Transpose A 2d Array Matrix In Ruby
Flip a matrix over its diagonal — rows become columns and columns become rows. Ruby’s Array#transpose handles this in one call.
Description
Array#transpose swaps rows and columns of a rectangular 2D array. All inner arrays must have equal length, or IndexError is raised.
For non-rectangular (jagged) arrays, use zip with a splat: matrix.first.zip(*matrix[1..]).
Common uses: rotating tabular data, converting column-major to row-major layout, and preparing data for CSV output where rows and columns need swapping.
Sample input:
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
Sample Output:
[[1, 4, 7], [2, 5, 8], [3, 6, 9]]
Answer
matrix = [[1, 2, 3],
[4, 5, 6],
[7, 8, 9]]
# Built-in transpose
matrix.transpose
# => [[1, 4, 7], [2, 5, 8], [3, 6, 9]]
# Non-square matrix
[[1, 2, 3], [4, 5, 6]].transpose
# => [[1, 4], [2, 5], [3, 6]]
# Jagged array (unequal row lengths) — use zip
jagged = [[1, 2, 3], [4, 5], [6]]
jagged.first.zip(*jagged[1..])
# => [[1, 4, 6], [2, 5, nil], [3, nil, nil]]
# Rotate 90° clockwise — transpose then reverse each row
matrix.transpose.map(&:reverse)
# => [[7, 4, 1], [8, 5, 2], [9, 6, 3]]
# Rotate 90° counter-clockwise — reverse each row then transpose
matrix.map(&:reverse).transpose
# => [[3, 6, 9], [2, 5, 8], [1, 4, 7]]
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