array output from apply() in R

Question

Suppose I have:

aa <- seq(2,10, length=3)
bb <- seq(20,30, length=2)
cc <- seq(10,11, length=2)

fun <- function(a,b,c) {return(a+b-c)}

out <- array(dim=c(length(aa), length(bb), length(cc)))
for(i in 1:length(aa)) {
  for(j in 1:length(bb)) {
    for(k in 1:length(cc)) {
      out[i,j,k] <- fun(aa[i], bb[j], cc[k])
    }
  }
}

Aiming for a faster alternative, I then tried apply() as follows

b2 <- rep(bb, each=2)
abc <- rbind(rep(aa, each=2*2), rep(bb, each=2), rep(cc,6))
out2 <- apply(abc, 2, function(x){ fun(x[1], x[2], x[3]) } )

This basically does the same calculation as before but I couldn't get the format of "out2" to be in the array as "out". Can you correct the below code so that out and out2 are exactly the same? Thanks so much in advance

out2 <- array(out2, dim=dim(out))

I think making it into an array via dim(out2) <- (...) and then using aperm to permute the dimensions might work, but I haven't figured out the proper incantation — Ben Bolker
– Ben Bolker, Commented May 9, 2014 at 2:23

thelatemail · Accepted Answer · 2014-05-09 02:21:31Z

2

Though not hugely flexible, you could rearrange the entire problem like:

outer(outer(aa,bb,"+"),cc,"-")

identical(out,outer(outer(aa,bb,"+"),cc,"-"))
#[1] TRUE

answered May 9, 2014 at 2:21

thelatemail

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1 Comment

T_D Over a year ago

Thanks @thelatemail But I was using that fun() as a simplest example and have something else much more complicated.. Nice trick btw, perhaps there is something similar about parallelizing that chunk of code using some shorts of of parrallel apply that you can share?

Csislander · Accepted Answer · 2014-05-09 13:35:40Z

2

Not sure if there's a way to do it in one step, but here it is in two more steps:

#your original code
b2 <- rep(bb, each=2)
abc <- rbind(rep(aa, each=2*2), rep(bb, each=2), rep(cc,6))
out2 <- apply(abc, 2, function(x){ fun(x[1], x[2], x[3])})

out2 = matrix(out2,ncol=4,nrow=3,byrow=T)
out2 = array(c(out2[,c(1,3)],out2[,c(2,4)]),dim=dim(out))
identical(out,out2)
#[1] TRUE

edited May 9, 2014 at 13:35

answered May 9, 2014 at 1:58

Csislander

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