# [R] set seed for random draws

Patrick Burns pburns at pburns.seanet.com
Sat Nov 5 10:00:47 CET 2011

```I'm suspecting this is confusion about
default behavior.

R automatically updates the random seed
when random numbers are generated (or
other random operations are performed).

The original poster may have experienced
systems where it is up to the user to
change the seed.

I'd suggest two rules of thumb when coming
up against something in R that you aren't

1. If it is a mundane task, R probably
takes care of it.

2. Experiment to see what happens.

Of course you could read documentation, but
no one does that.

On 05/11/2011 00:59, R. Michael Weylandt wrote:
> This might be more fundamental, but why do you feel the need to reset
> the seed each loop? There's nothing that suggests you need to...
>
> Michael
>
> On Fri, Nov 4, 2011 at 8:38 PM, Md Desa, Zairul Nor Deana Binti
> <zndeana at ku.edu>  wrote:
>> Hello, all!
>> I need help on these two problems:
>>
>> 1) If I want to randomly draw numbers from standard normal (or other distributions) in loops e.g.:
>>   ty=0; ks=0
>> for (i in 1:5) {
>>         set.seed(14537+i)
>>         k<-rnorm(1)
>>         ks[i]<-.3*k+.9
>>         if (ty==0) {
>>             while ((ks<.2)||(ks>3)) {
>>             #set.seed(13237+i*100)
>>             k<-rnorm(1)
>>             ks[i]-.3*k+.9 }
>>         }
>>      }
>> ....
>> ....
>> ....
>>      }
>>
>> Question: Here I draw initial a, then if the drawn initial a satisfied 2 conditions I redraw a. I set.seed(13237) in the first draw of a, should I set.seed() in the redraw part?
>>
>> 2) I also have more loops after this i loop that also draw from normal(0,1). I want to randomly draws from normal(0,1) for loop j (inside loop j I draw another random numbers from N(0,1))
>> My question: Should I or shouldn't I set seed again and again for each loop? Why or why not.
>>
>> I guess this problem concerned about setting seed as I want to have different number for each i.
>>
>> Thanks!
>>
>> Deana
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> and provide commented, minimal, self-contained, reproducible code.
>

--
Patrick Burns
pburns at pburns.seanet.com