Question

I’m trying to link two string together recursively, but not getting the expected results:

For the two strings "abcd" and "xyz" – expected output should be "axbyczd":

def strr(str1, str2):

    def recstr(str1, str2, prn):
        if str1 == '':
            return str2
        if str2 == '':
            return str1
        else:
            return prn + recstr(str1[:len(str1)-len(prn)],str2[:len(str2)-len(prn)],prn)
    return recstr(str1, str2, '')

print strr("abcdef","12345")

Answer 1

When you ran out of characters in either string, you returned the other string without concatenating it to a running accumulator. Look at what I do when s1 or s2 is empty.

Also, in your recursive case, you have a very complex slicing of s1 and s2. You should really only need to slice s1[1:] and s2[1:]

This should do it

def recstr(s1, s2, answer=''):
    if not s1:
        return answer+s2
    if not s2:
        return answer+s1
    return recstr(s1[1:], s2[1:], answer+s1[0]+s2[0])

In [15]: s1,s2 = 'abcd', 'xyz'

In [16]: print recstr(s1,s2)
axbyczd

Of course, a much cleaner way to do this would be to use itertools.izip_longest and itertools.chain.from_iterable:

In [23]: zips = itertools.izip_longest(s1,s2, fillvalue='')

In [24]: ''.join(itertools.chain.from_iterable(zips))
Out[24]: 'axbyczd'

[Thanks @AshwiniChaudhary for pointing out the fillvalue param in izip_longest]

Hope this helps