Unexpected python recursive multiplication return

Question

It is my intention to recursively multiply two numbers with this function. I realize this is quite possibly far from optimal. Why does my call, print rec_mult(15, 1111), to this function print None rather than 16665?

def rec_mult(x, y, tot=0, inc=0):
    if int(x) ==  0:
        return tot
    else:
        x = str(x)
        tot += int(x[(-1+inc):]) * y
        x = x[:(-1+inc)] + "0"; inc -= 1
        rec_mult(x, y, tot, inc)

Jared · Accepted Answer · 2013-07-02 23:05:19Z

5

You have to return when recursively calling your function like,

def rec_mult(x, y, tot=0, inc=0):
    if int(x) ==  0:
        return tot
    else:
        x = str(x)
        tot += int(x[(-1+inc):]) * y
        x = x[:(-1+inc)] + "0"; inc -= 1
        return rec_mult(x, y, tot, inc)  # <-- return here

print rec_mult(2, 10)  # 20

answered Jul 2, 2013 at 23:05

Jared

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