extracting rows in a matrix

[Francesco Potorti`]

> it doesn't work:
>octave:16> extract_row = rand(nrows,1) < 0.2
>extract_row =
>
>   0
>   0
>   0
>   0
>   0
>
>octave:17> B = A(extract_row, :)
>B = [](0x4)

It does work.  You chose nrows=5, which in many cases gives you a choice
of zero rows, as in this particular case.  Try many times with nrows=5,
and look at the results.  Also, try with a bigger nrows.

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