Procedures and BYREF

CALL runs a procedure

• A procedure is a named block. It does a job and returns no value.
• Define it with PROCEDURE … ENDPROCEDURE, and run it with CALL.
• Parentheses are written even when there are no parameters.

PROCEDURE Greet()
  OUTPUT "Hi"
ENDPROCEDURE
CALL Greet()

By value is the default

• A parameter with no mode is passed by value: the procedure gets a copy.
• Changing the copy does not change the caller's variable.
• You may also write BYVAL in front of the name. It means the same thing.

PROCEDURE Show(N : INTEGER)
  N ← N + 1
  OUTPUT N
ENDPROCEDURE
DECLARE A : INTEGER
A ← 5
CALL Show(A)
OUTPUT A

BYREF passes the real variable

• BYREF means the parameter is the caller's variable. A change is still there after CALL.
• If several parameters in a row use the same mode, the guide lets you write the keyword once.
• Do not pass a parameter to a function by reference. Use BYREF on a procedure.

PROCEDURE Swap(BYREF X : INTEGER, BYREF Y : INTEGER)
  DECLARE Temp : INTEGER
  Temp ← X
  X ← Y
  Y ← Temp
ENDPROCEDURE
DECLARE A : INTEGER
DECLARE B : INTEGER
A ← 1
B ← 2
CALL Swap(A, B)
OUTPUT A & "," & B

Common mistakes

• CALL before a procedure. Never CALL a function.
• IGCSE 0478 has no BYREF. On that paper every parameter is a copy.

Now you try

• Write a procedure that swaps two integers.