Transpose

Problem Link

Problem

The transpose of a matrix is obtained by flipping a matrix over its diagonal, switching its row and column indices.

type Matrix = Transpose<[[1, 2, 3], [4, 5, 6], [7, 8, 9]]>
// [[1, 4, 7], [2, 5, 8], [3, 6, 9]]

Solution

type Transpose<M extends number[][]> =
  M extends [infer First extends number[], ...infer Rest extends number[][]]
    ? First extends [infer _, ...infer ___]
      ? {
          [K in keyof First]: [
            First[K & keyof First],
            ...Transpose<Rest>[K & keyof Transpose<Rest>]
          ]
        }[keyof First] extends infer R
        ? R extends any[]
          ? R[]
          : never
        : never
      : []
    : []

A cleaner approach using index-based column extraction:

type ColAt<M extends any[][], I extends number> = {
  [K in keyof M]: M[K][I]
}

type Transpose<M extends any[][]> =
  M extends []
    ? []
    : M[0] extends []
      ? []
      : {
          [I in keyof M[0]]: ColAt<M, I & number>
        }

How it works:

1. M[0] gives the first row — its indices are the column indices of the matrix.
2. For each column index I, build a tuple by collecting M[K][I] for every row K.
3. ColAt<M, I> uses a mapped type over keyof M (row indices) to gather the I-th element from every row.
4. Mapping over keyof M[0] produces one output row per original column.

Key Takeaways

• Transpose maps M[row][col] → M[col][row], so the output has M[0].length rows and M.length columns.
• Using keyof M[0] as the outer iterator elegantly handles rectangular matrices.
• ColAt is a reusable "slice a column" helper that collects the same index across all rows.