Lecture 6Decision_Control_OR_Conditional_Control_Structures.ppt

©LPU CSE101 C Programming
CSE101-Lec#6-Part-1
• Control structures(Decision control
statements/ or Condition Statements)

1. Wake up;
2. Get ready;
3. If you have enough time, then
eat breakfast;
4. Go to school.
Program
• Program is a set of instruction executed one by one.
• Depending upon the circumstances sometimes it is
desirable to alter the sequence of execution of
statements.

Control Statements
• The C language programs until now follows a
sequential form of execution of statements.
• C language provides statements that can alter the
flow of a sequence of instructions. These statements
are called control statements.
• These statements help to jump from one part of the
program to another. The control transfer may be
conditional or unconditional.

Control Structure
• A control structure refers to the way in which the
programmer specifies the order of executing the
statements.
• Three control structures
– Sequence structure
• Programs are executed sequentially by default.
– Selection structures(Condition)
• if, if…else, if-else-if, Nested-if ,switch
– Repetition structures (iteration)
• while, do…while, for

Condition Statements(or Decision control
statements or Branching statements)
• The C condition statements or the decision statements,
checks the given condition
• Based upon the state of the condition, a sub-block is
executed.
• Decision statements are the:
– if statement
– if-else statement
– If-else-if statement
– Nested if statement
– switch statement

Daily routine
Start
Where
To
Go?
Go!!!
Class Movie
Stop Stop

if statement
If you have
time?
Yes No

if Statement
• If statement
– It is decision making statement uses keyword if.
– It allows the computer to evaluate the expression
first
• and then, depending on whether the value is ‘true’
or ‘false’, i.e. non zero or zero it transfers the
control to a particular statement.
A decision can be made on any expression.
zero - false
nonzero - true
Example:
3 < 4 is true

if Statement
Syntax
if (expression)
statement;
or
if (expression)
{
block of statements;
}

if Statement
• The if statement has the following syntax:
if ( condition )/* no semi-colon */
statement;
if is a C
reserved word
The condition must be a
boolean expression. It must
Evaluate to either non-zero or zero.
If the condition is non-zero, the statement is executed.
If it is zero, the statement is skipped.

Rain ???
Clouds?
Is it going to rain?
Look up sky for clouds
yes no
No rain
Raining

Program to
check
whether
number is
less than 10.
#include<stdio.h>
int main()
{
int v;
printf(“Enter the number :”);
scanf(“%d”, &v);
if(v<10)
printf(“number is less than 10”);
return 0;
}
Enter the number: 6
Number is less than 10

Control Flow

if..else statement
If you have
time?
Yes No
Grab
something to
eat along

if..else statement
• The if statement executes only when the
condition following if is true.
• It does nothing when the condition is false.
• The if..else statement takes care of the
true and false conditions.

if..else statement
• if..else has two blocks.
• One block is for if and it is executed when
condition is non-zero(true).
• The other block is of else and its executed when
condition is zero (false).
if (expression)
{
}
else
{
}
Syntax

if..else statement
• The else statement cannot be used without
if.
• No multiple else statements are allowed
with one if.
• else statement has no expression.
• Number of else cannot be greater than
number of if.

Example :
Program to
check
whether
number is
less than 10.
#include<stdio.h>
int main()
{
int a;
if(v<10)
else
printf(“number is greater than 10”);
return 0;
}
Enter the number: 7
Enter the number: 100
Number is greater than 10
or

Control Flow
MESSAG
E
DISPLAY

Q1
What will be the output of the following C code?
#include <stdio.h>
int main()
{
int x = 5;
if (x < 1)
printf("hello");
if (x == 5)
printf("hi");
else
printf("no");
return 0;
}
A. hi
B. hello
C. no
D. error

Q2
What will be the output of the following C code?
#include <stdio.h>
int main()
{
int x = 0;
if (x == 0)
printf("hi");
else
printf("how are u");
printf("hello");
return 0;
}
A. hi
B. how are you
C. hello
D. hihello

If-else-if
• if-else-if statement is used when program requires
more than one test expression.
• We can check multiple conditions, and what so ever condition
is true, that part will work
• Here, a user can decide among multiple options. The C if
statements are executed from the top down. As soon as one
of the conditions controlling the if is true, the statement
associated with that if is executed, and the rest of the C else-if
ladder is bypassed. If none of the conditions are true, then
the final else statement will be executed.

If-else-if ladder
if ( condition ) {
}
else if ( condition ) {
}
else {
}
Syntax

Program to
check
whether
number is
less than 10.
#include<stdio.h>
int main()
{
int a;
if(v<10){
}
else if(v<100){
}
return 0;
}
Enter the number: 1
Enter the number: 56
or

Program to
print grades
of students
marks.
#include<stdio.h>
int main()
{
float marks;
scanf(“%f”, &marks);
if (marks>90){
printf(“Grade A”);
}
else if (marks>80) {
printf(“Grade B”);
}
else if(marks>70){
printf(“Grade C”);
}
else if (marks >60) {
printf(“Grade D”);
}
return 0;
}
66.70
Grade D
78.00
Grade C
or

Q1
#include <stdio.h>
int main()
{
int x = 1;
if (x > 0)
printf("inside ifn");
else if (x > 0)
printf("inside elseifn");
}
A. inside if
B. inside elseif
C. inside if
inside elseif
D. Compile time error

Nested if
• A nested if in C is an if statement that is the
target of another if statement. Nested if
statements means an if statement inside
another if statement. C allows us to nested if
statements within if statements, i.e, we can
place an if statement inside another if
statement.

Syntax
if (condition1)
{
// Executes when condition1 is true
if (condition2)
{
// Executes when condition2 is true
}
}

Program example
// C program to illustrate nested-if statement
#include <stdio.h>
int main()
{
int i = 10;
if (i == 10)
{
if (i < 15)
printf("i is smaller than 15n");
if (i < 12)
printf("i is smaller than 12 toon");
else
printf("i is greater than 15");
}
return 0;
}
Output
• i is smaller than 15
• i is smaller than 12
too

What will be the output of following code?
#include <stdio.h>
int main()
{
int x = 0;
if (x == 1)
if (x >= 0)
printf("truen");
else
printf("falsen");
}
A. true
B. false
C. Depends on the
compiler
D. Nothing will be printed

What will be the output of the
following C code?
#include <stdio.h>
int main()
{
int x = 0;
if (x == 1)
if (x == 0)
printf("inside ifn");
else
printf("inside else ifn");
else
printf("inside elsen");
return 0;
}
A.inside if
B.inside else if
C.inside else
D.Compile time error

break statement
• break is a keyword.
• break allows the programmer to terminate
the loop.
• A break statement causes control to transfer
to the first statement after the loop or block.
• The break statement can be used in nested
loops. If we use break in the innermost loop
then the control of the program is terminated
only from the innermost loop.

switch Statement
Day=
Monday
Yes
No Day=
Sunday

switch Statement
• The control statement that allows to make a decision
from the number of choices is called switch.
• Also called switch-case-default.
• The switch statement provides another way to decide
which statement to execute next.
• The switch statement evaluates an expression, then
attempts to match the result to one of several possible
cases.
• Each case contains a value and a list of statements.
• The flow of control transfers to statement associated
with the first case value that matches.

switch Statement
switch (expression)
{
case constant1:
statements;
break;
case constant2:
statements;
break;
case constant3:
statements;
break;
default:
statements;
}
Syntax

Rules of using switch case
1. Case label must be unique
2. Case label must end with colon
3. Case label must have constant expression
4. Case label must be of integer, character type like
case 2, case 1+1, case ‘a’
5. Case label should not be floating point
6. Default can be placed anywhere in switch
7. Multiple cases cannot use same expression
8. Nesting of switch is allowed.
9. Variables are not allowed in switch case label..

Syntax error in switch statement
switch(pt){
case count:
printf(“%d”, count);
break;
case 2.5:
printf(“A line”);
break;
case 3 + 7.7:
printf(“A triangle”);
case 3 + 7.7:
printf(“A triangle”);
break;
case count+5:
printf(“A pentagon”);
break;
}
Variable cannot be
used as label
Floating point number
cannot be used
Floating point number
cannot be used and
same expression cannot
be used
constant expression
should be used

#include<stdio.h>
int main()
{
int pt;
printf("Enter the number of nodes:");
scanf("%d", &pt);
switch(pt){
case 0:
printf("nNo Geometry");
break;
case 1:
printf("nA point");
break;
case 2:
printf("nA line");
break;
case 3:
printf("nA triangle");
break;
case 4:
printf("nA rectangle");
break;
case 5:
printf("nA pentagon");
break;
default:
printf("Invalid input");
break;
}
return 0;
}
Program to
show switch
statement in
geometry
Enter the number of nodes: 2
A line

Q1
#include <stdio.h>
int main()
{
double ch;
printf("enter a value between 1 to 2:");
scanf("%lf", &ch);
switch (ch)
{
case 1:
printf("1");
break;
case 2:
printf("2");
break;
}
return 0;
}
A. Compile time error
B. 1
C. 2
D. Nothing will be displayed

Q2
What will be the output of the following C
code? (Assuming that we have entered the
value 1 in the standard input)
#include <stdio.h>
int main()
{
int ch;
scanf("%d", &ch);
switch (ch)
{
case 1:
printf("1 ");
default:
printf("2");
}
return 0;
}
A. 1
B. 2
C. 1 2

Q3
What will be the output of the following C
code? (Assuming that we have entered the
value 1 in the standard input)
#include <stdio.h>
int main()
{
int ch;
scanf("%d", &ch);
switch (ch)
{
case 1:
printf("1 ");
printf("hi");
break;
default:
printf("2n");
}
}
A. 1 hi
B. 2
C. hi
D. 1

Q4
following C code?
#include <stdio.h>
int main()
{
int x = 97;
switch (x)
{
case 'a':
printf("yes ");
break;
case 97:
printf("no");
break;
}
}
A. yes
B. yes no
C. Duplicate case value error
D. Nothing will be displayed

Q5
following C code?
#include <stdio.h>
int main()
{
int a = 1;
switch (a)
{
case a:
printf("Case A ");
default:
printf("Default");
}
return 0;
}
A. Output: Case A
B. Output: Default
C. Output: Case A Default

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