How to replace values between 2 arrays in R?

Question

I have three arrays that have the same dimensions but what I need is to replace values in dat1 by the corresponding values from dat2 when only corresponding values in dat3 are higher than 0.2.

data:

dat1 <- array(1:60, c(3,5,4)) 
dat2 <- array(rnorm(60), c(3,5,4)) 
dat3 <- array(rnorm(60), c(3,5,4))

Answer 1

Another option, creating an index first and then using it to subset:

idx <- dat3 > 0.2
dat1[idx] <- dat2[idx]

Edit after comment - a small performance comparison:

set.seed(2015)
N <- 1e6
dat1 <- array(N, c(3,5,4)) 
dat2 <- array(rnorm(N), c(3,5,4)) 
dat3 <- array(rnorm(N), c(3,5,4))

library(microbenchmark)
microbenchmark(
  ifelse = dat1 <- ifelse(dat3 > 0.2, dat2, dat1),
  index = {idx <- dat3 > 0.2
           dat1[idx] <- dat2[idx]},
  unit = "relative"
)

Unit: relative
   expr      min       lq   median       uq      max neval
 ifelse 5.131963 6.460236 5.545135 5.467555 33.86001   100
  index 1.000000 1.000000 1.000000 1.000000  1.00000   100

For the sample data, indexing is ~5 times faster than ifelse.

If you have two restrictions for the replacement index just use the following index instead:

idx <- dat3 > 0.2 & dat3 < 0.5

Answer 2

ifelse(dat3 > 0.2, dat2, dat1)