R help - Mar 2008 - 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

Mary Black wrote:
> 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
> 
You didn't tell us which package has this 'rmultinomial' function. 
The
function 'rmultinom' sounds like it (part of the stats package), but has
no argument 'long'.

You can just use the 'sample' function to do this.

sample(0:2, 100, prob = c(.06,.38,.56), replace = TRUE)

Best,
Erik Iverson
> 
> 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
> 
> ______________________________________________ R-help at r-project.org
> mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do
> read the posting guide http://www.R-project.org/posting-guide.html 
> and provide commented, minimal, self-contained, reproducible code.

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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PLEASE do read the posting guide
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and provide commented, minimal, self-contained, reproducible code.