Re: [julia-users] Swapping two columns (or rows) of an array efficiently

[Steven G. Johnson]

On Thursday, March 12, 2015 at 10:08:47 AM UTC-4, Ján Dolinský wrote:
>
> Hi,
>
> Is this an efficient way to swap two columns of a matrix ?
> e.g. 1st column with the 5th
>
> X = rand(10,5)
> X[:,1], X[:,5] = X[:,5], X[:,1]
>

It is not optimal, because it allocates temporary arrays.  Instead, you can 
just write:

function swapcols(X, i, j)
     for k = 1:size(X,1)
           X[k,i],X[k,j] = X[k,j],X[k,i]
     end
     return X
end

which allocates no temporary arrays and should be quite efficient.   You 
could gain a bit more efficiency by moving all of the bounds checks outside 
of the loop:

function swapcols(X, i, j)
     m, n = size(X)
     if (1 <= i <= n) && (1 <= j <= n)
         for k = 1:m
               @inbounds X[k,i],X[k,j] = X[k,j],X[k,i]
         end
         return X
     else
         throw(BoundsError())
     end
end

but these kinds of micro-optimizations are rarely worthwhile.