Programming algorithms and flowchart.ppt

PROGRAMMING,
ALGORITHMS AND
FLOWCHARTS

What is Programming?
 Series of instructions to a computer to
accomplish a task
 Instructions must be written in a way the
computer can understand
 Programming languages are used to write
programs

ALGORITHMS AND FLOWCHARTS
 A typical programming task can be divided into
two phases:
 Problem solving phase
 produce an ordered sequence of steps that describe
solution of problem
 this sequence of steps is called an algorithm
 Implementation phase
 implement the program in some programming
language

Devise an algorithm for cooking a sausage on a barbecue

Steps in Problem Solving
 First produce a general algorithm.
 Refine the algorithm successively to get step by
step detailed algorithm that is very close to a
computer language.
 Pseudocode is an artificial and informal
language that helps programmers develop
algorithms. Pseudocode is very similar to
everyday English.

Pseudocode & Algorithm
 Example 1: Write an algorithm to
determine a student’s final grade and
indicate whether it is passing or failing.
The final grade is calculated as the
average of four marks.
 Start with a general algorithm.

General Algorithm
Pseudocode:
 Input a set of 4 marks
 Calculate their average by summing and dividing
by 4
 if average is below 50
Print “FAIL”
else
Print “PASS”

Detailed Algorithm
Step 1: Input M1,M2,M3,M4
Step 2: GRADE  (M1+M2+M3+M4)/4
Step 3: if (GRADE < 50) then
Print “FAIL”
else
Print “PASS”
endif

Task: add two numbers
 Pseudocode:
Start
Get two numbers
Add them
Print the answer
End

Next Step - The Flowchart
 A graphical representation of the sequence of
operations in an information system or program.
Information system flowcharts show how data flows from
source documents through the computer to final
distribution to users. Program flowcharts show the
sequence of instructions in a single program or
subroutine. Different symbols are used to draw each
type of flowchart.
 The sequence of the flowchart and the algorithm MUST
match.

The Flowchart
A Flowchart
shows logic of an algorithm
emphasizes individual steps and their
interconnections
e.g. control flow from one action to the next

Flowchart Symbols
Basic
Oval
Parallelogram
Rectangle
Diamond
Hybrid
Name Symbol Use in Flowchart
Denotes the beginning or end of the program
Denotes an input operation
Denotes an output operation
Denotes a decision (or branch) to be made.
The program should continue along one of
two routes. (e.g. IF/THEN/ELSE)
Denotes a process to be carried out
e.g. addition, subtraction, division etc.
Flow line Denotes the direction of logic flow in the program
Module
Denotes a self-contained section of the
program

Example
PRINT
“PASS”
Step 1: Input M1,M2,M3,M4
Step 2: GRADE  (M1+M2+M3+M4)/4
Step 3: if (GRADE <50) then
Print “FAIL”
else
Print “PASS”
endif
START
Input
M1,M2,M3,M4
GRADE(M1+M2+M3+M4)/4
IS
GRADE<50
PRINT
“FAIL”
STOP
Y
N

Control Structures
Represent the flow of logic through the programme.
There are four main control structures:
• Sequence
• Decision – incorporating if-then-else
• Repetition
• Case

Sequence Structure
• a series of actions are performed in sequence
START
Display message
“How many
hours did you
work?”
Read Hours
Display message
“How much do
you get paid per
hour?”
Read Pay Rate
Multiply Hours
by Pay Rate.
Store result in
Gross Pay.
Display Gross
Pay
END
This is the flowchart for a program which
calculates an employee’s gross pay.

Example 1
 Write an algorithm and draw a flowchart to
convert the length in feet to centimetres.
Pseudocode:
 Input the length in feet (Lft)
 Calculate the length in cm (Lcm) by
multiplying LFT with 30
 Print length in cm (LCM)

Example
Algorithm
 Step 1: Input Lft
 Step 2: Lcm  Lft x 30
 Step 3: Print Lcm
START
Input
Lft
Lcm  Lft x 30
STOP
Flowchart
Print
Lcm

Example 2
Write an algorithm and draw a flowchart that
will read the two sides of a rectangle and
calculate its area.
Pseudocode
 Input the width (W) and Length (L) of a rectangle
 Calculate the area (A) by multiplying L with W
 Print A

Example 2
Algorithm
 Step 1: Input W,L
 Step 2: A  L x W
 Step 3: Print A
START
Input
W, L
A  L x W
STOP
Print
A

DECISION STRUCTURES
 One of two possible actions is taken, depending
on a condition.
Decision Structure
A new symbol, the diamond, indicates a yes/no question.

Decision Structures
 Sometimes only one choice is necessary
YES
NO

Example 1
 Pseudocode:
Start
Get year born
Calculate age
Print age
If age > 50 print OLD
End

 Flowchart
 Start
 Get year born
 Calculate age
 Print age
 If age > 50 print OLD
 End
Get yr
Calc age
Print age
Age>50
OLD
Y
N
Start
End

Decision Structure
 Sometimes two choices are offered. If the answer to the
question is yes, the flow follows one path. If the answer
is no, the flow follows another path
YES
NO

Decision Structure
 In the flowchart segment below, the question “is x < y?”
is asked. If the answer is no, then process A is
performed. If the answer is yes, then process B is
performed.
YES
NO
x < y
Process B
Process A

IF–THEN–ELSE STRUCTURE
 The structure is as follows
If condition then
true alternative
else
false alternative

IF–THEN–ELSE STRUCTURE
 The algorithm for the flowchart is as
follows:
If A>B then
print A
else
print B
is
A>B
Print
A
Print
B
N Y

Example 1
A ticket seller is issuing show tickets at the gate. When the patrons arrive,
he asks how old each one is. It they are under 12, they pay half price. If they
are 12 or over, they pay full price.
Algorithm
Get age
Age < 12?
If Age < 12
Then
Pay half price
Else
Pay full price

Flowchart
YES
NO
Begin
Input age
Age < 12
Half price
Full price
End

Example 2
 Write an algorithm that reads two values, determines the
largest value and prints the largest value with an
identifying message.
ALGORITHM
Step 1: Input VALUE1, VALUE2
Step 2: if (VALUE1 > VALUE2) then
MAX  VALUE1
else
MAX  VALUE2
endif
Step 3: Print “The largest value is”, MAX

Example 2
MAX  VALUE2
STOP
N Y
START
Input
VALUE1,VALUE2
MAX  VALUE1
is
VALUE1>VALUE2
Print
MAX

Example 3
 Write the algorithm and draw the flowchart
for the following:
 Enter two numbers, x and y. If x is less
than y, square x to give the result a,
otherwise, add x + y to give the result a.

YES
NO
x < y
Calculate a
as x times 2.
Calculate a
as x plus y.
Algorithm
Input x
Input y
x < y?
If x <y
Then
a = x^
Else
a = x+y

NESTED IFS
 One of the alternatives within an IF–
THEN–ELSE statement
may involve further IF–THEN–ELSE
statement

Programming algorithms and flowchart.ppt

Repetition Structure
Repetition (Iteration) Structure
 A repetition structure represents part of the program
that repeats. This type of structure is commonly
known as a loop.

 Notice the use of the diamond symbol. A loop tests a
condition, and if the condition exists, it performs an
action. Then it tests the condition again. If the
condition still exists, the action is repeated. This
continues until the condition no longer exists.

 In the flowchart segment, the question “is x < y?”
is asked. If the answer is yes, then Process A is
performed. The question “is x < y?” is asked
again. Process A is repeated as long as x is less
than y. When x is no longer less than y, the
repetition stops and the structure is exited.
x < y Process A
YES

x < y Add 1 to x
YES
So this might look like…
When would this loop exit?

The action performed by a repetition structure must eventually cause the loop to
terminate. Otherwise, an infinite loop is created.
In this flowchart segment, x is never changed. Once the loop starts, it will never
end.
QUESTION: How can this flowchart be modified so it is no longer an infinite
loop?
Out of Control!
x < y Display x
YES

This type of structure is known as a pre-test
repetition structure. The condition is tested
BEFORE any actions are performed.
Pre-test ( Before) Loop
x < y? Display x
Add 1 to x
YES

Post-test (After) Loop
This flowchart segment shows a
post-test repetition structure.
The condition is tested AFTER
the actions are performed.
A post-test repetition structure
always performs its actions at
least once.
Display x
Add 1 to x
YES
x < y

Case Structure
 One of several possible actions is taken, depending on
the contents of a variable.
 Used for MORE THAN TWO decisions

 The structure below indicates actions to
perform depending on the value in
years_employed.
CASE
years_employed
1 2 3 Other
bonus = 100 bonus = 200 bonus = 400 bonus = 800

CASE
years_employed
1 2 3 Other
bonus = 100 bonus = 200 bonus = 400 bonus = 800
If years_employed = 1,
bonus is set to 100
bonus is set to 200
bonus is set to 400
If years_employed is
any other value, bonus
is set to 800

Case Algorithm
Input a Grade
Case based on Grade
Case =100
Report “Perfect Score” (or suitable command eg Grade = Perfect)
Case > 89
Report “Grade = A”
Case > 79
Report “Grade = B”
Case > 69
Report “Grade = C”
Case > 59
Report “Grade = D”
Default (or Else)
Report “Grade = F”
End Case
Draw the Flowchart for the above example

CASE
grade
>89 >79 >69 > 59
Grade =A Grade = B Grade = C Grade = D
Flowchart
Grade = Perfect Grade = F
100 other

Construct the algorithm and the flowchart for the
following procedure:
An egg packing business needs to devise a
process to sort eggs for delivery to supermarkets.
If the eggs weigh 50 grams, they are Class A
eggs. If they weigh 60 grams, they are Class B
eggs. If they weigh 70 grams, they are Class C
eggs. If they weigh more than 70 grams, they are
Class D eggs.

Input a Weight
Case based on Weight
Case =50
Class = A
Case =60
Class = B
Case = 70
Class = C
Case > 70
Class = D
OR
Default/ Else Class = D
End Case
Algorithm
CASE
weight
50 60 70 > 70
Class =A Class = B Class = C Class = D
Flowchart

Modules (stepwise refinement)
A program module is a part of a program that
makes sense on its own, just like a paragraph
in an essay should make sense on its own. In
large programs, such as those used in
industry, programs are developed as
separate modules and then put together.
The process of representing modules in
flowcharts is known as stepwise refinement.
You will look at this more closely when you
start programming.

•The position of the module
symbol indicates the point the
module is executed.
•A separate flowchart can be
constructed for the module.
START
END
Read Input.
Call calc_pay
function.
Display results.
START
Display message
“How many
hours did you
work?”
Read Hours
Display message
“How much do
you get paid per
hour?”
Read Pay Rate
Multiply Hours
by Pay Rate.
Store result in
Gross Pay.
Display Gross
Pay
END

A simpler example… could look like
CASE
weight
50 60 70 > 70
Class =A Class = B Class = C Class =
D
Input weight
Calculate class
Display class

A complete program example
When you use stepwise refinement, the complete, refined flowchart is drawn first.
Then the modules are named and drawn in order.
This program lists the AFL team name and points total for all teams that have a full-forward from the AFL competition
who has kicked at least 10 goals.

MainLoop
AFL
Goals >= 10
Check
goals
Check pos
Check goals
Check conf

Finish
Although this procedure is listed as a module in the opening flow chart, it’s
unsuitable. Why?

Programming algorithms and flowchart.ppt

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