Iteration (Cambridge (CIE) IGCSE Computer Science)

Revision Note

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Written by: Robert Hampton

Reviewed by: James Woodhouse

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` `END WHILE`	`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

END WHILE

temperature = float(input("Enter temperature: "))

while temperature > 37:

print("Patient has a fever")

temperature = float(input("Enter temperature: "))

Nested Iteration

What is nested selection?

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

Last updated: 18 April 2024

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