Leading substrings exercise

Question

I have a couple of answers that I wrote up for this exercise. Both of my methods work. I'm hoping to get some feedback on each and how I could improve them, if needed.

The idea is to take a string, such as 'abc' and return an array like this: ['a', 'ab', 'abc']

For the second method, I used #map, but I'm not sure if this is the best way to achieve my desired result.

def substrings(string)
  subbed = []
  (0..string.length-1).each do |i|
    subbed.push(string.slice(0..i))
  end
  return subbed
end

def substrings2(string)
  string.split('').map.with_index do |c,i|
    string.slice(0..i)
  end
end


print substrings2('abc')

Answer 1

• In Ruby, there are enough Enumerable and Array methods that you virtually never need to initialize an empty array and iterate to push values.
• Your method doesn't return every substring in the string. 'bc' isn't present for example. Your method shouldn't be called substrings. Your title suggests leading_substrings.

Here's a clear, short way to initialize this array with Array.new :

def leading_substrings(string)
  Array.new(string.size) { |i| string[0..i] }
end

p leading_substrings('abc')
# ["a", "ab", "abc"]

Answer 2

What you're looking for is known as a "scan" in functional programming. Surprisingly, ruby's Enumerable, which is where this method belongs (since what you're doing is not, ultimately, specific to strings), does not provide a scan method or its equivalent.

But we can write one easily enough:

module Enumerable
  def scans
    reduce([]) {|m,x| m << (m.last || []) + [x]}
  end  
end

Now we have a general purpose method which we can use like so:

[9] pry(main)> (1..5).to_a.scans
=> [[1], [1, 2], [1, 2, 3], [1, 2, 3, 4], [1, 2, 3, 4, 5]]

Your original problem now becomes a simple delegation to scans:

[14] pry(main)> 'abc'.split('').scans.map(&:join)
=> ["a", "ab", "abc"]

Answer 3

Your second method is closer to idiomatic Ruby. In general, you should never need to initialize an array. chars is a nice alternative to split(''). And, you can open String and add a new method. I would do:

class String
  def substrings
    chars.map.with_index { |_char, index| slice(0..index) }
  end
end

Then:

'abc'.substrings