Off diagonal elements of Matrix

General Usage
8

Thanks for all the answers so far, @L_Adam, @Tamas_Papp! In the initial post I ordered the matrix entries rowwise, but everybody correctly assumed columnwise ordering :smiley:

I came up with a quick and dirty way for 1-based indexing and quadratic matrices, which does not need an if-statement:

function offdiag(A::Matrix)
    @assert size(A)[1] == size(A)[2]
    D = size(A)[1]
    v = zeros(D*(D-1))
    for i in 1:D
        for j in 1:(i-1)
            v[(i-1)*(D-1)+j] = A[j,i]
        end
        for j in (i+1):D
            v[(i-1)*(D-1)+j-1] = A[j,i]
        end
    end
    v
end

I know from direct SIMD programming that conditions in your code can decrease performance gains from SIMD by a factor (2?). Would this apply here as well? Or is Julia smart enough or not using SIMD at all?

PS: In my code example, one has off course disable bounds checking to make automatic SIMD available.