Selection: IF

IF … THEN on one line

• Selection chooses what to run.
• The 2026 guide puts THEN on the same line as the condition.
• The block ends at ENDIF.

DECLARE Age : INTEGER
Age ← 20
IF Age >= 18 THEN
  OUTPUT "You may vote"
ENDIF

ELSE is the other path

• ELSE runs when the condition is FALSE.
• Exactly one of the two blocks runs.
• Nested IF statements keep the same shape.

DECLARE Mark : INTEGER
Mark ← 72
IF Mark >= 50 THEN
  OUTPUT "Pass"
ELSE
  OUTPUT "Fail"
ENDIF

Relational operators

• = equal, <> not equal, < > <= >=.
• AND, OR and NOT are logic operators. Their operands are BOOLEAN.

DECLARE N : INTEGER
N ← 7
IF N MOD 2 = 0 THEN
  OUTPUT "even"
ELSE
  OUTPUT "odd"
ENDIF

AND, OR and NOT

• AND is true only when both tests are true. A mark is in range when it is at least 0 and at most 100.
• OR is true when either test is true. NOT flips TRUE and FALSE.
• Write the variable on both sides. Mark >= 0 AND Mark <= 100 is the test. Mark >= 0 AND <= 100 is not.

DECLARE Mark : INTEGER
Mark ← 80
IF Mark >= 0 AND Mark <= 100 THEN
    OUTPUT "valid"
ELSE
    OUTPUT "invalid"
ENDIF

Watch out

• THEN stays on the IF line. IGCSE 0478 puts THEN on the following line. Do not mix them.
• In a test, = means "is equal to". Storing a value still uses ←.
• "Pass" and "Fail" need quotes. OUTPUT Pass looks up a name called Pass.

Common mistakes

• Every IF needs an ENDIF.
• ELSE belongs to the IF above it, not to a nested one, unless you indent it that way.

Now you try

• Read a number and choose with IF … THEN … ELSE … ENDIF.