Iteration, Selection and Operators

Iteration: Repeating Instructions

Iteration allows a program to repeat a block of code without the programmer writing it out multiple times. There are two categories: definite iteration, where the number of repetitions is known in advance, and indefinite iteration, where the loop continues until a condition becomes true or false. You need a theoretical understanding of all four loop types, even if your school’s chosen programming language does not support all of them.

Definite Iteration — The FOR Loop

Definite iteration uses a FOR loop. The loop variable counts from a start value to an end value, incrementing by one on each pass. In AQA pseudo-code:

FOR i ← 1 TO 5
  OUTPUT i
ENDFOR
This outputs 1, 2, 3, 4, 5 — five iterations, one for each value of i.

Use a FOR loop when you know exactly how many times the loop should run — for example, processing each of ten exam answers, or generating a times table from 1 to 12.

Indefinite Iteration — Condition at the Start: WHILE

Indefinite iteration continues until a condition changes. The WHILE loop checks its condition before each iteration. If the condition is false when the loop is first reached, the body never executes at all.

WHILE password ≠ “secret”
  password ← USERINPUT
ENDWHILE
OUTPUT “Access granted”
This keeps asking for input while the password is wrong. If the user gets it right on the first attempt, the loop body runs once; if they never get it right, it runs indefinitely.

Indefinite Iteration — Condition at the End: REPEAT...UNTIL and DO...WHILE

Two loop forms check their condition after the body has run, which guarantees the body executes at least once.

REPEAT…UNTIL continues the body until the condition becomes true:

REPEAT
  pin ← USERINPUT
UNTIL pin = 1234
OUTPUT “PIN accepted”
The user is always asked for a PIN at least once, then again until they enter the correct value.

DO…WHILE continues the body while the condition remains true:

DO
  answer ← USERINPUT
WHILE answer ≠ “quit”
This is logically identical to REPEAT…UNTIL except for whether the condition is expressed as “stop when X” or “continue while X”.

Nested Iteration and Nested Selection

Loops and conditionals can be placed inside other loops and conditionals. This is called nesting. Nested structures are used for grid operations, table printing, and multi-condition checks.

A common exam mistake is incorrectly handling the boundary condition in a nested IF. For example:

IF score >= 40 THEN
  OUTPUT “Pass”
  IF score >= 70 THEN
    OUTPUT “Merit”
  ENDIF
ENDIF
Students often accidentally output “Merit” without checking that the outer IF’s condition was also met first. Always trace through nested conditions carefully.

Putting It Together — A Real Example

Consider a simple number-guessing game. The target number is 42. The player keeps guessing until they get it right or run out of attempts. This combines a WHILE loop (indefinite, condition at start), selection (IF to check the guess), Boolean operators, and counter variables:

target ← 42
attempts ← 0
maxAttempts ← 5
guess ← 0
WHILE guess ≠ target AND attempts < maxAttempts
  guess ← USERINPUT
  attempts ← attempts + 1
ENDWHILE
IF guess = target THEN
  OUTPUT “Correct!”
ELSE
  OUTPUT “Out of attempts”
ENDIF