multicool (version 0.1-11)

# multinom: Calculate multinomial coefficients

## Description

This function calculates the number of permutations of a multiset, this being the multinomial coefficient. If a set $$X$$ contains $$k$$ unique elements $$x_1, x_2, \ldots, x_k$$ with associate counts (or multiplicities) of $$n_1, n_2, \ldots, n_k$$, then this function returns $$\frac{n!}{n_1!n_2!\ldots n_k!}$$ where $$n = \sum_{i=1}{k}n_i$$.

## Usage

multinom(x, counts = FALSE, useDouble = FALSE)

## Arguments

x

Either a multiset (with one or more potentially non-unique elements), or if counts is TRUE a set of counts of the unique elements of $$X$$. If counts is FALSE and x is not numeric, then x will be coerced into an integer vector internally. If counts is TRUE then x must be a vector of integers that are greater than, or equal to zero.

counts

if counts is TRUE, then this means x is the set of counts $$n_1, n_2, \ldots, n_k$$ rather than the set itself

useDouble

if useDouble is TRUE then the computation will be done using double precision floating point arithmetic. This option was added because the internal code cannot handle integer overflow. The double precision code will may a result that is closer to the truth for large values, but this is not guaranteed. Ideally something like the GMP library should be used, but this is not a priority at this point in time.

## Value

A single integer representing the multinomial coefficient for the given multiset, or given set of multiplicities.

## Details

multinom depends on C++ code written by Dave Barber which can be found at http://tamivox.org/dave/multinomial/code.html. The code may require the STL algorithm library to be included in order to compile it.

## References

http://tamivox.org/dave/multinomial/code.html

## Examples

# NOT RUN {
## An example with a multiset X = (a,a,a,b,b,c)
## There are 3 a s, 2 b s and 1 c, so the answer should be
## (3+2+1)!/(3!2!1!) = 6!/3!2!1! = 60
x = rep(letters[1:3],3:1)
multinom(x)

## in this example x is a vector of counts
## the answer should be the same as above as x = c(3,2,1)
x = rep(letters[1:3],3:1)
x = as.vector(table(x)) #coerce x into a vector of counts
multinom(x, counts = TRUE)

## An example of integer overflow. x is a vector of counts
## c(12,11,8,8,6,5). The true answer from Maple is
## 11,324,718,121,789,252,764,532,876,767,840,000
## The error in the integer based answer is obvious.
## The error using floating point is not, but from Maple is
## 0.705057123232160000e+10
## Thanks to Lev Dashevskiy for calling my attention to this.
# }
# NOT RUN {
x = c(12,11,8,8,6,5)
multinom(x, counts = TRUE, useDouble = FALSE)
multinom(x, counts = TRUE, useDouble = TRUE)
# }
# NOT RUN {
# }