set.seed ( ) function

BTW, Ken Kleinman recently wrote a post on how to get a "real" random
numbers (into R) from a web-service:
http://www.r-bloggers.com/example-8-35-grab-true-not-pseudo-random-numbers-passing-api-urls-to-functions-or-macros/

<http://www.r-bloggers.com/example-8-35-grab-true-not-pseudo-random-numbers-passing-api-urls-to-functions-or-macros/>
Cheers,
Tal

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On Fri, Apr 22, 2011 at 6:47 AM, Joshua Wiley<jwiley.psych at gmail.com>wrote:

On Thu, Apr 21, 2011 at 8:34 PM, Penny Bilton<pennybilton at xnet.co.nz>
wrote:

Hi Josh,

Thanks for your reply.

The problem is have is in trying to retain the proportions of 2 groups in

my

data while sampling into training and test sets. I find that different
  arguments for set.seed give very different proportions of my 2 groups in
the training and test sets.

Sure, just because numbers are random does not guarantee that equal
numbers from both groups will be sampled.  Perhaps you are looking for
some sort of constrained random sampling like sampling x from group 1
and x from group 2?  If so, try calling sample() separately on each
group (for help applying the same function to different groups, take a
look at ?by or ?tapply for example).

Josh

PS cced back to list

Penny.



On 22/04/2011 3:27 p.m., Joshua Wiley wrote:

Hi,

On Thu, Apr 21, 2011 at 8:18 PM, Penny Bilton<pennybilton at xnet.co.nz>
  wrote:

I am using /set.seed()/   before the /sample/   function.

How does the length of the argument of /set.seed()/   and order of the
digits affect how the sampling is carried out?

You can use set.seed() to specify a particular seed so that while
pseudo-random numbers are sampled, you can repeat it.  For example:

set.seed(10)
rnorm(10)
set.seed(10)
rnorm(10)

Specifically, I have used set.seed(123456789). Will this configuration
give me a genuinely random sampling??

You will never get truly random sampling from a computer algorithm,
but it is darn close and more than adequate in the majority of cases.
123456789 is just a length 1 vector containing the number 123456789,
not 9 separate numbers.

Google will be able to give you a lot of information on pseudo-random
number algorithms as well as the concept of "seeds".  Also see
?set.seed

Cheers,

Josh

Thank you in anticipation.

Penny.


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--
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/

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