Faster and More Efficient `squareform()`

I'm using squareform(pdist(mX)) fairly often.

The problem is squareform() is so slow it makes use of pdist2(mX, mX) faster.

This is basically what squareform() does in the case above:

function [ mY ] = MySquareForm( vX )


numElements = numel(vX);

numDist = (1 + sqrt(1 + (8 * numElements))) / 2;

mY = zeros(numDist, numDist, class(vX));
mY(tril(true(numDist, numDist), -1)) = vX;
mY = mY + mY.';


end

Basically, imagine you have a symmetric matrix mX then the vector vx above is it lower tringular matrix vectorized.

Something like:

mX = randn(100, 100);
mX = mX + mX.';

mL = tril(mX, -1);
vX = mL(mL ~= 0);

If the diagonal of is zerod then one could reproduce mX from vX using MySquareForm().

Any ideas how to do the same more efficiently and faster?

[Royi Avital]

Anyone?