Pythonic way to insert every 2 elements in a string

Question

Is there a pythonic way to insert an element into every 2nd element in a string?

I have a string: 'aabbccdd' and I want the end result to be 'aa-bb-cc-dd'.

I am not sure how I would go about doing that.

Answer 1

Assume the string's length is always an even number,

>>> s = '12345678'
>>> t = iter(s)
>>> '-'.join(a+b for a,b in zip(t, t))
'12-34-56-78'

The t can also be eliminated with

>>> '-'.join(a+b for a,b in zip(s[::2], s[1::2]))
'12-34-56-78'

---

The algorithm is to group the string into pairs, then join them with the - character.

The code is written like this. Firstly, it is split into odd digits and even digits.

>>> s[::2], s[1::2]
('1357', '2468')

Then the zip function is used to combine them into an iterable of tuples.

>>> list( zip(s[::2], s[1::2]) )
[('1', '2'), ('3', '4'), ('5', '6'), ('7', '8')]

But tuples aren't what we want. This should be a list of strings. This is the purpose of the list comprehension

>>> [a+b for a,b in zip(s[::2], s[1::2])]
['12', '34', '56', '78']

Finally we use str.join() to combine the list.

>>> '-'.join(a+b for a,b in zip(s[::2], s[1::2]))
'12-34-56-78'

The first piece of code is the same idea, but consumes less memory if the string is long.

Answer 2

>>> s = 'aabbccdd'
>>> '-'.join(s[i:i+2] for i in range(0, len(s), 2))
'aa-bb-cc-dd'

Answer 3

This one-liner does the trick. It will drop the last character if your string has an odd number of characters.

"-".join([''.join(item) for item in zip(mystring1[::2],mystring1[1::2])])

Answer 4

If you want to preserve the last character if the string has an odd length, then you can modify KennyTM's answer to use itertools.izip_longest:

>>> s = "aabbccd"
>>> from itertools import izip_longest
>>> '-'.join(a+b for a,b in izip_longest(s[::2], s[1::2], fillvalue=""))
'aa-bb-cc-d'

or

>>> t = iter(s)
>>> '-'.join(a+b  for a,b in izip_longest(t, t, fillvalue=""))
'aa-bb-cc-d'

Answer 5

Here is one list comprehension way with conditional value depending of modulus of enumeration, odd last character will be in group alone:

for s  in ['aabbccdd','aabbccdde']:
    print(''.join([ char if not ind or ind % 2 else '-' + char
                    for ind,char in enumerate(s)
                    ]
                  )
          )
""" Output:
aa-bb-cc-dd
aa-bb-cc-dd-e
"""