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.
The main process is identifying what instruction
should be repeated.
Loops can be written using combination of previous
logic structures:

If/Then/Else structure.
Example:
Write an Algorithm that calculate the average of 7
students grade.
Solution without looping structure
Real St1_Grade, St2_Grade, St3_Grade,
St4_Grade, St5_Grade, St6_Grade,
St7_Grade,
Control ()
1. Process Read()
2. Process Average()
3. End
Read()
1.
2.
3.
4.
5.
6.
7.
8.
Enter St1_Grade
Enter St2_Grade
Enter St3_Grade
Enter St4_Grade
Enter St5_Grade
Enter St6_Grade
Enter St7_Grade
Exit
Average()
1.
2.
3.
4.
Real Sum, Average
Sum = St1_Grade+St2_Grade+St3_Grade+ St4_Grade+St5_Grade+St6_Grade+ St7_Grade
Average = Sum/ 7
Exit
Solution with looping structure
Real Student_Grade
Control ()
1. Process Average()
2. End
Read()
1. Enter Student_Grade
2. Exit
Average()
1. Integer Count=1
2. Real Sum=0
3. While ( count <= 7)
1. process Read()
2. Sum=Sum +Student_Grade
3. Count=Count+1
4. WhileEnd
5. Real Average = Sum / count
6. Exit
Average Module Algorithm Flowchart
Flowchart
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?
Average()
Integer Count=1
Sum has no initial
value, its value is
Unknown
Real Sum=0
F
While
Count<=7
T
Read()
Real Average = Sum / 7
Sum=Sum
+Student_Grade
Exit
Count=Count+1
What will
Happen If
we
remove
this?
This will lead to
an infinite loop
The loop logic structure
The loop logic structure is the repeating structure .
Through the loop structure , a program can process:

many payroll records .
 inventory items .
put a mailing list in alphabetical or zip code order .

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
 Which
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
Note: Remember Variable Must be
Initialized before starting the loop.


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
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
Note: Remember Variable Must be
Initialized to zero.
Product

Is similar to finding the sum of a series of number with
two exceptions :
 The
plus sign is replaced by the multiplication sign (*).
 The product variable must be initialized to 1 instead of 0.
 Example :
product = 1
Product = Product * Number
While/ While End Loop.

Repeats the instructions between the While & While End, if the
condition is true.
While <Condition (s)>
Instruction
Instruction
.
.
.
WhileEnd
While/ While End Loop.


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 .
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
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 and continue with the rest of the
solution 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 the set of instructions between the Repeat
and the Until , until the condition is true.
Use it when you do not know the number of times
the instruction will be executed .
While/WhileEnd
•Repeat the loop until the resultant of
condition is false.
•The condition is processed at the beginning
Repeat/Until
•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.
Repeat/Until

The Whil/WhilEnd
 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 .

The Repeat/Until
 You
can set the operand of the conditions anywhere within
the loop since the condition is processed at the end .

So , if there any reason that the loop instruction should
not be processed the first time the Whil/WhilEnd must
be used .
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
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
Loop: counter=begin To End Step S
Instruction
Instruction
Instruction
.
.
Loop –End: Counter
Variable name
Automatic-Counter Loop General Rules



When the computer executes the Loop instruction its seats 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
2
3
4
( J -1 )
J,
5
( J – 1)
J,
Example 2:

Create the algorithm & flowchart to:


Prints the even numbers.
The series of numbers will:
Start From 0 .
 Ends to 50.

Example 2 Solution
Algorithm
PrintEvenNum()
1. Loop: counter=0 To 50 Step 2
1. Print counter
2. Loop-End: counter
3. End
Nested Loops

Loops can be nested like decisions can.
 Cannot
use the same counter-control variable for the
inner loop as is used for the outer loop.


This is used when we want to do something multiple
times , and then do that multiple times
The inner loops do not have to be the same types of
loop structure as the outer loops.
Example
1:
Example 2
Nested Loop Example
Create the algorithm to find the average age of all the students
in five classes
Nested Loop Example Solution
Algorithm
AveragAge()
1. Integer age, StudentNum = 0
2. Real sum = 0
3. Loop: counter=0 To 5
1. Print “If you finish all student in one class enter: 0 or any negative value”
2. Enter age
3. While age > 0
1. StudentNum =StudentNum +1
2. sum=sum + age
3. Enter age
4. WhileEnd
4. Loop-End: counter
5. Real average= sum / StudentNum
6. Print “The average of age is : “ + average
7. End
Loop : counter=1 to 5
Stnymber=0
Sum=0
Enter age
While age >0
Stnumber=stnumber+1
Sum= sum + age
Enter age
Whileend
Average=sum/stnumber
Loop:counter
Indicators





Logical variables that change the processing path
or to control when the processing of a loop should
end .
Called ( flags , switches , trip value )
An error indicator designates that an error has
occurred in the input or the output.
An end-of-data indicator designate that there are
no more data to be entered .
They are set internally to change the processing
Enter grade
If grade >0
Then
Else
Print “ try agine , you are enter rong





Indicators


an indicator can be a variable of logical data or a
value that the variable can never equal .
Example :
 Indicator
for age = 0 or 500 or 300
 Indicator for a n error =true or false
Algorithm Instructions and Flowchart Symbols
Algorithm Instructions and Flowchart Symbols
Algorithm Instructions and Flowchart Symbols
Algorithm Instructions and Flowchart Symbols
Recursion



Another type of loop structure .
A module or a function calls itself .
Example :
Factorial(N)
1. If N > 1
then
Factorial = N * Factorial (N-1)
Else
Factorial = 1
2. Exit