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copied!<p>In my experience, when working with arrays (2D and/or matrix) I found that working with arrays internally is often the fastest way to go.</p> <p>For example, combining Daniel's and Ankur's approaches in a mutable way:</p> <pre><code>let mutableSortByCol f (m:Matrix<'T>) = let columns = [| for c in 0 .. m.NumCols - 1 -> m.Column c |> Vector.Generic.toArray |] for c in 0 .. m.NumCols - 1 do columns.[c] |> Array.sortInPlaceBy f Matrix.Generic.init (m.NumRows) (m.NumCols) (fun r c -> columns.[c].[r]) </code></pre> <p>I converted the matrix to an array of columns ('a[][], not 'a[,]), and performed an in-place sort on each column. After that, I filled a new matrix with the sorted result. Note that the original matrix remains unmodified: the columns array is populated with copies of the column vectors (Vector.toArray creates a new array).</p> <p>This approach is faster because it needs no transposes, sorts columns in place, and needs no conversion to and from intermediate list structures by keeping everything array-oriented. I suspect it could be made even faster if the Matrix module supported conversion to/from 'a[][] as well, although it's perhaps not really suited for matrices.</p> <p>Also, in case you didn't know: you can make use of F#'s structural comparison of tuples to sort by second element descending, first element ascending:</p> <p>Example:</p> <pre><code>> mutableSortByCol (fun (a,b) -> (-b,a)) M;; val it : Matrix<float * float> = matrix [[(1.0, 145.0); (1.0, 45.0); (3.0, 145.0); (3.0, 45.0); (5.0, 145.0)] [(3.0, 130.0); (2.0, 45.0); (1.0, 130.0); (4.0, 45.0); (1.0, 130.0)] [(5.0, 130.0); (3.0, 30.0); (5.0, 130.0); (1.0, 30.0); (3.0, 130.0)] [(2.0, 45.0); (4.0, 30.0); (4.0, 45.0); (2.0, 30.0); (2.0, 30.0)] [(4.0, 30.0); (5.0, 30.0); (2.0, 30.0); (5.0, 30.0); (4.0, 30.0)]] </code></pre>

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