Reverse a string each two characters with Ruby

Question

I want to reverse a string each two characters with Ruby.

The input:

\"0123456789abcdef\"

The output that I expect:

\"e         


        

Answer 1

I like all the solutions, especially @sawa's, but now, six hours later, its a challenge to come up with something different. In the interest of diversity, I offer a way that uses pairwise substitution. It's destructive, so I start by making a copy of the string.

Code

str = "0123456789abcdef"
s = str.dup
(0...s.size/2).step(2) { |i| s[i,2], s[-i-2,2] = s[-i-2,2], s[i,2] }
  s #=> "efcdab8967452301"

Explanation

Letting

enum = (0...s.size/2).step(2) #=> #

we can see what is enumerated:

enum.to_a                     #=> [0, 2, 4, 6]

The string

s #=> "0123456789abcdef"

is as follows after each call to the block:

i = 0: "ef23456789abcd01" 
i = 2: "efcd456789ab2301" 
i = 4: "efcdab6789452301" 
i = 6: "efcdab8967452301"