Summing objects in recursive python function

Question

I need to sum objects (strings, ints, etc.) in one function (don't create other function, it can be done within one). It should work this way: When given my_sum([[['s'],'ta'],['c',['k']]]), it should return 'stack'.

I came up with this:

def my_sum(array):

    if not array: #empty array
        return None

    else:

        for item in array:

            if type(item) == list:
                my_sum(item)

            else:
                print(item)

It of course is not doing what it should be, I was just playing around with it trying to come up with something. This code above returns this:

s
ta
c
k

I think I am not that far from result as I have what I need, but here is the problem how can I sum up those items ? I can't write result = '' anywhere in the function and then return it, because it would be deleting every time there would be recursion call. Also I don't want global variables (if anyone would think of that). Maybe I am just being stupid and can't see that it is one simple thing, pardon me if it is so.

Thank you for every answer!

You need another argument to your my_sum function, try an accumulator value so that instead of returning None you can return that value. def my_sum(array, acc=None): ... — bheklilr
– bheklilr, Commented Mar 3, 2015 at 21:08

georg · Accepted Answer · 2015-03-03 21:18:34Z

4

The common accumulating pattern is:

result = <init value>
for item in argument:
     result = result <operator> item
return result

(this can be written more concisely, but that's not the point for now).

Applied to your problem:

def my_sum(items):
    result = ''
    for item in items:
        if type(item) == list:
            result += my_sum(item)
        else:
            result += item
    return result

Note that type(x) == y is frowned upon in Python, isinstance is considered better style.

Homework: extend the function so that it works for these arguments too:

print my_sum([[['s'],'ta'],('c',('k')), {('over'), ('flow')}])

edited Mar 3, 2015 at 21:18

answered Mar 3, 2015 at 21:13

georg

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1 Comment

aruisdante Over a year ago

And even then, isinstance should be use sparingly, as it will, for example, fail if the input is a tuple even though it would be perfectly valid (as your followup question is designed to point out, I assume)

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