Iteration (Cambridge International IGCSE Computer Science)

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

Spec point — "c) Understand and use the concept of iteration 5 Understand and use nested statements"

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 ← TO NEXT
• 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 ← TO STEP NEXT
• A detailed look at a Python FOR statement:

Examples

Condition Controlled Loops

Spec point — "c) Understand and use the concept of iteration 5 Understand and use nested statements"

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 ** ** UNTIL

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 DO ** ** ENDWHILE
• A detailed look at a Python WHILE statement:

Nested Iteration

Spec point — "c) Understand and use the concept of iteration 5 Understand and use nested statements"

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