Selection: IF

IF makes a decision

• Selection chooses what to do, based on a test.
• IF runs its THEN block only when the test is TRUE.
• The block ends at ENDIF.

IF Age >= 18 THEN
    OUTPUT "You may vote"
ENDIF

IF ... ELSE

• ELSE gives the path to take when the test is FALSE.
• Exactly one of the two blocks runs.

IF Mark >= 50 THEN
    OUTPUT "Pass"
ELSE
    OUTPUT "Fail"
ENDIF

Comparison operators

• = equal to, <> not equal to.
• < > <= >= compare sizes.

IF N MOD 2 = 0 THEN
    OUTPUT "even"
ELSE
    OUTPUT "odd"
ENDIF

Watch out

• In a test, = means "is equal to". (The arrow ← is for storing a value.)
• Don't forget THEN after the test and ENDIF to close the block.

Common mistakes

• Write IF ... THEN ... ELSE ... ENDIF — do not forget ENDIF.
• Compare with = in pseudocode conditions.

Now you try

• Read the input, then choose with IF ... ELSE ... ENDIF.
• Press Check answer to test it.