Selection and Iteration

GCSE Computer Science · Programming

Selection and iteration

Two of the three programming constructs let a program make decisions (selection) and repeat work (iteration). Together they give programs their logic and power.

Selection

Selection chooses which code to run based on a condition (something that is True or False).

IF / ELSE IF / ELSE

if age < 13 then
   output "Child"
elseif age < 18 then
   output "Teenager"
else
   output "Adult"
endif

The conditions are checked in order; the first true one runs and the rest are skipped.
ELSE catches everything not already matched.

Nested selection

An IF inside another IF — used when a decision depends on more than one thing.

Comparison & logical operators in conditions

Use ==, !=, <, >, <=, >= and combine with AND, OR, NOT:

if age >= 13 AND age < 18 then ...

Iteration (loops)

Iteration repeats a block of code. There are two families: count-controlled and condition-controlled.

Count-controlled: FOR

Repeats a set number of times — use when you know how many repeats are needed.

for i = 1 to 5
   output i
next i

This prints 1 2 3 4 5.

Condition-controlled: WHILE

Repeats while a condition is True — the condition is checked before each pass, so the loop may run zero times.

while password != "letmein"
   input password
endwhile

Condition-controlled: DO … UNTIL / REPEAT

Repeats until a condition becomes True — the condition is checked after each pass, so the loop always runs at least once.

repeat
   input number
until number > 0

FOR vs WHILE — which to use?

Use FOR when the number of repeats is known (e.g. process 10 items).
Use WHILE when repeats depend on a condition that may change (e.g. keep asking until valid input). A WHILE can run 0 times; a REPEAT/UNTIL runs at least once.

Worked example

How many times does this run? for i = 2 to 8 — outputs each i.

From 2 to 8 inclusive = 2,3,4,5,6,7,8 → 7 times. ✓

Common mistakes

Infinite loops — a WHILE whose condition never becomes False (forgetting to change the variable inside).
Off-by-one errors — forgetting FOR loops are usually inclusive of the end value.
Using a WHILE when you know the exact count (a FOR is clearer), or vice versa.

Exam tips

Be able to trace a loop with a trace table, writing the variable values each pass.
State clearly: FOR = known number of repeats; WHILE = check condition first (may run 0 times); REPEAT/UNTIL = check after (runs ≥ 1 time).
Watch the loop's start and end values for off-by-one slips.

Key facts to remember

Selection = IF / ELSE IF / ELSE, using conditions and AND/OR/NOT.
Count-controlled (FOR) = set number of repeats; condition-controlled (WHILE/REPEAT) = repeat based on a condition.
WHILE checks before (0+ runs); REPEAT…UNTIL checks after (1+ runs).

Selection and Iteration