 Functions are useful for top-down design
 break a big problem down into smaller pieces

def main (): a1 = 5 a2 = 33 a3 = a2+5 partone(a1, a2) a2 = a2 + parttwo (a1, a3) partThree (a1, a2, a3, 23) main()

 The program in the previous slide won't run until all the other functions have definitions
 If you just want to write one of them, you have to fill in the others with "dummies" =
"stubs"
 A stub has the right header but doesn't do anything useful towards solving the problem

def partThree (p1, p2, p3, p4): print("in the partThree function") print( p1, p2 , p3) def partTwo (p1, p2): print ("in partTwo") return 1

 suppose you have written a function that solves a problem, but...
 you don't have the main function written yet
 how can you test your function without it?
 write a "driver"

 Usually a main function
 just calls the function to be tested with arguments that make sense
 reports the results
 does not try to solve the whole problem

def main (): x = 5 z= 23 print ("calling fun1 with x =", x, " and z = ", z) y = fun1 (x, z) print( "result of calling fun1 is ", y)

 both stubs and drivers known as
"scaffolding"
 not part of the finished program but very useful when testing parts of the program
 usually removed when program is finished