Re: NxN random matrix much cheaper than N^2 random scalars

[Andreas Weber]

Hi Daniel,

Am 17.11.2014 um 16:55 schrieb Daniel Molina García:
> I am using GNU Octave, version 3.8.1. I found that generating a matrix,
> for example as
> 
> for i=1:1
>   normrnd(0, 1, [n, 1]);
> endfor
> 
> is much fast than
> 
> for i=1:n
>   normrnd(0, 1);
> endfor
> 
> Why?

Because for loops are slow in octave. JIT would help in the future but
it's still experimental.

In the first case the for-loop is executed only once, therefore you get
only a small runtime penalty. Try to avoid large (high number of runs)
loops whenever possible.

See also this msg:
http://octave.1599824.n4.nabble.com/slow-for-loops-td1609594.html

> By the way, I found that specifying the 2 dimensions with a vector (as
> in the above example) instead of using two parameters is quicker.

How have you exactly measured this?
-- Andy