set seed for random draws

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

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

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

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.

Thank you everybody for the helpful advices.
Basically, I try to figure out why I get different numbers as there are more than one seed for a loop within a loop. Well, I guest I got it now. Because every time random seed is called or specified it'll output different random numbers, as it's requested.

Thanks!

D

I think it's easier than you are making it: the random seed is created
in a "pretty-random" way when you first use it and then it is updated

with each call to rDIST().

For example,

set.seed(1)
x1 <- .Random.seed
rnorm(1)
x2 <- .Random.seed
rnorm(1)
x3 <- .Random.seed

identical(x1, x2)
FALSE

identical(x1, x3)
FALSE

identical(x2, x3)
FALSE

set.seed(1)
identical(x1, .Random.seed)
TRUE

rnorm(2)
identical(x3, .Random.seed)
TRUE

But the period for the random seed to repeat is very, very long so you
don't have to think about it unless you really need to (or for
reproducible simulations)

Michael

On Sat, Nov 5, 2011 at 7:22 PM, Md Desa, Zairul Nor Deana Binti
<zndeana at ku.edu> wrote:

Thank you everybody for the helpful advices.
Basically, I try to figure out why I get different numbers as there are more than one seed for a loop within a loop. Well, I guest I got it now. Because every time random seed is called or specified it'll output different random numbers, as it's requested.

Thanks!

D