Little trick: random positive definite matrix

Sometimes we want to test a model with some random data. When dealing with quadratic problems we may need to invent a positive definite matrix $Q$. One simple way to do this is as follows:

I.e. in matrix terms: $Q:=Q^TQ$ (i.e. the assignment above is not done one-by-one but rather in parallel). To be precise: $Q$ will be positive semi-definite, i.e. $x^TQx\ge 0$.

This simple device can help generating random data for convex quadratic models.