CAIE 0478 & 9618 Subroutines

8. Procedures & Functions

Modular Cambridge pseudocode subroutines: PROCEDURE with CALL, FUNCTION with RETURN, and parameter passing by value (BYVAL) and reference (BYREF).

Procedures perform actions and are invoked with CALL. Functions return a single value via RETURN and are called as part of an expression (not with CALL). IGCSE uses ordinary typed parameters. Explicit pass-by-value (BYVAL, default) and pass-by-reference (BYREF) modes are AS & A Level (9618) extensions.

Procedures (CALL)

PSEUDOCODE

// Without parameters
PROCEDURE DefaultSquare()
   CALL Square(100)
ENDPROCEDURE

// With parameters
PROCEDURE Square(Size : INTEGER)
   FOR Side <- 1 TO 4
      CALL MoveForward(Size)
      CALL Turn(90)
   NEXT Side
ENDPROCEDURE

CALL Square(50)

Functions (RETURN)

PSEUDOCODE

FUNCTION Max(Number1 : INTEGER,
             Number2 : INTEGER)
             RETURNS INTEGER
   IF Number1 > Number2 THEN
      RETURN Number1
   ELSE
      RETURN Number2
   ENDIF
ENDFUNCTION

// Called as part of an expression
OUTPUT "Penalty = ", Max(10, Distance*2)

Passing by Reference (BYREF)

PSEUDOCODE

// BYREF allows the procedure to modify the caller's variables directly
PROCEDURE SWAP(BYREF X : INTEGER, BYREF Y : INTEGER)
   Temp <- X
   X <- Y
   Y <- Temp
ENDPROCEDURE

// Note: If not specified, BYVAL (pass by value) is assumed.
// Parameters should NOT be passed BYREF to a function.