Selection with IF and CASE

IF ... THEN ... ELSE

Selection lets a program choose which statements to run. An IF statement checks a condition. If it is TRUE, the THEN block runs. If it is FALSE, the optional ELSE block runs instead.

DECLARE Mark : INTEGER
INPUT Mark
IF Mark >= 50
  THEN
    OUTPUT "Pass"
  ELSE
    OUTPUT "Try again"
ENDIF

Comparison operators

Conditions are built from comparisons, which always give TRUE or FALSE:

AND, OR, NOT

Boolean operators combine conditions. AND is TRUE only if both sides are TRUE. OR is TRUE if at least one side is TRUE. NOT flips TRUE and FALSE.

DECLARE Age : INTEGER
DECLARE Ticket : STRING
INPUT Age
INPUT Ticket
IF Age >= 12 AND Ticket = "Y"
  THEN
    OUTPUT "Enjoy the film"
  ELSE
    OUTPUT "Sorry, you cannot go in"
ENDIF

Nested IF

An IF can sit inside the THEN or ELSE block of another IF. The inner condition is only checked when the program reaches the branch that contains it: here, only when Mark >= 70 is FALSE. Indent each level so you can see which ENDIF closes which IF.

DECLARE Mark : INTEGER
INPUT Mark
IF Mark >= 70
  THEN
    OUTPUT "Grade A"
  ELSE
    IF Mark >= 50
      THEN
        OUTPUT "Grade B"
      ELSE
        OUTPUT "Grade C"
    ENDIF
ENDIF

CASE OF

When one variable is compared against several fixed values, CASE OF is shorter and clearer than many nested IFs. OTHERWISE catches every value not listed.

DECLARE Choice : INTEGER
OUTPUT "1 Play  2 Settings  3 Quit"
INPUT Choice
CASE OF Choice
    1 : OUTPUT "Starting game"
    2 : OUTPUT "Opening settings"
    3 : OUTPUT "Goodbye"
    OTHERWISE : OUTPUT "Invalid choice"
ENDCASE

Check yourself