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rubyarrays
Steven L.
Steven L.

Reputation: 2155

Ruby's Array Combination Method

I am going through the problems on Ruby Monk's Ruby Primer.

Problem Statement Given a 3 or 4 digit number with distinct digits, return a sorted array of all the unique numbers that can be formed with those digits. Example: Given: 123 Return: [123, 132, 213, 231, 312, 321]

I thought that the Array#combination method would do the trick. My code looks like this:

def number_shuffle(number)
  # take integer and turn it into an array of digits
  digits = Array.new

  number.to_s.split('').each do |element|
    digits << element.to_i
  end

  # shuffle the elements
  return digits.combination(digits.length).to_a
end

puts number_shuffle(123)

But the code above returns:

1
2
3

Not sure what I'm doing wrong here. I thought the documentation made it clear:

http://www.ruby-doc.org/core-2.2.0/Array.html#method-i-combination

Any help is appreciated.

Upvotes: 5

Views: 4229

Answers (3)

rakeshbs
rakeshbs

Reputation: 24572

For the ruby monk question what u need is the Array.permutations. Array.permutation(n) is the number of possible arrangements of an array taking n at a time. [1,2,3] with n = 1 will be 1, 2, 3 [1,2,3] with n = 2 will be [1,2] [2,1] [1,3] [3,1] [2,3] [3,2]

What you need is 

   Array.permutations(Array.length)

Array.combination(n) returns the number of unique selections that can be made from the array when taking n objects out of the array.

for an Array [1,2,3] if n = 1. You can only take out one element at a time the possible selections are 1 ,2 and 3.

for an Array [1,2,3] if n= 2. You can take out two elements at a time. the possible selections are [1,2] , [1,3] and [2,3]

You have given the length of the Array as N (N = Array.Length)

So in the case of [1,2,3] if n = 3, There is only one way to make a selection using all the elements. That is [1,2,3]. This is why your code only returns one combination.

Upvotes: 0

August
August

Reputation: 12558

You can get the permutations of the character array using Array#permutation:

def number_shuffle(number)
  number.to_s.chars.permutation.map { |x| x.join.to_i }.sort
end

Upvotes: 1

Matt
Matt

Reputation: 20766

Instead of Array#combination, you want Array#permutation:

number = 123
number.to_s.split('').permutation.map(&:join).uniq.sort
# => ["123", "132", "213", "231", "312", "321"]

number = 122
number.to_s.split('').permutation.map(&:join).uniq.sort
# => ["122", "212", "221"]

Upvotes: 2

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