Slice string from end in Ruby

Question

How can I split a string into multiple substrings of equal length but from back?

For example if string is: "ABCDEFGH", then I want an array of each string of length 3 as:

["FGH", "CDE", "AB"]

Answer 1

I think that this does what you're asking:

> "ABCDEFGH".reverse.scan(/.{1,3}/).each { |x| x.reverse! }
=> ["FGH", "CDE", "AB"]

Here's a quick explanation:

.reverse reverses the string so that it is "HGFEDCBA" instead of "ABCDEFGH".

.scan(/.{1,3}/) converts the string into an array with each element of the array containing 3 characters (if the string isn't divisible by 3 then the last element of the array may have 1 or 2 characters).

.each { |x| x.reverse! } reverses the characters in each element of the array.

You could define a function like this:

def slice_string_from_end(s)
  s.reverse.scan(/.{1,3}/).each { |x| x.reverse! }
end

Then you can use:

slice_string_from_end("ABCDEFGH")

Answer 2

You can accomplish this using each_slice, but you'll need to reverse the string first, and then re-reverse each individual slice:

x = "ABCDEFGH"
x.chars.reverse.each_slice(3).map(&:reverse).map(&:join)
=> ["FGH", "CDE", "AB"]

• split the string into a character array (x.chars)
• reverse the array (.reverse)
• slice the array into sub-arrays of 3 characters (.each_slice(3))
• reverse each sub-array (.map(&:reverse))
• join each sub-array back into a string (.map(&:join))