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Applying a function to a multidimensional array with a grouping variable
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Applying a function to a multidimensional array with a grouping variable
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I have what I thought would be a simple problem, but I haven't been able to find an appropriate answer. I have a multidimensional array v[x,y,z] and I would like to apply a function to the array along the z dimension using a grouping variable (group). Here is an example (in R):
v<-1:81
dim(v)<-c(3,3,9)
group<-c('a','a','a','b','b','b','c','c','c')
Given that the grouping variable has 3 levels (a, b and c), the result (out) I'm looking for is an array of dimension 3x3x3. I can obtain out using the following code for the above example:
out1<-apply(v[,,c(1:3)],c(1,2),sum)
out2<-apply(v[,,c(4:6)],c(1,2),sum)
out3<-apply(v[,,c(7:9)],c(1,2),sum)
library(abind)
out<-abind(out1, out2, out3, along=3)
My question is if there is a a general means of obtaining the above result, which can be applied to large dimensional arrays and long grouping vectors.
Easy:
out <- apply(v, c(1, 2), by, group, sum)
But to get the data in exactly the same order as you want:
out <- aperm(apply(v, c(1, 2), by, group, sum), c(2, 3, 1))
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