[R] Efficient way to fill a matrix

[Philipp Pagel]

	Dear R experts,

Suppose I have a data frame of three variables:

> foo <- data.frame(row=1:5, col=1:3, val=rnorm(15))
> foo
   row col         val
1    1   1 -1.00631642
2    2   2  0.77715344
3    3   3  0.17358793
4    4   1 -1.67226988
5    5   2  1.08218836
6    1   3  1.32961329
7    2   1 -0.51186267
8    3   2 -1.20990127
9    4   3 -0.57786899
10   5   1  0.67102887
11   1   2  0.05646411
12   2   3  0.01146612
13   3   1 -3.12094409
14   4   2 -1.01932191
15   5   3  0.76736702


I want to turn this into a matrix of val according to row and col. Let's also
assume that some combinations of row and col are missing - i.e. there will be
NAs in the resulting Matrix. My current approach is simple and works but is
slow for large datasets:

mat <- matrix(nrow=max(foo$row), ncol=max(foo$col))
for (line in 1:dim(foo)[1]) {
	mat[foo[line, 'row'], foo[line, 'col']] <- foo[line, 'val']
}

> mat
           [,1]        [,2]        [,3]
[1,] -1.0063164  0.05646411  1.32961329
[2,] -0.5118627  0.77715344  0.01146612
[3,] -3.1209441 -1.20990127  0.17358793
[4,] -1.6722699 -1.01932191 -0.57786899
[5,]  0.6710289  1.08218836  0.76736702


Can anyone think of a more efficient way?

cu
	Philipp

-- 
Dr. Philipp Pagel
Lehrstuhl für Genomorientierte Bioinformatik
Technische Universität München
Wissenschaftszentrum Weihenstephan
85350 Freising, Germany
http://mips.gsf.de/staff/pagel