[R] uniform integer RNG 0 to t inclusive

[Sean O'Riordain]

Good morning,

I'm trying to concisely generate a single integer from 0 to n
inclusive, where n might be of the order of hundreds of millions.
This will however be used many times during the general procedure, so
it must be "reasonably efficient" in both memory and time... (at some
later stage in the development I hope to go vectorized)

The examples I've found through searching RSiteSearch() relating to
generating random integers say to use : sample(0:n, 1)
However, when n is "large" this first generates a large sequence 0:n
before taking a sample of one... this computer doesn't have the memory
for that!

When I look at the documentation for runif(n, min, max) it states that
the generated numbers will be min <= x <= max.  Note the "<= max"...

How do I generate an x such that the probability of being (the
integer) max is the same as any other integer from min (an integer) to
max-1 (an integer) inclusive... My attempt is:

urand.int <- function(n,t) {
  as.integer(runif(n,min=0, max=t+1-.Machine$double.eps))
}
# where I've included the parameter n to help testing...

is floor() "better" than as.integer?

Is this correct?  Is the probability of the integer t the same as the
integer 1 or 0 etc... I have done some rudimentary testing and this
appears to work, but power being what it is, I can't see how to
realistically test this hypothesis.

Or is there a a better way of doing this?

I'm trying to implement an algorithm which samples into an array,
hence the need for an integer - and yes I know about sample() thanks!
:-)

{ incidentally, I was surprised to note that the maximum value
returned by summary(integer_vector) is "pretty" and appears to be
rounded up to a "nice round number", and is not necessarily the same
as max(integer_vector) where the value is large, i.e. of the order of
say 50 million }

Is version etc relevant? (I'll want to be portable)
> version               _
platform       i386-pc-mingw32
arch           i386
os             mingw32
system         i386, mingw32
status
major          2
minor          3.1
year           2006
month          06
day            01
svn rev        38247
language       R
version.string Version 2.3.1 (2006-06-01)
>

Many thanks in advance for your help.
Sean O'Riordain
affiliation <- NULL