[R] removing loop in array recalculation

From: Colin Beale <c.beale_at_macaulay.ac.uk>
Date: Wed 18 Jan 2006 - 04:07:52 EST


I'm looking for a more elegant (and faster) solution to my current problem than the code at the end. I'm sure there is one, but can't think where to look - any pointers would be very welcome. The problem is one of resampling within an array. This array consists of 0s, 1s and NAs. For each level of dimension z, I would like to rewrite the array such that it looks up the values in the matrix on either side (through a variable number of cells) and determines if there is a 1 in any of them - if so, the new value is 1, otherwise 0. This process is repeated across the entire matrix: I will obviously end up with a lot more 1s in the new array than I did before. The code at the end works, but is very slow for large arrays (it needs package (magic) to work). Any suggestions/pointers would be gratefully recieved


An example dataset could be:

size = 1 #determines how many values to sample around the focal
library (magic)
A <- array (sample (c (0,0,0,1), 35, replace = T), dim = c (5,4,2))

    temp <- apad (apad (A, c (size,size,0)), c(size,size,0), post = FALSE) # pads array

    temp[,c (1, (4 + 2 * size)),] <- NA # makes additional rows/cols = NA

    temp[c (1, (5 + 2 * size)),,] <- NA

    for (y in 1: 5) {                               # recalculates 
within size
      for (x in 1: 4) {
        for (z in 1: 2) {
          A[y,x,z] <- ifelse (sum (temp[(y):(y+2 * size),(x):(x+2 *
size),z], na.rm = TRUE) == 0, 0, 1)


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