Faster and More Efficient `squareform()`

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Royi Avital
Royi Avital il 2 Ott 2019
Modificato: Royi Avital il 15 Ott 2019
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.';
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?

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