pushing array to array in ruby have a wrong result

Question

I have problem when pushing array to array, the result is not as I expected.

I run this code below :

 @arr = ["e", "s", "l", "e"]

    def permutations(array, i=0)
      combined_array = Array.new
      (i..array.size-1).each do |j|
        array[i], array[j] = array[j], array[i]
        puts "ARRAY : #{array}"
        combined_array << array.to_a
      end
      return combined_array
    end

permutations(@arr)

I got the output :

ARRAY : ["e", "s", "l", "e"]
ARRAY : ["s", "e", "l", "e"]
ARRAY : ["l", "e", "s", "e"]
ARRAY : ["e", "e", "s", "l"]
=> [["e", "e", "s", "l"], ["e", "e", "s", "l"], ["e", "e", "s", "l"], ["e", "e", "s", "l"]]

Result expected :

ARRAY : ["e", "s", "l", "e"]
ARRAY : ["s", "e", "l", "e"]
ARRAY : ["l", "e", "s", "e"]
ARRAY : ["e", "e", "s", "l"]
=> [["e", "s", "l", "e"], ["s", "e", "l", "e"], ["l", "e", "s", "e"], ["e", "e", "s", "l"]]

How to solve this problem ?

Do you know there is a predefined method permutation in Array class? — Rajagopalan
– Rajagopalan, Commented Apr 19, 2018 at 6:50
You don't have to write separate method for this. You simply write p arr.permutation.map{|s|s} that would do the job for you. — Rajagopalan
– Rajagopalan, Commented Apr 19, 2018 at 6:53
Hello @Rajagopalan, i knew that method, but for the big array, it will out of memory — robbywh
– robbywh, Commented Apr 19, 2018 at 6:56

Cristian Lupascu · Accepted Answer · 2018-04-19 06:51:54Z

2

According to the documentation, #to_a called on an Array returns self (the array itself, not a copy).

You are adding the same array to combined_array multiple times.

Change the .to_a to .dup and it will work fine.

answered Apr 19, 2018 at 6:51

Cristian Lupascu

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Comments

Rajagopalan · Accepted Answer · 2018-04-19 07:17:30Z

1

I think @GolfWolf has solved your problem.

But you don't have to write such a function to solve your problem in Ruby, Ruby has permutation method which you can use it.

p arr.permutation.to_a

If you want to get first 4 element then you can do this,

p arr.permutation.take(4)

edited Apr 19, 2018 at 7:17

answered Apr 19, 2018 at 6:59

Rajagopalan

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2 Comments

mu is too short Over a year ago

You'd probably be better off saying arr.permutation.to_a instead of the map{|s|s}. And the second one doesn't need any arrayification, just arr.permutation.take(4) would be sufficient.

Rajagopalan Over a year ago

@muistooshort Ah Okay! Thank you, updated the answer!

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