$ %done
lucky***


Declaring Variables
• Before using a variable, you must give the
compiler some information about the variable;
i.e., you must declare it.
• The declaration statement includes the data
type of the variable.
• They must be declared just after the start of
block (i.e. start of a function) and before any
other executable statement.
• Examples of variable declarations:
int meatballs ;
float area ;


Declaring Variables (con’t)
• When we declare a variable
– Space is set aside in memory to hold a value of
the specified data type
– That space is associated with the variable name
– That space is associated with a unique address
• Visualization of the declaration
int meatballs ;
meatballs
garbage

FE07 int

Notes About Variables

• You must not use a variable until you
somehow give it a value.
• You can not assume that the variable will
have a value before you give it one.
– Some compilers do, others do not! This is the
source of many errors that are difficult to find.


Simple Data Types
Type Typical Size in Bits Minimal Range
char 8 –128 to 127
unsigned char 8 0 to 255
signed char 8 –128 to 127
int 16 –32,768 to 32,767
unsigned int 16 0 to 65,535
signed int 16 Same as int
short int 16 –32,768 to 32,767
unsigned short int 16 0 to 65,535
signed short int 16 Same as short int
long int 32 –2,147,483,648 to 2,147,483,647
signed long int 32 Same as long int
unsigned long int 32 0 to 4,294,967,295
float 32 1E–37 to 1E+37 with six digits of precision
double 64 1E–37 to 1E+37 with ten digits of precision
long double 80 1E–37 to 1E+37 with ten digits of precision

Using Variables: Initialization
• Variables may be be given initial values, or
initialized, when declared. Examples:
length
int length = 7 ; 7

diameter
float diameter = 5.9 ; 5.9

initial
char initial = ‘A’ ; ‘A’


Using Variables: Assignment
• Variables may have values assigned to them through
the use of an assignment statement.
• Such a statement uses the assignment operator =
• This operator does not denote equality. It assigns
the value of the right-hand side of the statement (the
expression) to the variable on the left-hand side.
• Examples:
diameter = 5.9 ;
area = length * width ;
Note that only single variables (LValue) may appear
on the left-hand side of the assignment operator.

Functions
• It is necessary for us to use some functions to
write our first programs.
• Functions are parts of programs that perform a
certain task and we have to give them some
information so the function can do the task.
• We will show you how to use the functions as we
go through the course and later on will show you
how to create your own.


Getting Input from User
• Every process requires some input from
the user. Variables hold the input values.
• We have a function called scanf( ) that will
allow us to do that.
• The function scanf needs two pieces of
information to display things.
– The data type of input values
– Address where to store the values
• scanf( “%f”, &diameter );

Displaying Variables
• Variables hold values that we occasionally
want to show the person using the
program.
• We have a function called printf( ) that will
allow us to do that.
• The function printf needs two pieces of
information to display things.
– How to display it
– What to display
• printf( “%fn”, &diameter );

printf( “%fn”, diameter );

• The name of the function is “printf”.
• Inside the parentheses are:
– print specification, where we are going to
display:
• a floating point value (“%f”)
• We want to have the next thing started on a new
line (“n”).
– We want to display the contents of the
variable diameter.
• printf( ) has many other capabilities.

Backslash Codes
Code Meaning
b Backspace
f Form feed
n New line
r Carriage return
t Horizontal tab
" Double quote
' Single quote
Backslash
v Vertical tab
a Alert
? Question mark
N Octal constant (where N is an octal constant)
xN Hexadecimal constant (where N is a hexadecimal constant)

Format Specifiers for printf and
scanf
Data Type Printf specifier Scanf specifier

long double %Lf %Lf
double %f %lf
float %f %f
unsigned long int %lu %lu
long int %ld %ld
unsigned int %u %u
int %d %d
short %hd %hd
char %c %c

Both printf and scanf Returns a
Value
• We can call printf as
i=810;
n=printf(“%d”,i);

• We also can call a scanf
m=scanf(“%d%f”,&i,&f)

What will be the value of n & m if every thing
goes fine.

Example: Declarations and
Assignments
#include <stdio.h> inches
garbage
int main( void ) feet
garbage
{
fathoms
int inches, feet, fathoms ; garbage
fathoms
fathoms = 7 ; 7
feet = 6 * fathoms ; feet
42
inches = 12 * feet ; inches
– 504
–
–


Example: Declarations and
Assignments (cont’d)
–
–
–
printf (“Its depth at sea: n”) ;
printf (“ %d fathoms n”, fathoms) ;
printf (“ %d feet n”, feet) ;
printf (“ %d inches n”, inches) ;

return 0 ;
}


Enhancing Our Example
• What if the depth were really 5.75
fathoms? Our program, as it is, couldn’t
handle it.
• Unlike integers, floating point numbers can
contain decimal portions. So, let’s use
floating point, rather than integer.
• Let’s also ask the user to enter the number
of fathoms, by using the scanf( ) function.


Enhanced Program
#include <stdio.h>
int main ( void )
{
float inches, feet, fathoms ;
printf (“Enter the depth in fathoms : ”) ;
scanf (“%f”, &fathoms) ;
feet = 6 * fathoms ;
inches = 12 * feet ;
printf (“ %f fathoms n”, fathoms) ;
printf (“ %f feet n”, feet) ;
printf (“ %f inches n”, inches) ;
return 0 ;
}


• The scanf( ) function also needs two items:
– The input specification “%f”. (Never put a “n”
into the input specification.)
– The address of where to store the information.
(We can input more than one item at a time if
we wish, as long as we specify it correctly.)
• Notice the “&” in front of the variable name.
It says to use the address of the variable to
hold the information that the user enters.


Final “Clean” Program
#include <stdio.h>

#define FEET_PER_FATHOM 6
#define INCHES_PER_FOOT 12

int main( void )
{
float inches ; /* number of inches deep */
float feet ; /* number of feet deep */
float fathoms ; /* number of fathoms deep */

/* Get the depth in fathoms from the user */
printf (“Enter the depth in fathoms : ”) ;

Final “Clean” Program (con’t)

/* Convert the depth to inches */
feet = FEET_PER_FATHOM * fathoms ;
inches = INCHES_PER_FOOT * feet ;

/* Display the results */
printf (“ %f fathoms n”, fathoms) ;
printf (“ %f feet n”, feet);
printf (“ %f inches n”, inches);
return 0 ;
}


Good Programming Practices
• Place each variable declaration on its own
line with a descriptive comment.
• Place a comment before each logical
“chunk” of code describing what it does.
• Do not place a comment on the same line as
code (with the exception of variable
declarations).
• Use spaces around all arithmetic and
assignment operators.
• Use blank lines to enhance readability.

(con’t)
• Place a blank line between the last
variable declaration and the first
executable statement of the program.
• Indent the body of the program 3 to 5
spaces -- be consistent!
• Comments should explain why you are
doing something, not what you are doing
it.
a = a + 1 /* add one to a */ /* WRONG */
/* count new student */ /* RIGHT*/

Another Sample Program
#include <stdio.h>

#define PI 3.14159

int main ( void )
{
float radius = 3.0;
float area;

area = PI * radius * radius;
printf( “The area is %f.n”, area );
return 0 ;
}


Arithmetic Operators in C
Name Operator Example

Addition + num1 + num2
Subtraction - initial - spent
Multiplication * fathoms * 6
Division / sum / count
Modulus % m%n


Division
• If both operands of a division expression are
integers, you will get an integer answer.
The fractional portion is thrown away.
• Examples : 17 / 5 = 3
4 / 3 = 1
35 / 9 = 3


Division (con’t)

• Division where at least one operand is a
floating point number will produce a
floating point answer.
• Examples : 17.0 / 5 = 3.4
4 / 3.2 = 1.25
35.2 / 9.1 = 3.86813
• What happens? The integer operand is
temporarily converted to a floating point,
then the division is performed.

Division By Zero
• Division by zero is mathematically
undefined.
• If you allow division by zero in a program,
it will cause a fatal error. Your program
will terminate execution and give an error
message.
• Non-fatal errors do not cause program
termination, just produce incorrect results.


Modulus
• The expression m % n yields the integer
remainder after m is divided by n.
• Modulus is an integer operation -- both
operands MUST be integers.
• Examples : 17 % 5 = 2
6%3 = 0
9%2 = 1
5%8 = 5


Uses for Modulus
• Used to determine if an integer value is
even or odd
5 % 2 = 1 odd 4 % 2 = 0 even
If you take the modulus by 2 of an integer,
a result of 1 means the number is odd and
a result of 0 means the number is even.


Arithmetic Operators
Rules of Operator Precedence
Operator(s) Precedence & Associativity
() Evaluated first. If nested,
innermost first. If on same level,
evaluated left to right.
* / % Evaluated second. If there are
several, evaluated left to right.
+ - Evaluated third. If there are
several, evaluated left to right.
= Evaluated last, right to left.


Using Parentheses
• Use parentheses to change the order in
which an expression is evaluated.
a+b*c Would multiply b * c first,
then add a to the result.
If you really want the sum of a and b to be
multiplied by c, use parentheses to force the
evaluation to be done in the order you want.
(a + b) * c
• Also use parentheses to clarify a complex
expression.

Practice With Evaluating
Expressions
Given integer variables a, b, c, d, and e,
where a = 1, b = 2, c = 3, d = 4,
evaluate the following expressions:
a+b-c+d
a*b/c
1+a*b%c
a+d%b-c
e=b=d+c/b-a

Relational Operators
< less than
> greater than
<= less than or equal to
>= greater than or equal to
== is equal to
!= is not equal to

Relational expressions evaluate to the integer
values 1 (true) or 0 (false).
All of these operators are called binary operators
because they take two expressions as operands.

Practice with Relational
Expressions
int a = 1, b = 2, c = 3 ;
Expression Value Expression Value
a < c a + b >= c
b <= c a + b == c
c <= a a != b
a>b a + b != c
b >= c


Arithmetic Expressions: True or
False
• Arithmetic expressions evaluate to
numeric values.

• An arithmetic expression that has a value
of zero is false.

• An arithmetic expression that has a value
other than zero is true.


Practice with Arithmetic
Expressions
int a = 1, b = 2, c = 3 ;
float x = 3.33, y = 6.66 ;
Expression Numeric Value True/False
a+b
b-2*a
c-b-a
c-a
y-x
y-2*x

Increment and Decrement
Operators
• The increment operator ++
• The decrement operator --
• Precedence: lower than (), but higher than
* / and %
• Associativity: right to left
• Increment and decrement operators can
only be applied to variables, not to
constants or expressions


Increment Operator
• If we want to add one to a variable, we can
say:
count = count + 1 ;
• Programs often contain statements that
increment variables, so to save on typing, C
provides these shortcuts:
count++ ; OR ++count ;
Both do the same thing. They change the
value of count by adding one to it.


Postincrement Operator
• The position of the ++ determines when the value
is incremented. If the ++ is after the variable, then
the incrementing is done last (a postincrement).
int amount, count ;
count = 3 ;
amount = 2 * count++ ;

• amount gets the value of 2 * 3, which is 6, and
then 1 gets added to count.
• So, after executing the last line, amount is 6 and
count is 4.

Preincrement Operator
• If the ++ is before the variable, then the
incrementing is done first (a preincrement).
int amount, count ;
count = 3 ;
amount = 2 * ++count ;

• 1 gets added to count first, then amount gets the
value of 2 * 4, which is 8.
count is 4.


Code Example Using ++
#include <stdio.h>
int main ( )
{
int i = 1 ;
/* count from 1 to 10 */
while ( i < 11 )
{
printf (“%d ”, i) ;
i++ ; /* same as ++i */
}
return 0 ;
}


Decrement Operator
• If we want to subtract one from a variable, we
can say:
count = count - 1 ;
• Programs often contain statements that
decrement variables, so to save on typing, C
provides these shortcuts:
count-- ; OR --count ;
Both do the same thing. They change the
value of count by subtracting one from it.


Postdecrement Operator
• The position of the -- determines when the value is
decremented. If the -- is after the variable, then
the decrementing is done last (a postdecrement).
int amount, count ;
count = 3 ;
amount = 2 * count-- ;

• amount gets the value of 2 * 3, which is 6, and
then 1 gets subtracted from count.
count is 2.

Predecrement Operator
• If the -- is before the variable, then the
decrementing is done first (a predecrement).
int amount, count ;
count = 3 ;
amount = 2 * --count ;

• 1 gets subtracted from count first, then amount
gets the value of 2 * 2, which is 4.
count is 2.


A Hand Trace Example
int answer, value = 4 ;
Code Value Answer
4 garbage
value = value + 1 ;
value++ ;
++value ;
answer = 2 * value++ ;
answer = ++value / 2 ;
value-- ;
--value ;
answer = --value * 2 ;


Lvalue Required

answer++ = value-- / 3 ;

• In C any value that is having an address is
called an Lvalue.


Practice
Given
int a = 1, b = 2, c = 3 ;

What is the value of this expression?

++a * b - c--

What are the new values of a, b, and c?


More Practice
Given
int a = 1, b = 2, c = 3, d = 4 ;

What is the value of this expression?

++b / c + a * d++

What are the new values of a, b, c, and d?


Assignment Operators
= += -= *= /= %=
Statement Equivalent Statement
a=a+2; a += 2 ;
a=a-3; a -= 3 ;
a=a*2; a *= 2 ;
a=a/4; a /= 4 ;
a=a%2; a %= 2 ;
b=b+(c+2); b += c + 2 ;
d=d*(e-5); d *= e - 5 ;

Practice with Assignment
Operators
int i = 1, j = 2, k = 3, m = 4 ;

Expression Value
i += j + k

j *= k = m + 5

k -= m /= j * 2


Code Example Using /= and ++
Counting the Digits in an Integer
#include <stdio.h>
int main ( )
{
int num, temp, digits = 0 ;
temp = num = 4327 ;
while ( temp > 0 )
{
printf (“%dn”, temp) ;
temp /= 10 ;
digits++ ;
}
printf (“There are %d digits in %d.n”, digits, num) ;
return 0 ;
}


Operator Precedence and
Associativity
Precedence Associativity

() left to right/inside-out
++ -- ! + (unary) - (unary) (type) right to left
* / % left to right
+ (addition) - (subtraction) left to right
< <= > >= left to right
== != left to right
&& left to right
|| left to right
= += -= *= /= %= right to left
, (comma) right to left

Review: Structured
Programming
• All programs can be written in terms of
only three control structures
– The sequence structure
• Unless otherwise directed, the statements are
executed in the order in which they are written.
– The selection structure
• Used to choose among alternative courses of
action.
– The repetition structure
• Allows an action to be repeated while some
condition remains true.

Selection: the if statement

if ( condition )
{
statement(s) /* body of the if statement */
}

The braces are not required if the body contains
only a single statement. However, they are a
good idea and are required by the 104 C Coding
Standards.


Examples
if ( age >= 18 )
{
printf(“Vote!n”) ;
}

if ( value == 0 )
{
printf (“The value you entered was zero.n”) ;
printf (“Please try again.n”) ;
}

Good Programming Practice

• Always place braces around the body of
an if statement.
• Advantages:
– Easier to read
– Will not forget to add the braces if you go
back and add a second statement to the body
– Less likely to make a semantic error
• Indent the body of the if statement 3 to 5

Selection: the if-else statement
if ( condition )
{
statement(s) /* the if clause */
}
else
{
statement(s) /* the else clause */
}

Example
if ( age >= 18 )
{
}
else
{
printf(“Maybe next time!n”) ;
}

Example

if ( value == 0 )
{
printf(“Please try again.n”) ;
}
else
{
printf (“Value = %d.n”, value) ;
}

• Always place braces around the bodies of
the if and else clauses of an if-else
statement.
• Advantages:
– Easier to read
– Will not forget to add the braces if you go back
and add a second statement to the clause
• Indent the bodies of the if and else clauses
3 to 5 spaces -- be consistent!

The Conditional Operator
expr1 ? expr2 : expr3

If expr1 is true then expr2 is executed, else expr3 is evaluated,
i.e.:

x = ((y < z) ? y : z);

OR

(y < z) ? printf(“%d is smaller”,y): printf(“%d is smaller”,y);


Nesting of if-else Statements
if ( condition1 )
{
statement(s)
}
else if ( condition2 )
{
statement(s)
}
... /* more else clauses may be here */
else
{
statement(s) /* the default case */
}


Example
if ( value == 0 )
{
}
else if ( value < 0 )
{
printf (“%d is negative.n”, value) ;
}
else
{
printf (“%d is positive.n”, value) ;
}

Gotcha! = versus ==
int a = 2 ;

if ( a = 1 ) /* semantic (logic) error! */
{
printf (“a is onen”) ;
}
else if ( a == 2 )
{
printf (“a is twon”) ;
}
else
{
printf (“a is %dn”, a) ;
}

Gotcha (con’t)
• The statement if (a = 1) is syntactically correct,
so no error message will be produced. (Some
compilers will produce a warning.) However, a
semantic (logic) error will occur.

• An assignment expression has a value -- the
value being assigned. In this case the value
being assigned is 1, which is true.

• If the value being assigned was 0, then the
expression would evaluate to 0, which is false.

Logical Operators
• So far we have seen only simple conditions.
if ( count > 10 ) . . .
• Sometimes we need to test multiple conditions in
order to make a decision.
• Logical operators are used for combining simple
conditions to make complex conditions.
&& is AND if ( x > 5 && y < 6 )
|| is OR if ( z == 0 || x > 10 )
! is NOT if (! (bob > 42) )

Example Use of &&

if ( age < 1 && gender == ‘m’)
{
printf (“Infant boyn”) ;
}


Truth Table for &&

Expression1 Expression2 Expression1 && Expression2

0 0 0
0 nonzero 0
nonzero 0 0
nonzero nonzero 1

Exp1 && Exp2 && … && Expn will evaluate to 1 (true)
only if ALL subconditions are true.


Example Use of ||

if (grade == ‘D’ || grade == ‘F’)
{
printf (“See with your Juniors !n”) ;
}


Truth Table for ||

Expression1 Expression2 Expression1 || Expression2

0 0 0
0 nonzero 1
nonzero 0 1
nonzero nonzero 1

Exp1 && Exp2 && … && Expn will evaluate to 1
(true) if only ONE subcondition is true.


Example Use of !

if ( ! (x == 2) ) /* same as (x != 2) */
{
printf(“x is not equal to 2.n”) ;
}


Truth Table for !

Expression ! Expression

0 1
nonzero 0


Gotcha! && or ||
int a = 0 ;
int b=1;
if ( (a++ == 1) && (b++==1 ) ) /* semantic (logic) error! */
{
printf (“First Gotchan”) ;
}
else if ( (a++ == 0) || (b++==1 ) )
{
printf (“Second Gotchan”) ;
}
else
{
printf (“a is %dn”, a) ;
}

Gotcha (con’t)
• While evaluating a condition if first subpart of a
Complex condition having && operator is false
than the remaining subpart will not be evaluated.

• Similarly While evaluating a condition if first
subpart of a Complex condition having || operator
is true than the remaining subpart will not be
evaluated.


Some Practice Expressions
int a = 1, b = 0, c = 7;

Expression Numeric Value True/False
a
b
c
a+b
a && b
a || b
!c
!!c
a && !b
a < b && b < c
a > b && b < c
a >= b || b > c

More Practice
Given
int a = 5, b = 7, c = 17 ;
evaluate each expression as True or False.
1. c / b == 2
2. c % b <= a % b
3. b + c / a != c - a
4. (b < c) && (c == 7)
5. (c + 1 - b == 0) || (b = 5)


Review: Repetition Structure

• A repetition structure allows the programmer
to specify that an action is to be repeated while
some condition remains true.
• There are three repetition structures in C, the
while loop, the for loop, and the do-while loop.


The while Repetition Structure
while ( condition )
{
statement(s)
}

The braces are not required if the loop body
contains only a single statement. However, they
are a good idea and are required by the 104 C
Coding Standards.


Parts of a While Loop

• Every while loop will always contain
three main elements:
– Priming: initialize your variables.
– Testing: test against some known condition.
– Updating: update the variable that is tested.

125

Simple While Loop
OUTPUT:
Index: 1

#include <stdio.h> Index: 2
Index: 3
#define MAX 10 Index: 4
main () 1. Priming
Index: 5
{ Index: 6
2. Test Condition
Index: 7
int index =1;
Index: 8
while (index <= MAX) { Index: 9
printf ("Index: %dn", index);Index: 10
index = index + 1;
3. Update
}
}

126

Example
while ( children > 0 )
{
children = children - 1 ;
cookies = cookies * 2 ;
}



• Always place braces around the body of a
while loop.
• Advantages:
– Easier to read
– Will not forget to add the braces if you go
back and add a second statement to the loop
body
• Indent the body of a while loop 3 to 5

Another while Loop Example
• Problem: Write a program that calculates
the average exam grade for a class of 10
students.
• What are the program inputs?
– the exam grades
• What are the program outputs?
– the average exam grade


The Pseudocode
<total> = 0
<grade_counter> = 1

While (<grade_counter> <= 10)
Display “Enter a grade: ”
Read <grade>
<total> = <total> + <grade>
<grade_counter> = <grade_counter> + 1
End_while
<average> = <total> / 10
Display “Class average is: “, <average>


The C Code
#include <stdio.h>
int main ( )
{
int counter, grade, total, average ;
total = 0 ;
counter = 1 ;
while ( counter <= 10 )
{
printf (“Enter a grade : “) ;
scanf (“%d”, &grade) ;
total = total + grade ;
counter = counter + 1 ;
}
average = total / 10 ;
printf (“Class average is: %dn”, average) ;
return 0 ;
}

Versatile?

• How versatile is this program?
• It only works with class sizes of 10.
• We would like it to work with any class size.
• A better way :
– Ask the user how many students are in the
class. Use that number in the condition of the
while loop and when computing the average.


New Pseudocode
<total> = 0
<grade_counter> = 1
Display “Enter the number of students: “
Read <num_students>
While (<grade_counter> <= <num_students>)
Display “Enter a grade: ”
Read <grade>
End_while
<average> = <total> / <num_students>


New C Code
#include <stdio.h>
int main ( )
{
int numStudents, counter, grade, total, average ;
total = 0 ;
counter = 1 ;
printf (“Enter the number of students: “) ;
scanf (“%d”, &numStudents) ;
while ( counter <= numStudents) {
printf (“Enter a grade : “) ;
scanf (“%d”, &grade) ;
}
average = total / numStudents ;
return 0 ;
}

Why Bother to Make It Easier?
• Why do we write programs?
– So the user can perform some task
• The more versatile the program, the more
difficult it is to write. BUT it is more useable.
• The more complex the task, the more difficult
it is to write. But that is often what a user
needs.
• Always consider the user first.


Using a Sentinel Value
• We could let the user keep entering grades
and when he’s done enter some special
value that signals us that he’s done.
• This special signal value is called a
sentinel value.
• We have to make sure that the value we
choose as the sentinel isn’t a legal value.
For example, we can’t use 0 as the sentinel
in our example as it is a legal value for an
exam score.

The Priming Read
• When we use a sentinel value to control a
while loop, we have to get the first value
from the user before we encounter the
loop so that it will be tested and the loop
can be entered.
• This is known as a priming read.
• We have to give significant thought to the
initialization of variables, the sentinel
value, and getting into the loop.


New Pseudocode
<total> = 0
<grade_counter> = 1
Display “Enter a grade: “
Read <grade>
While ( <grade> != -1 )
Display “Enter another grade: ”
Read <grade>
End_while
<average> = <total> / <grade_counter>


New C Code
#include <stdio.h>
int main ( )
{
int counter, grade, total, average ;
total = 0 ;
counter = 1 ;
printf(“Enter a grade: “) ;
scanf(“%d”, &grade) ;
while (grade != -1) {
printf(“Enter another grade: “) ;
}
average = total / counter ;
return 0 ;
}

Final “Clean” C Code
#include <stdio.h>
int main ( )
{
int counter ; /* counts number of grades entered */
int grade ; /* individual grade */
int total; /* total of all grades */
int average ; /* average grade */

/* Initializations */
total = 0 ;
counter = 1 ;


Final “Clean” C Code (con’t)
/* Get grades from user */
/* Compute grade total and number of grades */

printf(“Enter a grade: “) ;
while (grade != -1) {
printf(“Enter another grade: “) ;
}

/* Compute and display the average grade */
average = total / counter ;

return 0 ;
}

Using a while Loop to Check
#include <stdio.h>
User Input
int main ( )
{
int number ;
printf (“Enter a positive integer : “) ;
scanf (“%d”, &number) ;
while ( number <= 0 )
{
printf (“nThat’s incorrect. Try again.n”) ;
printf (“Enter a positive integer: “) ;
scanf (“%d”, &number) ;
}
printf (“You entered: %dn”, number) ;
return 0 ;
}


Counter-Controlled Repetition
(Definite Repetition)
• If it is known in advance exactly how many
times a loop will execute, it is known as a
counter-controlled loop.
int i = 1 ;
while ( i <= 10 )
{
printf(“i = %dn”, i) ;
i=i+1;
}


Counter-Controlled Repetition
(con’t)
• Is the following loop a counter-controlled
loop?

while ( x != y )
{
printf(“x = %d”, x) ;
x=x+2;
}


Event-Controlled Repetition
(Indefinite Repetition)
• If it is NOT known in advance exactly how
many times a loop will execute, it is known
as an event-controlled loop.
sum = 0 ;
printf(“Enter an integer value: “) ;
scanf(“%d”, &value) ;
while ( value != -1) {
sum = sum + value ;
printf(“Enter another value: “) ;
scanf(“%d”, &value) ;
}

Event-Controlled Repetition
(con’t)
• An event-controlled loop will terminate
when some event occurs.
• The event may be the occurrence of a
sentinel value, as in the previous example.
• There are other types of events that may
occur, such as reaching the end of a data
file.


The 3 Parts of a Loop
#include <stdio.h>
int main ()
{
int i = 1 ; initialization of loop control variable

/* count from 1 to 100 */
while ( i < 101 ) test of loop termination condition
{
printf (“%d “, i) ;
i=i+1; modification of loop control
} variable
return 0 ;
}


The for Loop Repetition
Structure
• The for loop handles details of the counter-controlled
loop “automatically”.
• The initialization of the the loop control variable, the
termination condition test, and control variable
modification are handled in the for loop structure.

for ( i = 1; i < 101; i = i + 1)
{
initialization modification
} test


When Does a for Loop Initialize, Test
and Modify?
• Just as with a while loop, a for loop
– initializes the loop control variable before
beginning the first loop iteration,
– modifies the loop control variable at the very
end of each iteration of the loop, and
– performs the loop termination test before each
iteration of the loop.
• The for loop is easier to write and read for
counter-controlled loops.


A for Loop That Counts From 0
to 9
for ( i = 0; i < 10; i = i + 1 )
{
printf (“%dn”, i) ;
}


We Can Count Backwards, Too

for ( i = 9; i >= 0; i = i - 1 )
{
}


We Can Count By 2’s ... or 7’s
… or Whatever

for ( i = 0; i < 10; i = i + 2 )
{
}


The do-while Repetition
Structure
do
{
statement(s)
} while ( condition ) ;

• The body of a do-while is ALWAYS
executed at least once. Is this true of a
while loop? What about a for loop?


Example
do
{
printf (“Enter a positive number: “) ;
scanf (“%d”, &num) ;
if ( num <= 0 )
{
printf (“nThat is not positive. Try againn”) ;
}
} while ( num <= 0 ) ;


An Equivalent while Loop
while ( num <= 0 )
{
}

• Notice that using a while loop in this case
requires a priming read.


An Equivalent for Loop
for ( ; num <= 0; )
{
}

• A for loop is a very awkward choice here because
the loop is event-controlled.


So, Which Type of Loop Should I
Use?
• Use a for loop for counter-controlled
repetition.
• Use a while or do-while loop for event-
controlled repetition.
– Use a do-while loop when the loop must
execute at least one time.
– Use a while loop when it is possible that the
loop may never execute.


Infinite Loop

• Infinite Loop: A loop that never ends.
– Generally, you want to avoid these!
– There are special cases, however, when
you do want to create infinite loops on
purpose.
• Common Exam Questions:
– Given a piece of code, identify the bug in
the code.
– You may need to identify infinite loops.


Infinite Loop Example #1
#include <stdio.h>
#define MAX 10
Index: 1
main ()
Index: 1
{
Index: 1
int index =1;
Index: 1
while (index <= MAX)
{ Index: 1

printf ("Index: %dn", index);
} …
[forever]
}


Infinite Loop, Example #2
#include <stdio.h>
Index: 1
/*no MAX here*/
main () Index: 2

{ Index: 3

int index = 1; Index: 4

while (index > 0) Index: 5
{
printf ("Index: %dn", index); … [forever] ?
index = index + 1;
}
}

Nested Loops

• Loops may be nested (embedded) inside
of each other.
• Actually, any control structure (sequence,
selection, or repetition) may be nested
inside of any other control structure.
• It is common to see nested for loops.


Nested for Loops
for ( i = 1; i < 5; i = i + 1 )
{
for ( j = 1; j < 3; j = j + 1 )
{
if ( j % 2 == 0 ) How many times is the “if”
{ statement executed?
printf (“O”) ;
}
What is the output ?
else
{
printf (“X”) ;
}
}
printf (“n”) ;
}


The break Statement
• The break statement can be used in
while, do-while, and for loops to
cause premature exit of the loop.


Example break in a for Loop
#include <stdio.h>
int main ( )
{ OUTPUT:
int i ;
for ( i = 1; i < 10; i = i + 1 ) 1234
{
if (i == 5)
Broke out of loop at i = 5.
{
break ;
}
printf (“%d “, i) ;
}
printf (“nBroke out of loop at i = %d.n”, i) ;
return 0 ;
}


The continue Statement

• The continue statement can be used
in while, do-while, and for loops.
• It causes the remaining statements in
the body of the loop to be skipped for
the current iteration of the loop.


Example continue in a for Loop
#include <stdio.h>
int main ( )
{ OUTPUT:
int i ;
for ( i = 1; i < 10; i = i + 1 ) 12346789
{
if (i == 5)
Done.
{
continue ;
}
printf (“%d ”, i) ;
}
printf (“nDone.n”) ;
return 0 ;
}

Debugging Tips
• Trace your code by hand (a hand trace),
keeping track of the value of each
variable.
• Insert temporary printf() statements so you
can see what your program is doing.
– Confirm that the correct value(s) has been
read in.
– Check the results of arithmetic computations
immediately after they are performed.


Multiple Selection

• So far, we have only seen binary
selection. if ( age >= 18 )
if ( age >= 18 )
{
{
}
}
else
{
printf(“Maybe next time!n”) ;
}


Multiple Selection (con’t)

• Sometimes it is necessary to branch in
more than two directions.
• We do this via multiple selection.
• The multiple selection mechanism in C is
the switch statement.


Multiple Selection with if
if (day == 0 ) {
printf (“Sunday”) ; if (day == 4) {
} printf (“Thursday”) ;
}
if (day == 1 ) { if (day == 5) {
printf (“Monday”) ; printf (“Friday”) ;
} }
if (day == 2) { if (day == 6) {
printf (“Tuesday”) ; printf (“Saturday”) ;
}
}
if ((day < 0) || (day > 6)) {
if (day == 3) { printf(“Error - invalid day.n”) ;
printf (“Wednesday”) ; }
}


Multiple Selection with if-else
if (day == 0 ) {
printf (“Sunday”) ;
} else if (day == 1 ) {
printf (“Monday”) ;
} else if (day == 2) { This if-else structure is more
printf (“Tuesday”) ;
} else if (day == 3) { efficient than the corresponding
printf (“Wednesday”) ;
} else if (day == 4) {
if structure. Why?
printf (“Thursday”) ;
} else if (day == 5) {
printf (“Friday”) ;
} else if (day = 6) {
printf (“Saturday”) ;
} else {
printf (“Error - invalid day.n”) ;
}


The switch Multiple-Selection
Structure
switch ( integer expression )
{
case constant1 :
statement(s)
break ;
case constant2 :
statement(s)
break ;

...
default:
statement(s)
break ;
}


switch Statement Details
• The last statement of each case in the
switch should almost always be a break.
• The break causes program control to jump
to the closing brace of the switch structure.
• Switch statement can only test for equality
condition (==).
• A switch statement will compile without a
default case, but always consider using one.



• Include a default case to catch invalid
data.
• Inform the user of the type of error that
has occurred (e.g., “Error - invalid day.”).
• If appropriate, display the invalid value.


switch Example
switch ( day )
{
case 0: printf (“Sundayn”) ;
break ;
case 1: printf (“Mondayn”) ; Is this structure more
break ;
case 2: printf (“Tuesdayn”) ; efficient than the
break ;
case 3: printf (“Wednesdayn”) ;
equivalent nested if-else
break ; structure?
case 4: printf (“Thursdayn”) ;
break ;
case 5: printf (“Fridayn”) ;
break ;
case 6: printf (“Saturdayn”) ;
break ;
default: printf (“Error -- invalid day.n”) ;
break ;
}


Why Use a switch Statement?

• A nested if-else structure is just as efficient
as a switch statement.
• However, a switch statement may be
easier to read.
• Also, it is easier to add new cases to a
switch statement than to a nested if-else
structure.


The char Data Type
• The char data type holds a single character.
char ch;
• Example assignments:
char grade, symbol;
grade = ‘B’;
symbol = ‘$’;
• The char is held as a one-byte integer in memory.
The ASCII code is what is actually stored, so we
can use them as characters or integers,
depending on our need.

The char Data Type (con’t)
• Use
scanf (“%c”, &ch) ;
to read a single character into the variable ch.
(Note that the variable does not have to be called
“ch”.”)
• Use
printf(“%c”, ch) ;
to display the value of a character variable.


char Example
#include <stdio.h>
int main ( )
{
char ch ;

printf (“Enter a character: “) ;
scanf (“%c”, &ch) ;
printf (“The value of %c is %d.n”, ch, ch) ;
return 0 ;
}

If the user entered an A, the output would be:
The value of A is 65.

The getchar ( ) Function
• The getchar( ) function is found in the stdio
library.
• The getchar( ) function reads one character
from stdin (the standard input buffer) and
returns that character’s ASCII value.
• The value can be stored in either a character
variable or an integer variable.


getchar ( ) Example
#include <stdio.h>
int main ( )
{
char ch ; /* int ch would also work! */

ch = getchar( ) ;
return 0 ;
}


Problems with Reading Characters
• When getting characters, whether using scanf( ) or
getchar( ), realize that you are reading only one
character.
• What will the user actually type? The character
he/she wants to enter, followed by pressing ENTER.
• So, the user is actually entering two characters,
his/her response and the newline character.
• Unless you handle this, the newline character will
remain in the stdin stream causing problems the next
time you want to read a character. Another call to
scanf() or getchar( ) will remove it.

Improved getchar( ) Example
#include <stdio.h>
int main ( )
{
char ch, newline ;

ch = getchar( ) ;
newline = getchar( ) ; /* could also use scanf(“%c”, &newline) ; */
return 0 ;
}



Additional Concerns with Garbage in
stdin
• When we were reading integers using scanf( ), we
didn’t seem to have problems with the newline
character, even though the user was typing ENTER
after the integer.
• That is because scanf( ) was looking for the next
integer and ignored the newline (whitespace).
• If we use scanf (“%d”, &num); to get an integer, the
newline is still stuck in the input stream.
• If the next item we want to get is a character, whether
we use scanf( ) or getchar( ), we will get the newline.
• We have to take this into account and remove it.

EOF Predefined Constant
• getchar( ) is usually used to get characters from
a file until the end of the file is reached.
• The value used to indicate the end of file varies
from system to system. It is system
dependent.
• But, regardless of the system you are using,
there is a #define in the stdio library for a
symbolic integer constant called EOF.
• EOF holds the value of the end-of-file marker for
the system that you are using.


getchar( ) Example Using EOF
#include <stdio.h>
int main ()
{
int grade, aCount, bCount, cCount, dCount, fCount ;
aCount = bCount = cCount = dCount = fCount = 0 ;
while ( (grade = getchar( ) ) != EOF ) {
switch ( grade ) {
case ‘A’: aCount++; break ;
case ‘B’: bCount++; break ;
case ‘C’ : cCount++; break ;
case ‘D’: dCount++; break ;
case ‘F’: fCount++; break ;
default : break ;
}
}
return 0 ;
}


Incremental Programming
Review
• Write your code in incomplete but working pieces.
• For example, for your projects,
– Don’t write the whole program at once.
– Just write enough to display the user prompt on
the screen.
– Get that part working first (compile and run).
– Next, write the part that gets the value from the
user, and then just print it out.


Increment Programming Review
(con’t)
– Get that working (compile and run).
– Next, change the code so that you use the
value in a calculation and print out the
answer.
– Get that working (compile and run).
– Continue this process until you have the final
version.
– Get the final version working.
• Bottom line: Always have a working
version of your program!

Example of Incremental
Programming
Problem:
• Write an interactive program that allows the
user to calculate the interest accrued on a
savings account. The interest is compounded
annually.
• The user must supply the principal amount,
the interest rate, and the number of years
over which to compute the interest.


Rough Algorithm
Print explanation of the program
Get <principal> from user
Get <interest rate> from user
Get <number of years> from user
<amount> = <principal>
While (<number of years> > 0 )
amount = amount + (amount X <interest rate>)
<number of years> = <number of year> + 1
End_while
<interest accrued> = <amount> - <principal>
Display report

Report Design

Interest rate : 7.0000 %
Period : 20 years

Principal at start of period : 1000.00
Interest accrued : 2869.68
Total amount at end of period : 3869.68


Version #1
/* Filename: interest.c
* Author: Hemant Mehta
* Date written: 11/14//06
* Description: This program computes the interest accrued in an account
* that compounds interest annually. */
#include <stdio.h>
int main ( )
{
/* Print Instructions */
printf (“This program computes the interest accrued in an account thatn”);
printf (“compounds interest annually. You will need to enter the amountn”);
printf (“of the principal, the interest rate and the number of years.nn”);

return 0;
}


Output #1

This program computes the interest accrued in an account that
compounds interest annually. You will need to enter the amount
of the principal, the interest rate and the number of years.


Version #2
* Author: ___________
#include <stdio.h>
int main ( )
{
float principal, rate ;
int years ;
printf (“This program computes the interest accrued in an account thatn”) ;
printf (“compounds interest annually. You will need to enter the amountn”) ;
printf (“of the principal, the interest rate and the number of years.nn”) ;
/* Get input from user */
printf (“Enter the principal amount : “) ;
scanf (“%f”, &principal) ;
printf (“Enter the interest rate as a decimal (for 7%% enter .07) : “) ;
scanf (“%f”, &rate) ;
printf (“Enter the number of years : “) ;
scanf (“%d”, &years) ;
printf (“nprincipal = %f, rate = %f, years = %dn”, principal, rate, years ) ;
return 0 ;
}


Output #2

Enter the principal amount : 1000.00
Enter the interest rate as a decimal (for 7% enter .07) : .07
Enter the number of years : 20

principal = 1000.000000, rate = 0.070000, years = 20


Version #3
* Author: ____________
#include <stdio.h>
int main ( )
{
float principal, rate, amount, interest ;
int years, i ;
printf (“Enter the principal amount : “);
scanf (“%f”, &principal);
scanf (“%f”, &rate);
printf (“Enter the number of years : “);
scanf (“%d”, &years);


Version #3 (con’t)
/* Save the original principal amount by varying another variable, amount */
amount = principal;
/* Calculate total amount in the account after the specified number of years */
for ( i = 0 ; i < 1 ; i++ )
{
amount += amount * rate ;
}
/* Calculate accrued interest */
interest = amount - principal ;

printf (“amount = %f, interest = %fn”);

return 0 ;
}


Output #3


amount = 1070.000000, interest = 70.000000


Version #4
* Author: ____________
#include <stdio.h>
int main ( )
{
int years, i ;


amount = principal;
for ( i = 0 ; i < 2 ; i++ )
{
}


return 0 ;
}


Output #4


amount = 1144.900000, interest = 144.900000


Version #5
* Author: ____________
#include <stdio.h>
int main ( )
{
int years, i ;


amount = principal;
for ( i = 0 ; i < years ; i++ )
{
}


return 0 ;
}


Output #5


amount = 3869.680000, interest = 2869.680000


Final Version
* Author: ____________
#include <stdio.h>
int main ( )
{
int years, i ;


Final Version (con’t)
amount = principal;
for ( i = 0 ; i < years ; i++ )
{
}
/* Print report */
printf (“Interest rate : %.4f %%n”, 100 * rate ) ;
printf (“ Period : %d yearsnn”, years ) ;
printf (“ Principal at start of period : %9.2f”, principal );
printf (“ Interest accrued : %9.2f”, interest );
printf (“Total amount at end of period : %9.2f”, amount);

return 0 ;
}


Final Output


Interest rate : 7.0000 %
Period : 20 years

Principal at start of period : 1000.00
Interest accrued : 2869.68
Total amount at end of period : 3869.68


Top-Down Design
• If we look at a problem as a whole, it may seem
impossible to solve because it is so complex.
Examples:
– writing a tax computation program
– writing a word processor
• Complex problems can be solved using top-
down design, also known as stepwise
refinement, where
– We break the problem into parts
– Then break the parts into parts
– Soon, each of the parts will be easy to do

Advantages of Top-Down
Design
• Breaking the problem into parts helps us to
clarify what needs to be done.
• At each step of refinement, the new parts
become less complicated and, therefore,
easier to figure out.
• Parts of the solution may turn out to be
reusable.
• Breaking the problem into parts allows more
than one person to work on the solution.


An Example of Top-Down
Design
• Problem:
– We own a home improvement company.
– We do painting, roofing, and basement
waterproofing.
– A section of town has recently flooded
(zip code 21222).
– We want to send out pamphlets to our
customers in that area.


The Top Level
• Get the customer list from a file.
• Sort the list according to zip code.
• Make a new file of only the customers with the zip
code 21222 from the sorted customer list.
• Print an envelope for each of these customers.

Main

Read Sort Select Print


Another Level?
• Should any of these steps be broken down
further? Possibly.
• How do I know? Ask yourself whether or
not you could easily write the algorithm for
the step. If not, break it down again.
• When you are comfortable with the
breakdown, write the pseudocode for each
of the steps (modules) in the hierarchy.
• Typically, each module will be coded as a
separate function.

Structured Programs
• We will use top-down design for all remaining
programming projects.
• This is the standard way of writing programs.
• Programs produced using this method and using
only the three kinds of control structures,
sequential, selection and repetition, are called
structured programs.
• Structured programs are easier to test, modify,
and are also easier for other programmers to
understand.


Another Example

• Problem: Write a program that draws this
picture of a house.


The Top Level
• Draw the outline of the house
• Draw the chimney
• Draw the door
• Draw the windows

Main

Draw Draw Draw Draw
Outline Chimney Door Windows


Pseudocode for Main

Call Draw Outline
Call Draw Chimney
Call Draw Door
Call Draw Windows


Observation
• The door has both a frame and knob. We
could break this into two steps.
Main

Draw Draw Draw Draw
Outline Chimney Door Windows

Draw Draw
Door Frame Knob


Pseudocode for Draw Door

Call Draw Door Frame
Call Draw Knob


Another Observation

• There are three windows to be drawn.

Main

Draw Draw
Outline . . . Windows

Draw Draw Draw
Window 1 Window 2 Window 3


One Last Observation
• But don’t the windows look the same?
They just have different locations.
• So, we can reuse the code that draws a
window.
– Simply copy the code three times and edit it to
place the window in the correct location, or
– Use the code three times, “sending it” the
correct location each time (we will see how to
do this later).
• This is an example of code reuse.

Reusing the Window Code

Main

Draw Draw
Outline . . . Windows

Draw a
Window


Pseudocode for Draw Windows

Call Draw a Window, sending in Location 1


Review of Structured
Programming
• Structured programming is a problem solving
strategy and a programming methodology that
includes the following guidelines:
– The program uses only the sequence,
selection, and repetition control structures.
– The flow of control in the program should be
as simple as possible.
– The construction of a program embodies top-
down design.


Review of Top-Down Design
• Involves repeatedly decomposing a
problem into smaller problems
• Eventually leads to a collection of small
problems or tasks each of which can be
easily coded
• The function construct in C is used to
write code for these small, simple
problems.


Functions
• A C program is made up of one or more functions,
one of which is main( ).
• Execution always begins with main( ), no matter
where it is placed in the program. By convention,
main( ) is located before all other functions.
• When program control encounters a function
name, the function is called (invoked).
– Program control passes to the function.
– The function is executed.
– Control is passed back to the calling function.


Sample Function Call
#include <stdio.h>

int main ( ) printf is the name of a predefined
{ function in the stdio library

printf (“Hello World!n”) ; this statement is
return 0 ; is known as a
} function call
this is a string we are passing
as an argument (parameter) to
the printf function


Functions (con’t)
• We have used few predefined functions such
as:
– printf
– scanf
– getchar
• Programmers can write their own functions.
• Typically, each module in a program’s design
hierarchy chart is implemented as a function.
• C function names follow the same naming rules
as C variables.

Sample Programmer-Defined
#include <stdio.h>
Function
void printMessage ( void ) ;

int main ( )
{
printMessage ( ) ;
return 0 ;
}

void printMessage ( void )
{
printf (“A message for you:nn”) ;
printf (“Have a nice day!n”) ;
}


Examining printMessage
#include <stdio.h>

void printMessage ( void ) ; function prototype

int main ( )
{
printMessage ( ) ; function call
return 0 ;
}

void printMessage ( void ) function header
{
printf (“A message for you:nn”) ; function
printf (“Have a nice day!n”) ; body
}

function definition

The Function Prototype
• Informs the compiler that there will be a function
defined later that:
returns this type
has this name
takes these arguments

void printMessage (void) ;

• Needed because the function call is made before
the definition -- the compiler uses it to see if the
call is made properly

The Function Call
• Passes program control to the function
• Must match the prototype in name, number of
arguments, and types of arguments
void printMessage (void) ;
int main ( ) same name no arguments
{
printMessage ( ) ;
return 0 ;
}

The Function Definition
• Control is passed to the function by the function
call. The statements within the function body will
then be executed.
void printMessage ( void )
{
printf (“A message for you:nn”) ;
}

• After the statements in the function have
completed, control is passed back to the calling
function, in this case main( ) . Note that the
calling function does not have to be main( ) .

General Function Definition
Syntax
type functionName ( parameter1, . . . , parametern )
{
variable declaration(s)
statement(s)
}
If there are no parameters, either
functionName( ) OR functionName(void)
is acceptable.
• There may be no variable declarations.
• If the function type (return type) is void, a return
statement is not required, but the following are permitted:
return ; OR return( ) ;

Using Input Parameters
void printMessage (int counter) ;
int main ( )
{
int num;
printf (“Enter an integer: “) ;
printMessage (num) ; one argument matches the one formal parameter
return 0 ; of type int of type int
}

void printMessage (int counter)
{
int i ;
for ( i = 0; i < counter; i++ )
{
}
}

Final “Clean” C Code
#include <stdio.h>

void printMessage (int counter) ;

int main ( )
{
int num ; /* number of times to print message */

printf (“Enter an integer: “) ;
printMessage (num) ;

return 0 ;
}


Final “Clean” C Code (con’t)
/*************************************************************************
** printMessage - prints a message a specified number of times
** Inputs: counter - the number of times the message will be
** printed
** Outputs: None
/*************************************************************************/
void printMessage ( int counter )
{
int i ; /* loop counter */

for ( i = 0; i < counter; i++ )
{
}
}


• Notice the function header comment before the
definition of function printMessage.
• Your header comments should be neatly formatted
and contain the following information:
– function name
– function description (what it does)
– a list of any input parameters and their meanings
– a list of any output parameters and their meanings
– a description of any special conditions


Functions Can Return Values
/****************************************************************************
** averageTwo - calculates and returns the average of two numbers
** Inputs: num1 - an integer value
** num2 - an integer value
** Outputs: the floating point average of num1 and num2
*****************************************************************************/
float averageTwo (int num1, int num2)
{
float average ; /* average of the two numbers */

average = (num1 + num2) / 2.0 ;
return average ;
}


Using averageTwo
#include <stdio.h>
float averageTwo (int num1, int num2) ;
int main ( )
{
float ave ;
int value1 = 5, value2 = 8 ;
ave = averageTwo (value1, value2) ;
printf (“The average of %d and %d is %fn”, value1, value2, ave) ;
return 0 ;
}

{
float average ;

average = (num1 + num2) / 2.0 ;
return average ;
}

Parameter Passing
• Actual parameters are the parameters that
appear in the function call.
average = averageTwo (value1, value2) ;

• Formal parameters are the parameters that
appear in the function header.

• Actual and formal parameters are matched by
position. Each formal parameter receives the
value of its corresponding actual parameter.

Parameter Passing (con’t)

• Corresponding actual and formal
parameters do not have to have the same
name, but they may.
• Corresponding actual and formal
parameters must be of the same data
type, with some exceptions.


Local Variables
• Functions only “see” (have access to) their own
local variables. This includes main( ) .
• Formal parameters are declarations of local
variables. The values passed are assigned to
those variables.
• Other local variables can be declared within the
function body.


Parameter Passing and Local
Variables
#include <stdio.h> float averageTwo (int num1, int num2)
float averageTwo (int num1, int num2) ; {
int main ( ) float average ;
{
float ave ; average = (num1 + num2) / 2.0 ;
int value1 = 5, value2 = 8 ; return average ;
}
ave = averageTwo (value1, value2) ;
printf (“The average of “) ;
printf (“%d and %d is %fn”,
value1, value2, ave) ;
return 0 ;
}
value1 value2 ave num1 num2 average

5 8

int int float int int float

Same Name, Still Different Memory
Locations
#include <stdio.h> float averageTwo (int num1, int num2)
float averageTwo (int num1, int num2) ; {
int main ( ) float average ;
{
float average ; average = (num1 + num2) / 2.0 ;
int num1 = 5, num2 = 8 ; return average ;
}
average = averageTwo (num1,
num2) ;
printf (“The average of “) ;
printf (“%d and %d is %fn”,
num1, num2, average) ;
return 0 ;
}
num1 num2 average num1 num2 average

5 8
int int float int int float

Changes to Local Variables Do NOT
Change Other Variables with the
#include <stdio.h> Same Name
void addOne (int number) ; void addOne (int num1)
{
int main ( ) num1++ ;
{ printf (“In addOne: “) ;
int num1 = 5 ; printf (“num1 = %dn”, num1) ;
addOne (num1) ; }
printf (“In main: “) ;
printf (“num1 = %dn”, num1) ; num1
return 0 ;
} int
num1
5 OUTPUT
int In addOne: num1 = 6
In main: num1 = 5

Call by Value and Call by Reference
By Default the function calling is by Value i.e. there is no
relation between actual and formal parameter. Change in
formal parameter doesn't affect actual parameter.

In case if we require the value updated by a function, there
is a provision for single updated value by using return
type of the function. But in case we require more than
one value the we must call the function by reference.

In case of call by reference we use pointers.


Header Files
• Header files contain function prototypes
for all of the functions found in the
specified library.
• They also contain definitions of constants
and data types used in that library.


Commonly Used Header Files
Header File Contains Function Prototypes for:
<stdio.h> standard input/output library functions
and information used by them
<math.h> math library functions
<stdlib.h> conversion of numbers to text, text to
numbers, memory allocation, random
numbers, and other utility functions
<time.h> manipulating the time and date
<ctype.h> functions that test characters for certain
properties and that can convert case
<string.h> functions that manipulate character strings


Using Header Files
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main ( )
{
float side1, side2, hypotenuse ;
printf(“Enter the lengths of the right triangle sides: “) ;
scanf(“%f%f”, &side1, &side2) ;
if ( (side1 <= 0) || (side2 <= 0) {
exit (1) ;
}
hypotenuse = sqrt ( (side1 * side1) + (side2 * side2) ) ;
printf(“The hypotenuse = %fn”, hypotenuse) ;
return 0 ;
}

User Defined Header Files
Save different functions in a file (extension .h). This file can
be used as header file. e.g. myheader.h

In order to include the this user defined header file we can
• Either put the file in the include folder.
• Or keep it in the current folder an include it with
#include “myheader.h”


The stack and the heap

• Local variables, function arguments, return value
are stored on a stack
• Each function call generates a new "stack frame"
• After function returns, stack frame disappears
– along with all local variables and function
arguments for that invocation



void contrived_example(int i, float f)
{
int j = 10;
double d = 3.14;
int arr[10];
/* do some stuff, then return */
return (j + i);
}


/* somewhere in code */
int k = contrived_example(42, 3.3);
• What does this look like on the stack?



(more frames)
return value 52
function i = 42
arguments f = 3.3
j = 10 stack frame
local d = 3.14
variables
arr[10] =
<garbage>



• Another example:
int factorial(int i)
{
if (i == 0) {
return 1;
} else {
return i * factorial (i - 1);
}
}



• Pop quiz: what goes on the stack for
factorial(3)?
• For each stack frame, have...
– no local variables
– one argument (i)
– one return value
• Each recursive call generates a new stack frame
– which disappears after the call is complete



return value ?
factorial(3) stack frame
i = 3



return value ?
i = 2
return value ?
i = 3



return value ?
i = 1
return value ?
i = 2
return value ?
i = 3



return value ?
i = 0
return value ?
i = 1
return value ?
i = 2
return value ?
i = 3



return value 1
i = 0
return value ?
i = 1
return value ?
i = 2
return value ?
i = 3



return value 1
i = 1
return value ?
i = 2
return value ?
i = 3



return value 2
i = 2
return value ?
i = 3



return value 6
i = 3



factorial(3)

result: 6



void foo() {
int arr[10]; /* local (on stack) */
/* do something with arr */
} /* arr is deallocated */
• Local variables sometimes called "automatic"
variables; deallocation is automatic



foo

local arr[10] =
stack frame
variables <whatever>



• The "heap" is the general pool of computer
memory
• Memory is allocated on the heap using
malloc() or calloc()
• Heap memory must be explicitly freed using
free()
• Failure to do so memory leak!



void foo2() {
int *arr;
/* allocate memory on the heap: */
arr = (int *)calloc(10, sizeof(int));
} /* arr is NOT deallocated */



void foo3() {
int *arr;
/* allocate memory on the heap: */
arr = (int *)calloc(10, sizeof(int));
free(arr);
}


0x1234
arr[0]
arr[1]
arr[2]
arr[3]
foo2 and arr[4]
foo3
(etc.)

heap
local arr =
stack frame
variables 0x1234
stack

0x1234
arr[0]
arr[1]
(after foo2
exits, arr[2]
without arr[3]
freeing memory
leak arr[4]
memory)
(etc.)

heap

stack


0x1234 arr[0]
arr[1]
(after foo3
exits, with arr[2]
freeing arr[3]
memory) arr[4]
(etc.)

heap

stack


C programming part1

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