[SOLVED] Easiest way to Swap Rows in Julia

Easiest way to Swap Rows in Julia

I want to swap two specific rows of a Matrix. I do not want to write a second function just for that and I am looking for the easiest/quickest way possible.

In Matlab or Octave in order to swap the k-th and r-th row of a Matrix A we could simply write:

A([k r],:) = A([r k],:)

When I try this on Julia (VsCode) I get this error: ERROR: LoadError: syntax: "[k r]" is not a valid function argument name around File Path

Is there another way? I do not want to have a function just to swap rows. Thanks in advance

Solution

In Julia, as in Numpy, you index into arrays using square brackets, unlike in Matlab where indexing uses regular parentheses.

So when you write A([k r],:), it is interpreted as a function call. Changing it to

A[[k r], :] = A[[r k], :]

works. But it is more idiomatic to index with vectors than with matrices, so add commas, like this:

A[[k, r], :] = A[[r, k], :]

You said 'easiest', but if you care at all for performance, then the following is much, much faster, and uses no memory:

function swaprows!(A, k, r)
    for i in axes(A, 2)
        (A[k, i], A[r, i]) = (A[r, i], A[k, i])
    end
end