[R] Random number generation

Duncan Murdoch murdoch at stats.uwo.ca
Thu Jun 1 21:52:42 CEST 2006

On 6/1/2006 3:13 PM, Carl wrote:
> Hi All.
> (This is probably an incredibly stupid question :))
> I have just noticed that every time I start an R2.3 session and request 
> a random number (say with rnorm) that I receive the same number. AFAIK I 
> have not set a seed at any point. Returning to an older version of 
> R(v1.8) I get the behaviour that I have come to expect, where a new seed 
> is automatically generated for each new session (actually it was a 
> student running bootstraps on multiple machines that has noticed this).
> I am running R2.3 for windows on winXP pro, AMD Athlon 2400+.

R saves the random seed in a variable called .Random.seed.  (Because of 
the initial dot, this is not normally shown by ls().  Use ls(all=T) to 
see it.

If you save this to an .RData file, it will be reloaded next time you 
run, and your RNG will continue on from before.

I'm guessing you saved a copy once, but are *not* saving your workspace 
each time you restart:  hence you get the same seed each time.

You can avoid this by deleting the saved workspace, or just deleting 
.Random.seed and saving it (but it will be resurrected next time you do 
a save), or by always saving your workspace (so you save a different 
seed every time).

My advice would be not to save the workspace; I save individual objects 
sometimes, but I like a nice clean workspace each time I start.

Duncan Murdoch

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