Joining pairs of elements of a list

Question

I know that a list can be joined to make one long string as in:

x = ['a', 'b', 'c', 'd'] print ''.join(x) 

Obviously this would output:

'abcd' 

However, what I am trying to do is simply join the first and second strings in the list, then join the third and fourth and so on. In short, from the above example instead achieve an output of:

['ab', 'cd'] 

Is there any simple way to do this? I should probably also mention that the lengths of the strings in the list will be unpredictable, as will the number of strings within the list, though the number of strings will always be even. So the original list could just as well be:

['abcd', 'e', 'fg', 'hijklmn', 'opq', 'r']  

Answer 1

You can use slice notation with steps:

>>> x = "abcdefghijklm" >>> x[0::2] #0. 2. 4... 'acegikm' >>> x[1::2] #1. 3. 5 .. 'bdfhjl' >>> [i+j for i,j in zip(x[::2], x[1::2])] # zip makes (0,1),(2,3) ... ['ab', 'cd', 'ef', 'gh', 'ij', 'kl'] 

Same logic applies for lists too. String lenght doesn't matter, because you're simply adding two strings together.

Answer 2

Use an iterator.

List comprehension:

>>> si = iter(['abcd', 'e', 'fg', 'hijklmn', 'opq', 'r']) >>> [c+next(si, '') for c in si] ['abcde', 'fghijklmn', 'opqr'] 

• Very efficient for memory usage.
• Exactly one traversal of s

Generator expression:

>>> si = iter(['abcd', 'e', 'fg', 'hijklmn', 'opq', 'r']) >>> pair_iter = (c+next(si, '') for c in si) >>> pair_iter # can be used in a for loop <generator object at 0x4ccaa8> >>> list(pair_iter)  ['abcde', 'fghijklmn', 'opqr'] 

• use as an iterator

Using map, str.__add__, iter

>>> si = iter(['abcd', 'e', 'fg', 'hijklmn', 'opq', 'r']) >>> map(str.__add__, si, si) ['abcde', 'fghijklmn', 'opqr'] 

next(iterator[, default]) is available starting in Python 2.6