PROBLEM SOLVING WITH

LOOPS

Chapter 7

Concept of Repetition Structure Logic

It is a computer task, that is used for Repeating a series of instructions many times.

Ex. The Process of calculating the Total Payment for more than one employee.

Loops can be written using combination of previous

logic structures

Example:

Write an Algorithm that calculate the average of 7

students grade.

Solution without looping structure

Average() 1. Real Sum, Average

2. Sum = St1_Grade+St2_Grade+St3_Grade+ St4_Grade+St5_Grade+St6_Grade+ St7_Grade

3. Average = Sum/ 7

4. Exit

Real St1_Grade, St2_Grade, St3_Grade, St4_Grade, St5_Grade, St6_Grade, St7_Grade,

Control () 1. Read() 2. Average() 3. End

Read() 1. Read St1_Grade 2. Read St2_Grade 3. Read St3_Grade 4. Read St4_Grade 5. Read St5_Grade 6. Read St6_Grade 7. Read St7_Grade 8. Exit

Solution with looping structure

Average()

1. Integer Count=1

2. Real Sum=0

3. While ( count <= 7)

1. Read()

2. Sum=Sum +Student_Grade

3. Count=Count+1

4. WhileEnd

5. Real Average = Sum / count

6. Exit

Real Student_Grade Control ()

1. Read() 2. Average() 3. End

Read() 1. Enter Student_Grade 2. Exit

Average Module Algorithm Flowchart

Flowchart

Average()

Integer Count=1

Real Sum=0

While

Count<=7

T

Read()

Sum=Sum+Student_Grade

Count=Count+1 Exit

F

Real Average = Sum / 7

What will

Happen If

we

remove

this?

This will lead to

an infinite loop

What will

Happen If

we

remove

this?

Count has no

data, its value is

Unknown, So the

value of the

condition is

unpredictable.

What will

Happen If

we

remove

this?

Sum has no initial

value, its value is

Unknown

The loop logic structure

There are three types of loop structures:

The While/WhileEnd loop

Which repeats instructions while a condition is True and stops repeating when a condition arises that is not True .

The Repeat/Until loop

Which repeats instructions while a condition is False or until a condition is True .

The automatic-counter loop

a variable is set equal to a given number and increases in equal given increments until it is greater than an ending number .

The loop logic structure

The algorithm and flowchart differ with each type of

loop structure .

Several standard types of tasks are accomplished

through the use of the loop structure .

Counting ( incrementing and decrementing)

Accumulating ( calculating a sum or a total)

In both task a number is added or subtracted from a variable

and the result is stored back into the same variable .

In each case the resultant variable is assigned the value of zero

before starting the loop( initializing the variable ) .

Initialization

The Initialization

set to an initial value

usually zero but not all the time

Examples:

Count = 0

Count = 1

Count = 100

Incrementing (or Decrement)

Is done by adding a constant , such as 1 or 2 to the value of a variable

Example :

Counter = counter + 1 or c = c + 1

The increment can be one , two , or any other constant , including negative number if you want to decrement rather than increment .

Example:

Counter = counter -1 or c = c - 1

Remember Variable Must be Note:

Initialized before starting the loop.

The Accumulating

Or summing , a group of numbers

Similar to incrementing, except a variable instead of a constant is added to another variable.

sum = sum + variable or s = s + v

Examples: Totalsales = Totalsales + Sales

Remember Variable Must be Note:

Initialized to zero.

While/ While End Loop.

Repeats the instructions between the While & While End, if the

condition is true.

While <Condition (s)>

Instruction

Instruction

.

.

.

WhileEnd

while/whileEnd

Use the While/ While End Loop structure when you do not know the number of times an instruction to be repeated.

Or if there are cases when the instructions in the loop

should not be processed .

Primer read The value must be entered before the loop start.

It gives the while/whileEnd loop a valid value for the variable in order for the conditions to be true the first time through the loop .

The value of variable that allow to control when to stop the looping process called a trip value .

Example 1:

Create the algorithm to find the average age of all the

students in class

How many

Students?

UNKOWN

How can I solve this

problem?

Average Age of a

Class –

While/WhileEnd

Repeat/Until

Repeat

Instruction

Instruction

.

.

.

Until< Condition(S)>

Example 1:

Create the algorithm to find the average age of all the

students in class

How many

Students?

UNKOWN

How can I solve this

problem?

Average Age of

a Class –

Repeat/Until

While/EndWhile & Repeat/Until

Repeat/Until While/WhileEnd

•You can set the operand of the conditions

anywhere within the loop since the

condition is processed at the end .

•Repeat the loop until the resultant of

condition is true.

•The condition is processed at the end

•Instructions in the loop are processed

entirely at least once.

•You must initialize the data so that the

resultant of the condition is true the first

time through the loop. Otherwise , the loop

instruction will never be processed .

•Repeat the loop until the resultant of

condition is false.

•The condition is processed at the beginning

Automatic-Counter Loop

Increments or decrements a variable each time the

loop is repeated .

Use it when you know from the start the number of

times the loop will be executed .

The beginning value , the ending value , and the

increment value may be constant , variable , or

expressions .

Automatic-Counter Loop

Variable name Loop: counter=begin To End Step S

Instruction

Instruction

Instruction

.

.

Loop –End: Counter

Automatic-Counter Loop:

General Rules

When the computer executes the Loop instruction its sets the counter equal to the beginning number.

When the computer executes the Loop-End instruction it increments the counter.

When the counter is less than or equal to the ending number The processing continues for the instructions that follows the loop.

When the counter is greater than the ending number. The processing continues for the instructions that follows the loop-End

instruction.

Automatic-Counter Loop

When decrementing the counter :

The counter is decremented at the end of the loop .

When the counter is greater than or equal to the ending

value , the loop continues.

Step value needs to be a negative number and begin must

be greater than end

Example 1: using Automatic-Counter Loop

Create the algorithm to find the average age of all the

students in class

( J -1 )

2

3

4

5

J ,

( J – 1)

J,

Example 2:

Create the algorithm & flowchart to:

Prints the even numbers.

The series of numbers will:

Start From 0 .

Ends at 50.

Example 2 Solution

Algorithm

PrintEvenNum()

1. Loop: counter=0 To 50 Step 2

1. Print counter

2. Loop-End: counter

3. End

Recursion

Another type of loop structure .

A module or a function calls itself .

Example :

FactorialFunction (N)

1. If N > 1

then

Factorial = N * FactorialFunction (N-1)

Else

Factorial = 1

2. Exit