Re: [R] difference between rnorm(1000, 0, 1) and running rnorm(500, 0, 1) twice

From: Duncan Murdoch <murdoch_at_stats.uwo.ca>
Date: Wed 08 Feb 2006 - 23:22:09 EST

On 2/8/2006 4:53 AM, Bjørn-Helge Mevik wrote:
> Why don't you test it yourself?
>
> E.g.,
>
> set.seed(42)
> bob1 <- rnorm(1000,0,1)
> set.seed(42)
> bob2 <- rnorm(500,0,1)
> bob3 <- rnorm(500,0,1)
> identical(bob1, c(bob2, bob3))
>
> I won't tell you the answer. :-)

This isn't really something that can be proved by a test. Perhaps the current implementation makes those equal only because 500 is even, or divisible by 5, or whatever...

I think the intention is that those should be equal, but in a quick search I've been unable to find a documented guarantee of that. So I would take a defensive stance and assume that there may be conditions where c(rnorm(m), rnorm(n)) is not equal to rnorm(m+n).

If someone can point out the document I missed, I'd appreciate it.

Duncan Murdoch



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