Substrings in Ruby

There are different way to find every substrings from a string in Ruby. The most straightforward way (in my opinion) is to call two Integer#upto method.

Let’s say we want every substring of “amethyst”. Using a multidimensional Arrays, the result will look like the following:

  ["a", "m", "e", "t", "h", "y", "s", "t"],
  ["am", "me", "et", "th", "hy", "ys", "st"],
  ["ame", "met", "eth", "thy", "hys", "yst"],
  ["amet", "meth", "ethy", "thys", "hyst"],
  ["ameth", "methy", "ethys", "thyst"],
  ["amethy", "methys", "ethyst"],
  ["amethys", "methyst"],

From this result we see we need two numbers:

  • A number to symbolize the length of the string
  • A number to symbolize the position where to start in the string

The former is just a matter of looping from one (or whatever minimum length you need) to the string number of characters (or, again, whatever the maximum String length you need).

This can easily be done in Ruby:

string = 'amethyst'

1.upto(string.length) do |chars_position|
  puts string[0...chars_position]

Note the use of the three dots (...): because Ruby uses 0 indexed collections, we need not to include the last number. It would be possible to use two dots and passing -1 to method upto (0.upto(string.length -1)) to achieve the same effect. This is hella ugly though.

For the second number, i.e. the number to symbolize the position where to start in the string, we can once again use method upto to iterate over the string. This time, we will use the parameter as the position in the string to start to.

string = 'amethyst'

0.upto(string.length) do |starting_chars|
  puts string[starting_chars...-1]

Now we have a way to control the string’s length and where we should start iterating over it. Combining those two snippets of codes, we can create a method to get every substrings, given a string (actually two methods for ease of reading).

def substrings_maker(str, substr_size)
  substr_collector =

  0.upto(str.length - substr_size) do |starting_chars|
    substr_collector <<  str[starting_chars...(starting_chars + substr_size)]


def find_all_substrings(str)
  substr_list =

  1.upto(str.length) do |chars_position|
    substr_list << substrings_maker(str, chars_position)


violet_rock = 'amethyst'

puts find_all_substrings(violet_rock)

All of that to learn there is meth in a_meth_yst!

Practice Problems