On May 14, 2012, at 10:09 AM, 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.
}
}
}
}
If you can create a data.frame or matrix that has the indices
x1,x2,y1,y2 and the values you can use the: K[cbind(index-vectors)]
<- values construction:
mtx<- data.matrix( expand.grid(x1=1:2,x2=1:2,y1=1:2,y2=1:2) )
K[mtx] <- apply(mtx, 1, function(x) x["x1"]*x["y2"] -
sin(x['x2']*x['y1']) )
#----------------
> K
, , 1, 1
[,1] [,2]
[1,] 0.158529 0.09070257
[2,] 1.158529 1.09070257
, , 2, 1
[,1] [,2]
[1,] 0.09070257 1.756802
[2,] 1.09070257 2.756802
, , 1, 2
[,1] [,2]
[1,] 1.158529 1.090703
[2,] 3.158529 3.090703
, , 2, 2
[,1] [,2]
[1,] 1.090703 2.756802
[2,] 3.090703 4.756802
Thank you in advance for any help.
--
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