Swapping either Columns or Rows in a Ruby Matrix

Question

I want to swap either rows or columns in an array of arrays (i.e. a matrix). I have found this swap method online, and I extended it with my mutate function, where the board.matrix is the array of arrays:

# Swap to elements of an Array
def swap!(a,b)
  self[a], self[b] = self[b], self[a]
  self
end

def mutate(board)
  matrix = board.matrix
  random = rand
  rand_a = rand(matrix.length-1)
  rand_b = rand(matrix.length-1)

  puts "Old one:"
  board.print_matrix
  # We have a 50:50 chance of swapping either columns or rows
  if random <= 0.5
    # Swap columns: Transpose first
    puts "Swapping columns #{rand_a} and #{rand_b}..."
    temp = matrix.transpose
    temp.swap!(rand_a, rand_b)
    matrix = temp.transpose
  else
    # Just swap rows
    puts "Swapping rows #{rand_a} and #{rand_b}..."
    matrix.swap!(rand_a, rand_b)
  end
  puts "New one:"
  board.print_matrix
end

Now it does what it should for rows:

Old one:
X  X  0  0  
0  0  0  0  
X  X  0  0  
0  0  0  0  

Swapping rows 1 and 0...
New one:
0  0  0  0  
X  X  0  0  
X  X  0  0  
0  0  0  0

But it doesn't for columns. Why is that?

Old one:
0  X  X  0  
0  0  X  0  
X  0  0  0  
0  0  0  0  

Swapping columns 1 and 0...
New one:
0  X  X  0  
0  0  X  0  
X  0  0  0  
0  0  0  0

Arnaud Le Blanc · Accepted Answer · 2011-01-30 20:32:12Z

2

Because in the following code you did not transformed the Array matrix is pointing to, you just have assigned a new Array to matrix, which is a local variable:

temp = matrix.transpose
temp.swap!(rand_a, rand_b)
matrix = temp.transpose

You can actually replace this code with this:

matrix.each do |row|
    row[rand_a], row[rand_b] = row[rand_b], row[rand_a]
end

answered Jan 30, 2011 at 20:32

Arnaud Le Blanc

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DigitalRoss · Accepted Answer · 2011-01-30 20:35:49Z

2

The problem is that you only set the local variable matrix to the new object created by #transpose.

Then you printed the original matrix object which is apparently board.matrix.

Although you also used the local variable to swap the rows, in that case the local was a reference to the original object, so they were the same, and it "worked" from the point of view of your output routine. But with the columns a new object was returned by transpose and board was never updated.

answered Jan 30, 2011 at 20:35

DigitalRoss

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