Iteration

Exam code: 2210

Written by: Ashika|Reviewed by: Caroline Carroll|Updated 2 July 2026

What is iteration?

Iteration is repeating a line or a block of code using a loop
Iteration can be:
- Count controlled
- Condition controlled
- Nested

Count Controlled Loops

What is a count controlled loop?

A count controlled loop is when the code is repeated a fixed number of times (e.g. using a for loop)
A count controlled loop can be written as:

FOR <identifier> ← <value1> TO <value2>
<statements>
NEXT <identifier>
Identifiers must be an integer data type
It uses a counter variable that is incremented or decremented after each iteration
This can be written as:

FOR <identifier> ← <value1> TO <value2> STEP <increment>
<statements>
NEXT <identifier>
A detailed look at a Python FOR statement:

xp66-srn-for-loop-pseudocode-computer-science-revision-notes

Examples

Iteration	Pseudocode	Python
Count controlled	`FOR X ← 1 TO 10 OUTPUT "Hello" NEXT X`	`for x in range(10): print("Hello")`
	This will print the word "Hello" 10 times
	`FOR X ← 2 TO 10 STEP 2 OUTPUT X NEXT X`	`for x in range(2,12,2): print(x) # Python range function excludes end value`
	This will print the even numbers from 2 to 10 inclusive
	`FOR X ← 10 TO 0 STEP -1 OUTPUT X NEXT X`	`for x in range(10,-1,-1): print(x) # Python range function excludes end value`
	This will print the numbers from 10 to 0 inclusive

Condition Controlled Loops

What is a condition controlled loop?

A condition controlled loop is when the code is repeated until a condition is met
There are two types of condition controlled loops:
- Post-condition (REPEAT)
- Pre-condition (WHILE)

Post-condition loops (REPEAT)

A post-condition loop is executed at least once
The condition must be an expression that evaluates to a Boolean (True/False)
The condition is tested after the statements are executed and only stops once the condition is evaluated to True
It can be written as:

REPEAT
<statement>
UNTIL <condition>

Iteration	Pseudocode	Python
Post-condition	`REPEAT INPUT Colour UNTIL Colour = "red"`	# NOT USED IN PYTHON
`REPEAT INPUT Guess UNTIL Guess = 42`	# NOT USED IN PYTHON

Iteration

Pseudocode

Python

Post-condition

REPEAT
    INPUT Colour
UNTIL Colour = "red"

# NOT USED IN PYTHON

REPEAT
    INPUT Guess
UNTIL Guess = 42

# NOT USED IN PYTHON

Pre-condition loops (WHILE)

The condition must be an expression that evaluates to a Boolean (True/False)
The condition is tested and statements are only executed if the condition evaluates to True
After statements have been executed the condition is tested again
The loop ends when the condition evaluates to False
It can be written as:

WHILE <condition> DO
<statements>
ENDWHILE
A detailed look at a Python WHILE statement:

Iteration	Pseudocode	Python
Pre-condition	`WHILE Colour != "Red" DO INPUT Colour ENDWHILE`	`while colour != "Red": colour = input("New colour")`
`INPUT Temperature WHILE Temperature > 37 DO OUTPUT "Patient has a fever" INPUT temperature ENDWHILE`	`temperature = float(input("Enter temperature: ")) while temperature > 37: print("Patient has a fever") temperature = float(input("Enter temperature: "))`

Iteration

Pseudocode

Python

Pre-condition

WHILE Colour != "Red" DO
    INPUT Colour
ENDWHILE

while colour != "Red":
    colour = input("New colour")

INPUT Temperature
WHILE Temperature > 37 DO
    OUTPUT "Patient has a fever"
    INPUT temperature
ENDWHILE

temperature = float(input("Enter temperature: "))
while temperature > 37:
    print("Patient has a fever")
    temperature = float(input("Enter temperature: "))

Nested Iteration

What is nested iteration?

Nested iteration is a loop within a loop, e.g. a FOR inside of another FOR
Nested means to be 'stored inside the other'

Example

Pseudocode
`Total ← 0 FOR Row ← 1 TO MaxRow RowTotal ← 0 FOR Column ← 1 TO 10 RowTotal ← RowTotal + Amount[Row, Column] NEXT Column OUTPUT "Total for Row ", Row, " is ", RowTotal Total ← Total + RowTotal NEXT Row OUTPUT "The grand total is ", Total`

Pseudocode

Total ← 0 

FOR Row ← 1 TO MaxRow 
    RowTotal ← 0 

    FOR Column ← 1 TO 10 
        RowTotal ← RowTotal + Amount[Row, Column] 
    NEXT Column 

    OUTPUT "Total for Row ", Row, " is ", RowTotal 
    Total ← Total + RowTotal 

NEXT Row 

OUTPUT "The grand total is ", Total

Python
# Program to print a multiplication table up to a given number # Prompt the user to enter a number number = int(input("Enter a number: ")) # Set the initial value of the counter for the outer loop outer_counter = 1 # Outer loop to iterate through the multiplication table while outer_counter <= number: # Set the initial value of the counter for the inner loop inner_counter = 1 # Inner loop to print the multiplication table for the current number while inner_counter <= 10: # Calculate the product of the outer and inner counters product = outer_counter * inner_counter # Print the multiplication table entry print(outer_counter, "x", inner_counter, "=", product) # Increment the inner counter inner_counter = inner_counter + 1 # Move to the next number in the multiplication table outer_counter = outer_counter + 1

Python

# Program to print a multiplication table up to a given number

# Prompt the user to enter a number
number = int(input("Enter a number: "))

# Set the initial value of the counter for the outer loop
outer_counter = 1

# Outer loop to iterate through the multiplication table
while outer_counter <= number:
    # Set the initial value of the counter for the inner loop
    inner_counter = 1

    # Inner loop to print the multiplication table for the current number
    while inner_counter <= 10:
        # Calculate the product of the outer and inner counters
        product = outer_counter * inner_counter

        # Print the multiplication table entry
        print(outer_counter, "x", inner_counter, "=", product)

        # Increment the inner counter
        inner_counter = inner_counter + 1

    # Move to the next number in the multiplication table
    outer_counter = outer_counter + 1

What is iteration?

Count Controlled Loops

Count Controlled Loops

What is a count controlled loop?

Examples

Condition Controlled Loops

Condition Controlled Loops

What is a condition controlled loop?

Post-condition loops (REPEAT)

Pre-condition loops (WHILE)

Nested Iteration

Nested Iteration

What is nested iteration?

Example

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