IF statements

IF statements allow your program to perform different actions depending on a condition. The condition can be any expression with a boolean type (TRUE or FALSE).

IF THEN

In the example below, the condition we test is AGE < 8. If that is TRUE, then we print a warning message. Otherwise the code between THEN and END IF is skipped.

MODULE MAIN
  FUNC SHOW_NOTICE(AGE: INT): INT
    IF AGE < 8 THEN
      # THIS CODE ONLY RUNS WHEN AGE IS LESS THAN 8
      CONSOLE::PRINT("YOU ARE TOO YOUNG TO RIDE.")
    END IF

    # THIS CODE ALWAYS RUNS
    CONSOLE::PRINT("{N}")
  END FUNC

  FUNC START()
    CONSOLE::INIT()

    SHOW_NOTICE(5)
  END FUNC
END MODULE

ELSE

Instead of skipping to END IF, we can instead use ELSE to provide an alternative behavior:

MODULE MAIN
  FUNC SHOW_NOTICE(AGE: INT): INT
    IF AGE < 8 THEN
      # ONLY RUNS WHEN AGE < 8
      CONSOLE::PRINT("YOU ARE TOO YOUNG TO RIDE.")
    ELSE
      # ONLY RUNS WHEN AGE >= 8
      CONSOLE::PRINT("YOU ARE OLD ENOUGH TO RIDE.")
    END IF

    # THIS CODE ALWAYS RUNS
    CONSOLE::PRINT("{N}")
  END FUNC

  FUNC START()
    CONSOLE::INIT()

    SHOW_NOTICE(5)
  END FUNC
END MODULE

ELSIF

We can use ELSIF to consider multiple conditions:

MODULE MAIN
  FUNC SHOW_NOTICE(AGE: INT): INT
    IF AGE < 8 THEN
      # LESS THAN 8
      CONSOLE::PRINT("YOU ARE TOO YOUNG TO RIDE.")
    ELSIF AGE <= 12 THEN
      # BETWEEN 8 AND 12
      CONSOLE::PRINT("ASK YOUR PARENT FOR PERMISSION.")
    ELSIF AGE >= 18 THEN
      # 18 OR OLDER
      CONSOLE::PRINT("YOU ARE TOO OLD TO RIDE.")
    ELSE
      # BETWEEN 13 AND 17
      CONSOLE::PRINT("YOU ARE OLD ENOUGH TO RIDE.")
    END IF

    # THIS CODE ALWAYS RUNS
    CONSOLE::PRINT("{N}")
  END FUNC

  FUNC START()
    CONSOLE::INIT()

    SHOW_NOTICE(5)
  END FUNC
END MODULE

You may wonder why we need ELSIF. Couldn't we just use ELSE and IF?

You can, but the END IF indentation will accumulate quickly:

    . . .
    IF AGE < 8 THEN
      # LESS THAN 8
      CONSOLE::PRINT("YOU ARE TOO YOUNG TO RIDE.")
    ELSE
      IF AGE <= 12 THEN
        # BETWEEN 8 AND 12
        CONSOLE::PRINT("ASK YOUR PARENT FOR PERMISSION.")
      ELSE
        IF AGE >= 18 THEN
          # 18 OR OLDER
          CONSOLE::PRINT("YOU ARE TOO OLD TO RIDE.")
        ELSE
          # BETWEEN 13 AND 17
          CONSOLE::PRINT("YOU ARE OLD ENOUGH TO RIDE.")
        END IF
      END IF
    END IF
    . . .

ELSIF is cleaner and simpler. (Of course ELSIF could be spelled out as ELSE IF, but then beginners might have trouble recognizing which IF matches the END IF.)

DO

Programs frequently use very simple IF statements, for example:

  . . .
  IF COUNT < 0 THEN
    0 -> COUNT
  END IF
  . . .

  . . .
  IF COUNT > 10 THEN
    RETURN RESULT
  END IF
  . . .

You can save a line of code by using DO instead of THEN. The examples below have identical meaning as above:

  . . .
  IF COUNT < 0
  DO 0 -> COUNT
  . . .

  . . .
  IF COUNT > 10
  DO RETURN RESULT
  . . .

DO can only be used with simple statements such as assignments or function calls. The following examples are NOT allowed:

  . . .
  # ⚠️ ERROR: "LOOP" IS NOT ALLOWED WITH "DO"
  IF COUNT < 0
  DO LOOP
  END LOOP
  . . .

  . . .
  # ⚠️ ERROR: "IF" IS NOT ALLOWED WITH "DO"
  IF COUNT < 0
  DO IF COUNT > 5
  DO 0 -> COUNT
  . . .