How to apply a function to a multidimensional array based on its indices

Hello,

Try

K <- array(0,dim=c(2,2,2,2)) #dimensions will be much larger
for(x1 in 1:2){
  for(y1 in 1:2)  {
    for(x2 in 1:2)    {
      for(y2 in 1:2)      {
        K[x1,y1,x2,y2] <-  x1*y2 - sin(x2*y1) #this is just a dummy
function.
      }  
    }
  }
}


fun <- function(x){
	x1 <- x[1]  # these are
	x2 <- x[2]  # not
	y1 <- x[3]  # really
	y2 <- x[4]  # needed
	x1*y2 - sin(x2*y1) # could have used x[1], etc
}


res <- apply(expand.grid(1:2, 1:2, 1:2, 1:2), 1, fun)
dim(res) <- c(2, 2, 2, 2)
res

all.equal(K, res)
[1] TRUE

See the help pages
?expand.grid
?apply


Hope this helps,

Rui Barradas

math_daddy wrote

Hello. I have a 4 dimensional array and I want to fill in the slots with
values which are a function of the inputs. Through searching the forums
here I found that the function "outer" is helpful for 2x2 matrices but
cannot be applied to general multidimensional arrays. Is there anything
which can achieve, more efficiently than the following code, the job I
want?

K <- array(0,dim=c(2,2,2,2)) #dimensions will be much larger
for(x1 in 1:2)
{
  for(y1 in 1:2)
  {
    for(x2 in 1:2)
    {
      for(y2 in 1:2)
      {
        K[x1,y1,x2,y2] <- x1*y2 - sin(x2*y1) #this is just a dummy
function.
      }   
    }
  }
}

Thank you in advance for any help.

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