m <- matrix(round(rnorm(4000 * 2000), 4), nr = 4000)
is.na(m) <- sample(8e6, 1e6)
system.time(
idx <- which(
matrix(m %in% head(sort(m, TRUE), 50),
nr = nrow(m)), arr.ind = TRUE))
# user system elapsed
# 3.12 0.19 3.18
-Peter Ehlers
On 2010-06-18 5:13, Dennis Murphy wrote:
Hi:
Here's a faked up example:
a<- matrix(rnorm(4000*2000), 4000, 2000)
# Generate some NAs in the matrix
nr<- sample(50, 1:4000)
nc<- sample(50, 1:2000)
a[nr, nc]<- NA
# convert to data frame:
b<- data.frame(row = rep(1:4000, 2000), col = rep(1:2000, each = 4000),
x = as.vector(a))
# relatively time consuming...about 13.5 s on my machine
bb<- b[rev(order(b$x, na.last = FALSE)), ]
bb[1:10, ]
row col x
691269 3269 173 5.103704
7815076 3076 1954 4.961544
4999621 3621 1250 4.953265
500469 469 126 4.937655
5878224 2224 1470 4.929150
4287270 3270 1072 4.913791
4442521 2521 1111 4.896869
4668867 867 1168 4.863504
5716575 575 1430 4.760778
3055274 3274 764 4.758995
HTH,
Dennis
On Thu, Jun 17, 2010 at 10:41 PM, uschlecht<ulrich.schle...@stanford.edu>wrote:
Hi,
I have a huge matrix (4000 * 2000 data points) and I would like to retrieve
the coordinates (column and row) for the top 50 (or x) values. Some
positions in the matrix have NA as a value. These should be discarded.
My current method is to replace all NAs by 0, then rank all the values and
then extract the positions with the 50 highest ranks. It is very
time-consuming!
Is there a simpler way to do this?
Thank you,
Ulrich
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