On Jun 10, 2009, at 7:05 PM, William Dunlap wrote:
Subscripting by a 2-column matrix
M[cbind(v, seq_len(ncol(M)))]
uses much less space (hence time) than making
the ncol(M) by ncol(M) intermediate matrix just
to extract its diagonal. E.g.
test <- function(n, seed) {
if (!missing(seed))
set.seed(seed)
M <- matrix(sample(LETTERS, 2*n, replace = TRUE), 2)
v <- sample(2, n, replace=T)
t1<-system.time(r1<-M[cbind(v,seq_len(ncol(M)))])
t2<-system.time(r2<-diag(M[v, 1:ncol(M)]))
list(identical=identical(r1,r2), "time(matrix subscript)"=t1,
"time(diag(big matrix))"=t2)
}
test(100)
$identical
[1] TRUE
$`time(matrix subscript)`
user system elapsed
0.000 0.000 0.001
$`time(diag(big matrix))`
user system elapsed
0.001 0.000 0.001
test(1000)
$identical
[1] TRUE
$`time(matrix subscript)`
user system elapsed
0.000 0.000 0.001
$`time(diag(big matrix))`
user system elapsed
0.082 0.021 0.103
test(5000)
$identical
[1] TRUE
$`time(matrix subscript)`
user system elapsed
0.001 0.000 0.001
$`time(diag(big matrix))`
user system elapsed
3.379 0.552 3.932
Nicely done comparison Bill.
Thanks,
Marc
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