Subroutines

Functions and procedures

A subroutine is a named block of code that runs when it is called.

• A function returns a value to the code that called it.
• A procedure does a job but returns nothing. (AQA style calls both SUBROUTINE; Python uses def for both.)
• Parameters are the names in the brackets of the definition; the values passed in when it is called are arguments.

function area(w, h)
    return w * h
endfunction
procedure banner(msg)
    print("== " + msg + " ==")
endprocedure
banner("Room " + str(area(3, 4)))

SUBROUTINE area(w, h)
    RETURN w * h
ENDSUBROUTINE
SUBROUTINE banner(msg)
    OUTPUT "== " + msg + " =="
ENDSUBROUTINE
banner("Room " + INT_TO_STRING(area(3, 4)))

def area(w, h):
    return w * h
def banner(msg):
    print("== " + msg + " ==")
banner("Room " + str(area(3, 4)))

1. The last line runs first, because the lines above it only define the subroutines.
2. To build the message it needs area(3, 4). Inside area, w = 3 and h = 4, so it returns 12.
3. str(12) is "12", so the argument passed to banner is "Room 12".
4. banner prints == Room 12 ==. A procedure returns nothing, so nothing comes back.

A function can call another function. The inner call is worked out first and its returned value is used where the call was: if double(n) returns n * 2, then double(double(3)) + 1 is double(6) + 1 = 13.