ClockRandSeed

Psychtoolbox › PsychProbability

[seed,whichGen] = ClockRandSeed([seed],[whichGen])

ClockRandSeed seeds the random number from the time-of-day clock. If seed
is passed this is used instead.

The multiplier 1e6 in fix(1e6*sum(clock)) is bigger than the 100
suggested by Mathworks (see help RAND). They suggest 100*sum(clock), but
this would only change the seed once every 1/100 sec, which might
correspond to many iterations of a loop. With the bigger multiplier it's
pretty sure (depending only on clock grain) that two successive calls
will generate different seeds.

Also unlike the Mathworks suggestion (see help RAND), we call FIX. This
has no effect on RAND and RANDN, but makes it easier to correctly print
the seed to a file. When a random seed generates an interesting result
one would like to be able to recreate the conditions that generated the
run. If the seed 1e6*sum(clock) is printed to a file, '%.0f' will often
round it differently than RAND would.

Note in that in Matlab 5 there are two random number generators, the
Matlab 5 generator and the Matlab 4 generator. (Matlab 4
has only the version 4 generator.) ClockRandSeed checks what version of
Matlab you're running and uses the version 5 generator if it can. The
string whichGen contains 'seed' for the version 4 generator and 'state'
for the version 5 generator.

See also: rand, randn.