CODEWITHSUNDEEP
arraysruby
Palma25
Palma25

Reputation: 13

Pushing an array into another array overwrites all the elements in the second array

I'm writing a method that modifies an array in a temporary variable and then pushes that variable into a multidimensional array. But when I push the temp variable into the multidimensional array, all the contents get overwritten with the value of temp. This is the method:

def possible_moves(start, moves = [])
  return if start.any?(nil)

  temp = []

  # add 2 to first, add 1 to last

  temp << start.first + 2  
  temp << start.last + 1

  moves << temp

  # add 2 to first, subtract 1 to last

  temp.pop 
  temp << start.last - 1

  moves << temp

  moves
end

If I run the method with start = [3, 4] then the result is

moves = [[5, 3], [5, 3]]

when the expected result should be

moves = [[5, 5], [5, 3]]

Also, using Array#push instead of Array#<< gives the same result.

I'm very new to programming and am having a hard time figuring out what's happening and how to solve it, so any help is very much appreciated. I'm working with Ruby 2.7.0.

Upvotes: 1

Views: 266

Answers (1)

Qaisar Nadeem
Qaisar Nadeem

Reputation: 2434

def possible_moves(start, moves = [])
  return if start.any?(nil)

  temp = []

  # add 2 to first, add 1 to last

  temp << start.first + 2  
  temp << start.last + 1

  moves << [*temp]
  #or moves << temp.dup

  # add 2 to first, subtract 1 to last

  temp.pop 
  temp << start.last - 1

  moves << [*temp]
  #or moves << temp.dup

  moves
end

The reason of above behaviour is due to the fact that when you push one array into another then pushed array is not copied , instead it's reference is copied so when you change the source array. Its value in other array changes. Here is a simple illustration to show that

ar1 = [1,2,3]
ar2 = [ar1,[5,6]] # [[1, 2, 3], [5, 6]]
ar1[0] = 99

now check ar2 , it would be [[99, 2, 3], [5, 6]]

You can verify it by checking object_id of any ruby object

ar1.object_id => somenumber

ar2[0].object_id => samenumberagain

Upvotes: 1

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