R array indexing for multidimensional arrays

Question

I have a simple question of indexing an array for multidimensional arrays in R. I do a lot of simulations, each of which gives a result in the form of a matrix where entries are classified by category. So, for example, the result looks like

aresult<-array(sample(1:3, 6, replace=T), dim=c(2,5), dimnames=list( c("prey1", "prey2"), c("predator1", "predator2", "predator3", "predator4", "predator5"))) 

Now I want to save the results of my experiments in a 3D matrix, where the first two dimensions are the same as in aresult , and in the third dimension the number of experiments that fall into each category. So my count arrays should look like

 Counts<-array(0, dim=c(2, 5, 3), dimnames=list( c("prey1", "prey2"), c("predator1", "predator2", "predator3", "predator4", "predator5"), c("n1", "n2", "n3"))) 

and after each experiment, I want to increase the numbers in the third dimension by 1, using the values ​​in aresults as indices.

How can I do this without using loops?

Answer 1

This sounds like typical matrix indexing work. By multiplying Counts with a three-column matrix, each row defining the indices of the element that we want to extract, we can extract and increase any elements that we like.

 # Create a map of all combinations of indices in the first two dimensions i <- expand.grid(prey=1:2, predator=1:5) # Add the indices of the third dimension i <- as.matrix( cbind(i, as.vector(aresult)) ) # Extract and increment Counts[i] <- Counts[i] + 1