Python recursion variable state

Question

I'm trying to write a Python function to take a string and a number and return a list containing repetitions of the string. For example

print func(3, 'aaa')

returns

['aaa', 'aaa', 'aaa']

This is what I've done so far:

def func(times, data):
    if times > 0:
        return data.split() + func(times-1, data)

However, it's giving me a TypeError:

can only concatenate list (not "NoneType") to list.

I'm still a novice and I just read about recursion.

I would also like to know how to "carry over" the state of a variable from successive function calls, without having to define a global variable. Help please!

So what happens when times == 0? You are not returning anything in that case. — Martijn Pieters
– Martijn Pieters, Commented Sep 18, 2016 at 13:03
While this could be done with recursion, this problem seems simple enough to do without it — OneCricketeer
– OneCricketeer, Commented Sep 18, 2016 at 13:03
I think he's trying to get some practice with recursion, nothing wrong with that. But yes, [data]*times is probably the best way to do it otherwise. — damjan
– damjan, Commented Sep 18, 2016 at 13:04
ok: so long for def func(nb,s): return [s]*nb print(func(3, 'aaa')) — Jean-François Fabre
– Jean-François Fabre ♦, Commented Sep 18, 2016 at 13:06

Padraic Cunningham · Accepted Answer · 2016-09-18 13:21:24Z

You need a base case where times is 0, there you can just return an empty list:

def func(times, data):
    if times == 0:
        return []
    # no need for split, just wrap data in a list.
    return [data] + func(times-1, data)

In your code when times == 0, your function implicitly returns None so you are trying to add the result of the recursive calls to None. We should also probably use <= in the base case to catch negative input for times:

def func(times, data):
    if times <= 0:
        return []
    return [data] + func(times-1, data)

If we don't we would recurse infinitely and hit a RuntimeError: maximum recursion depth exceeded as we would never hit the base case.

There is a nice online tool python tutor that can visualise the steps so you can see exactly what is happening which would highlight why and where your code erros.

When you have a working solution you could use rcviz which will create a nice png of the execution steps:

Note: 1. The edges are numbered by the order in which they were traversed by the execution. 2. The edges are colored from black to grey to indicate order of traversal : black edges first, grey edges last.

0xdead · Accepted Answer · 2016-09-18 13:54:12Z

0

While @Padraic Cunningham has a good answer, here is a simple way to do it:

def func(number, content):
    return [content] * number

Or list comprehension:

def func(number, content):
    return [content for _ in range(number)]

answered Sep 18, 2016 at 13:54

0xdead

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