#377 Split by Widths

Using ruby to split strings into chunks of specified widths; cassidoo’s interview question of the week (2025-10-20).

Notes

The interview question of the week (2025-10-20) should play to ruby’s strength and flexibility in array and string processing:

Given a string str and an array of positive integers widths, write a function that splits the string into lines, each with the exact number of characters as specified by the corresponding width. Return an array of the substrings. Use the last width for any remaining characters if the array is shorter than needed.

Example:
const str = "Supercalifragilisticexpialidocious";
const widths = [5, 9, 4];

> splitByWidths(str, widths);
> ['Super', 'califragi', 'list', 'icex', 'pial', 'idoc', 'ious']

Thinking about the Problem

The widths array could be just right for the given string, or it could have a surfeit or excess of elements.

Similarly, the string may not exactly divide according to the widths. We basically need to keep processing until all characters are consumed.

A First Go

We take advantage of ruby’s forgiving nature in two places:

shift will return nil if we’ve run out of elements, so we keep the previous value as a fallback: width = widths.shift || width
and input[index, width] will return up to width characters, but not complain if it runs short

def split_by_widths(input, widths)
  result = []
  index = 0
  while index < input.length do
    width = widths.shift || width
    result << input[index, width]
    index += width
  end
  result
end

Running some examples:

$ ./example.rb "Supercalifragilisticexpialidocious" "5, 9, 4"
["Super", "califragi", "list", "icex", "pial", "idoc", "ious"]
$ ./example.rb "Supercalifragilisticexpialidocious" "5, 9, 6"
["Super", "califragi", "listic", "expial", "idocio", "us"]
$ ./example.rb "abcdefg" "1"
["a", "b", "c", "d", "e", "f", "g"]
$ ./example.rb "abcdefghij" "3, 2, 4, 6, 6"
["abc", "de", "fghi", "j"]

Tests

I’ve setup some validation in test_example.rb:

$ ./test_example.rb
Run options: --seed 29972

# Running:

....

Finished in 0.000229s, 17467.2489 runs/s, 17467.2489 assertions/s.

4 runs, 4 assertions, 0 failures, 0 errors, 0 skips

Example Code

Final code is in example.rb:

#!/usr/bin/env ruby

def split_by_widths(input, widths)
  result = []
  index = 0
  while index < input.length do
    width = widths.shift || width
    result << input[index, width]
    index += width
  end
  result
end

if __FILE__ == $PROGRAM_NAME
  (puts "Usage: ruby #{$0} (string) (widths comma separated)"; exit) unless ARGV.length > 1
  input = ARGV[0]
  widths = ARGV[1].split(',').map(&:to_i)
  puts split_by_widths(input, widths).inspect
end

Credits and References

cassidoo’s interview question of the week (2025-10-20)