[R] table over a matrix dimension...

[Jonathan Greenberg]

R-helpers:

I'm trying to determine the frequency of characters for a matrix
applied to a single dimension, and generate a matrix as an output.
I've come up with a solution, but it appears inelegant -- I was
wondering if there is an easier way to accomplish this task:

# Create a matrix of "factors" (characters):
random_characters=matrix(sample(letters[1:4],1000,replace=TRUE),100,10)

# Applying with the table() function doesn't work properly, because not all rows
# have ALL of the factors, so I get a list output:
apply(random_characters,1,table)

# Hacked solution:
unique_values = letters[1:4]

countsmatrix <- t(apply(random_characters,1,function(x,unique_values)
{
counts=vector(length=length(unique_values))
for(i in seq(unique_values))
{
counts[i] = sum(x==unique_values[i])
}
return(counts)
},
unique_values=unique_values
))

# Gets me the output I want but requires two nested loops (apply and
for() ), so
# not efficient for very large datasets.

###

Is there a more elegant solution to this?

--j

-- 
Jonathan A. Greenberg, PhD
Assistant Professor
Global Environmental Analysis and Remote Sensing (GEARS) Laboratory
Department of Geography and Geographic Information Science
University of Illinois at Urbana-Champaign
259 Computing Applications Building, MC-150
605 East Springfield Avenue
Champaign, IL  61820-6371
Phone: 217-300-1924
http://www.geog.illinois.edu/~jgrn/
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