R help - Mar 2007 - Is there a quick way to count the number of times each element in a vector appears?

Hi there,


I'm writing a function that calculates the probability of different
outcomes of dice rolls (e.g., the sum of the highest three rolls of
five six-sided dice).  I'm using the "combinations" function from
the
"gtools" package, which is great: it gives me a matrix with all of the
possible combinations (with repetitions allowed).  Now I want to count
the number of times each element appears in each arrangement so I can
calculate the number of permutations of that arrangement.  E.g., if I
get output like:
> combinations(3,3, rep=TRUE)
      [,1] [,2] [,3]
 [1,]    1    1    1
 [2,]    1    1    2
 [3,]    1    1    3
 [4,]    1    2    2
 [5,]    1    2    3
 [6,]    1    3    3
 [7,]    2    2    2
 [8,]    2    2    3
 [9,]    2    3    3
[10,]    3    3    3

I'd like to be able to determine that the first row has 3 repetitions,
yielding 3!/3! = 1 permutation, while the second row has 3
repetitions, yielding 3!/2! = 3 permutations, etc.  (This gets harder
when there are large numbers of dice with many faces.)

I know there are simple things to do, like iterating over the rows
with for loops, but I've heard that for loops are sub-optimal in R,
and I'd like to see what an elegant solution would look like.

E.g., I might like to use sapply() with whatever function I come up
with; I thought of using something like duplicated() and just counting
the number of TRUEs that are returned for each vector (since the
elements are always returned in non-decreasing order), but I'm
optimistic that there is a better (faster/cleaner) way.

So here is my question in a nutshell:
Does anyone have ideas for how I might efficiently process a matrix
like that returned by a call to combinations(n, r, rep=TRUE) to
determine the number of repetitions of each element in each row of the
matrix?  If so, I'd love to hear them!


Thanks very much for your time,
Dylan Arena
(Statistics M.S. student)

is this what you mean?

tmp <- combinations(3, 3, rep=TRUE)
colSums(apply(tmp, 1, duplicated))+1

b

On Mar 6, 2007, at 1:16 AM, Dylan Arena wrote:
> Hi there,
>
>
> I'm writing a function that calculates the probability of different
> outcomes of dice rolls (e.g., the sum of the highest three rolls of
> five six-sided dice).  I'm using the "combinations" function
from the
> "gtools" package, which is great: it gives me a matrix with all
of the
> possible combinations (with repetitions allowed).  Now I want to count
> the number of times each element appears in each arrangement so I can
> calculate the number of permutations of that arrangement.  E.g., if I
> get output like:
>
>> combinations(3,3, rep=TRUE)
>       [,1] [,2] [,3]
>  [1,]    1    1    1
>  [2,]    1    1    2
>  [3,]    1    1    3
>  [4,]    1    2    2
>  [5,]    1    2    3
>  [6,]    1    3    3
>  [7,]    2    2    2
>  [8,]    2    2    3
>  [9,]    2    3    3
> [10,]    3    3    3
>
> I'd like to be able to determine that the first row has 3 repetitions,
> yielding 3!/3! = 1 permutation, while the second row has 3
> repetitions, yielding 3!/2! = 3 permutations, etc.  (This gets harder
> when there are large numbers of dice with many faces.)
>
> I know there are simple things to do, like iterating over the rows
> with for loops, but I've heard that for loops are sub-optimal in R,
> and I'd like to see what an elegant solution would look like.
>
> E.g., I might like to use sapply() with whatever function I come up
> with; I thought of using something like duplicated() and just counting
> the number of TRUEs that are returned for each vector (since the
> elements are always returned in non-decreasing order), but I'm
> optimistic that there is a better (faster/cleaner) way.
>
> So here is my question in a nutshell:
> Does anyone have ideas for how I might efficiently process a matrix
> like that returned by a call to combinations(n, r, rep=TRUE) to
> determine the number of repetitions of each element in each row of the
> matrix?  If so, I'd love to hear them!
>
>
> Thanks very much for your time,
> Dylan Arena
> (Statistics M.S. student)
>
> ______________________________________________
> R-help at stat.math.ethz.ch 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.

Dylan Arena wrote:
> 
> I'm writing a function that calculates the probability of different
> outcomes of dice rolls (e.g., the sum of the highest three rolls of
> five six-sided dice).
>
You know there are simpler ways to do this, don't you?

Alberto Monteiro

El lun, 05-03-2007 a las 22:16 -0800, Dylan Arena escribi?:
> So here is my question in a nutshell:
> Does anyone have ideas for how I might efficiently process a matrix
> like that returned by a call to combinations(n, r, rep=TRUE) to
> determine the number of repetitions of each element in each row of the
> matrix?  If so, I'd love to hear them!
> 
> 
here is an answer in a nutshell:

my.table <- combinations(3,3,rep=TRUE)

## one possibility is 
apply(my.table,1,table)

## or better, in plain 
table(my.table,row(my.table))

look at the help pages of 
?table
?apply
?row
> Thanks very much for your time,
> Dylan Arena
> (Statistics M.S. student)
> 
that took probably one minute of my time... so never mind


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