Two-dimensional array, how to create column array that modifies original array

Question

I have a two-dimensional array, and I want to be able to use another array which refers to the column of the original one. I can do it with rows, but I can't figure out how to do it with columns.

twodim = [[" "," "," "], [" "," "," "], [" "," "," "]]
row1 = twodim[0]
row1[0] = "X"

col1 = << twodim[0][0] << twodim[1][0] << twodim[2][0]
col1[2] = "X"

=> puts twodim
#actual result
=> [["X", " ", " "], [" ", " ", " "], [" ", " ", " "]]
#desired result
=> [["X", " ", " "], [" ", " ", " "], ["X", " ", " "]]

I've tried using map! but that ends up causing the original twodim array to change when I just try to assign the columns to variables. I've also tried transpose but that returns a new array. Is there a way to create an array using the object id's from the original array in order to be able to modify it? This is for a tictactoe program, and I'm writing the ai for it. I'm trying to check if there's a way to win the game in a given row or column, then have it fill in the winning spot if it exists. Here's the rest of it if it helps:

    rows = [row1, row2, row3]
    cols = [col1, col2, col3]
    rows.map do |row|
        if !row.include?(@player) && row.count(@ai) == 2 then
            row.fill(@ai)
            return
        end
    end
    cols.each do |col|
        if !col.include?(@player) && col.count(@ai) == 2 then
            col.fill(@ai)
            puts "col = #{col}"
            return
        end
    end

Community · Accepted Answer · 2017-05-23 10:33:29Z

Use String#replace

twodim = [[" "," "," "], [" "," "," "], [" "," "," "]]
row1 = twodim[0]
row1[0] = "X"
col1 = [twodim[0][0], twodim[1][0], twodim[2][0]]

col1[2].replace("X")

twodim
# => [["X", " ", " "], [" ", " ", " "], ["X", " ", " "]]

Explanation:

In your example, when doing col1[2] = "X" you are assigning to the object col1[2] a new variable with the value "X", that means that col1[2] is now storing the reference of the new object "X". However, when doing col1[2].replace("X") the method replace is changing the content of the object that is referenced by col12.

The following example will clarify the above explanation:

$> col1[1].object_id
# => 2161281980 
$> col1[1] = "X"
# => "X" 
$> col1[1].object_id # Different object_id !!!
# => 2161469100
$> col1[1].replace("X")
# => "X" 
$> col1[1].object_id
# => 2161469100 # Same object_id !!!

I also recommend you to read this post: Is Ruby pass by reference or by value?

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