[R] [PS] rmultinomial() function

[Ben Fairbank]

Mary --

The dmultinomial function (try ?Multinomial, noting that it is an upper
case M) has a "log" option, which, if set to TRUE, returns logarithms of
probabilities, but that is for computing probabilities, not generating
samples.  Perhaps the "long" you referred to is a misprint for "log?"

In any case, try ?Multinomial, and give rmultinom() another try.  Note,
however, that its output gives the _number_ of each of the sampled items
produced in a single sample, not the sequence of draws.  If you need the
sequence, then I think that the reply from Erik Iverson tells you how
best to proceed.

Ben 

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Mary Black
Sent: Tuesday, March 18, 2008 5:29 PM
To: r-help at r-project.org
Subject: [PS] [R] rmultinomial() function

After scouring the online R resources and help pages, I still need
clarification on the function rmultinomial().  I would like to create a
vector, say of 100 elements, where every element in the vector can take
on the value of 0, 1 or 2, and where each of those values have a
specific probability. ie. the probability a given element in the vector
= 0 is 0.06, 1 = 0.38, 2 = 0.56 (probabilities sum to 1). Can I use
rmultinomial() function to do this?  

The following code does not seem to produce the result I need, but this
sort of code is all I could find the R "help" pages:
 
> rmultinomial(100,c(0.06,0.38,0.56))
[1]  3 29 68
> rmultinomial(100,c(0.06,0.38,0.56),long=TRUE)
  [1] 3 3 2 2 3 2 3 3 2 3 2 3 3 3 2 2 3 3 2 3 1 2 3 3 3 3 3 3 2 3 3 2 3
3 3 2 3 2 2 3 2 3 2 3 3 3 2 1 3 3 1
 [52] 2 3 2 2 3 3 2 2 2 1 3 3 2 3 3 3 3 2 3 3 3 3 2 3 2 3 3 2 3 3 2 3 3
2 3 2 3 3 2 3 3 3 2 3 2 2 2 2 2

Also, I don't really understand the difference between the default
long=FALSE and long=TRUE.  The R "help" simply states that you use "long
TRUE to choose one generator, FALSE to choose another one"; however I
could not find any documentation that described what the difference
between those generators is.  Any clarification would be greatly
appreciated! 

Thanks,

Mary

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