What is the simplest way to swap each pair of adjoining chars in a string with Python?

Question

I want to swap each pair of characters in a string. \'2143\' becomes \'1234\', \'badcfe\' becomes \'abcdef\'.

How

Answer 1

A bit late to the party, but there is actually a pretty simple way to do this:

The index sequence you are looking for can be expressed as the sum of two sequences:

 0  1  2  3 ...
+1 -1 +1 -1 ...

Both are easy to express. The first one is just range(N). A sequence that toggles for each i in that range is i % 2. You can adjust the toggle by scaling and offsetting it:

         i % 2      ->  0  1  0  1 ...
     1 - i % 2      ->  1  0  1  0 ...
2 * (1 - i % 2)     ->  2  0  2  0 ...
2 * (1 - i % 2) - 1 -> +1 -1 +1 -1 ...

The entire expression simplifies to i + 1 - 2 * (i % 2), which you can use to join the string almost directly:

result = ''.join(string[i + 1 - 2 * (i % 2)] for i in range(len(string)))

This will work only for an even-length string, so you can check for overruns using min:

N = len(string)
result = ''.join(string[min(i + 1 - 2 * (i % 2), N - 1)] for i in range(N))

Basically a one-liner, doesn't require any iterators beyond a range over the indices, and some very simple integer math.