Iteration (Cambridge International A-Level 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

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:

• In a FOR loop, the increment must be an expression that evaluates to an integer

  • The loop starts at value1
  • The identifier is updated by the increment value on each iteration
  • The loop continues until the identifier passes value2

• The increment can be positive or negative, depending on the direction you want the loop to count

Nested count-controlled loops

• The outer loop runs i from 1 to 3
• For each value ofi, the inner loop runs j from 1 to 3
• It outputs all combinations of i × j

Count-controlled loops in different languages

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:

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:

Nested condition-controlled loops

• Ask the user to enter up to 3 usernames
• For each one, keep asking until a non-empty name is entered
• Outer WHILE runs 3 times (for 3 usernames)
• Inner WHILE ensures that each username is not blank

Condition-controlled loops in different languages

When to use each type of loop