efficient way to get values from a raster
Dear list, I want to extract cell values from a raster object given row and column numbers or cell numbers. The solution I used is not efficient (see the following) comparing with extracting values from a matrix. Any advice?
m <- matrix(rnorm(25),5) # matrix r <- raster(m) # raster
# --- case 1:
system.time(for (i in 1:1000) r[1:3,1:3]) # extracting values from raster given rows and columns
user system elapsed
0 0 0
system.time(for (i in 1:1000) m[1:3,1:3]) # extracting the same values from matrix
user system elapsed 8.41 0.03 9.58 #---- case 2:
cells <- c(1,2,3,6,7,8,11,12,13) # cell numbers
system.time(for (i in 1:1000) r[cells]) # extracting values from raster given cells number
user system elapsed 3.52 0.00 3.59
system.time(for (i in 1:1000) t(m)[cells]) # extracting the same values from matrix
user system elapsed 0.07 0.00 0.08 Best regards, Babak