Combine two strings (char by char) and repeat last char of shortest one

Question

I have two strings, (string1 and string2).

If they are equal in length, the function should return a string that is formed by alternating characters from each of the two strings.

If they are not equal in length, then the function extends the shorter string by repeating the last character until they are the same length and then alternates the characters of the two strings.

For example,

extendedString("abc", "def") =>  "adbecf" 
extendedString("ab", "defg") =>  "adbebfbg"

I have written the part where it returns if the strings are the same length, but I have no idea how to repeat the last character.

def extendedString(string1, string2):
    x = string1
    y = string2
    z = ""
    if len(x) == len(y):
        return "".join(i for j in zip(string1,string2) for i in j)

Answer 1

You can use the zip_longest function from itertools.
Works like zip, but gives you the ability to fill the blanks (default filler is None, but you can change it:

import itertools

def extendedString(string1,string2):
    filler = string2[-1] if len(string1)>len(string2) else string1[-1]
    return "".join(i for j in itertools.zip_longest(string1, string2, fillvalue=filler) for i in j)

update

added the filler to be the last char of the shortest string (in case it's needed)

In [50]: extendedString("abc","def")
Out[50]: 'adbecf'

In [51]: extendedString("ab","defg")
Out[51]: 'adbebfbg'

If you are using python2 the function is itertools.izip_longest

Answer 2

A one-liner solution that doesn't require itertools:

def extendedString(a,b):
    return ''.join(x+y for x,y in zip(*(s+s[-1]*(max(len(a),len(b))-len(s)) for s in (a,b))))

Output:

$ extendedString('abc','1234')
'a1b2c3c4'
$ extendedString('abc','12')
'a1b2c2'

Answer 3

First make both the strings of same length and then join. Something like:

def extendedString(string1,string2):
    x=string1
    y=string2

    if len(x) < len(y):
        x = x + x[-1] * (len(y) - len(x))
    elif len(x) > len(y):
        y = y + y[-1] * (len(x) - len(y))

    return "".join(i for j in zip(x, y) for i in j)

print extendedString("abc", "def")
print extendedString("ab","defg") 
print extendedString("defg","ab") 

Output:

$ python test.py
adbecf
adbebfbg
daebfbgb
$