Slow down

toeplitz.R

##R version 3.4.2 (2017-09-28)
##Platform: x86_64-pc-linux-gnu (64-bit)
##Running under: Ubuntu 16.04.3 LTS

bm_prog_toeplitz = function() {
  N = 3000
  ans = rep(0, N*N)
  dim(ans) = c(N, N)
  system.time({
    for (j in 1:N) {
      for (k in 1:N) {
        ans[k,j] = abs(j - k) + 1
      }
    }
  }
  )
}
## Takes around 7 seconds
bm_prog_toeplitz()


## Around 2 seconds
N = 3000
ans = rep(0, N*N)
dim(ans) = c(N, N)
system.time({
  for (j in 1:N) {
    for (k in 1:N) {
      ans[k,j] = abs(j - k) + 1
    }
  }
})