Python basic program understanding

Question

I am trying to understand this program but can't get my head around it. Could anyone suggest where i'm going wrong?

def mult(a, b):
    if b == 0:
        return 0
    rest = mult(a, b - 1)
    value = a + rest
    return value

print("3 * 2 = ", mult(3, 2))

In the above Python Script,

• The line print("3 * 2 = ", mult(3, 2)) is run
• The Mult function is called
• In the Mult Function b == 2 so the if condition return FALSE
• The Line rest = Mult(a, b -1) is next and calls the Mult function again on new values (3,1)
• In the Mult Function b == 1 so the if condition return FALSE
• The Line rest = Mult(a, b -1) is next and calls the Mult function again on new values (3,0)
• In the Mult Function b == 0 so the if condition return TRUE
• The value 0 is returned by the if condtion
• The program Prints "3 * 2 = 0"?

I followed your comments and put this together to make it easier to follow

Answer 1

You are using mult recursively, and it returns to rest, so you will be updating that value each time you come back from the function. A few print statements might help clear up what is going on.

def mult(a, b):
    if b == 0:
        print "b = 0"
        return 0
    rest = mult(a, b - 1)
    value = a + rest
    print "rest", rest, "value", value
    return value

mult(3,2)

Output

b = 0
rest 0 value 3
rest 3 value 6
6

Answer 2

The problem in your understanding lies with where the call to mult(3, 0) returns to. That call does not return to the line print..., it returns to the line rest = mult(a, b-1). So then value = a + rest will result in value having the value of 3 + 0 = 3. Then the return value line will return to rest = mult(a, b-1) again. When it hits return value again, that is when it will return to the print statement.

Answer 3

You have to build a call-stack in your step-by-step-list: when the b == 0 the function mult returns to the place where it was last called, that was the line rest = ... and not the line print ....