ken ken - 27 days ago 8
R Question

Return the indices of a 3D array in R based on multiple values

I would like to obtain the indices of a 3D array in R (ie. arr[x,y,z]) based on multiple values. Specifically, using the first z dimension to subset values in the second z dimension. Here is a example:

# create example array
> m1 <- matrix(c(rep("a",5), rep("b",5), rep("c",5)), nr = 5)
> m2 <- matrix(c(rep(100, 5), rep(10, 5), rep(10, 5)), nr = 5)
> arr <- array(c(m1, m2), dim = c(dim(m1), 2))

#use which() to return the indices in m2 that correspond to indices with
#"a" and "c" values in m1. This does not work as expected.
> ac.ind <- which(arr[,,1] %in% c("a", "c"), arr.ind = T)

> ac.ind
[1] 1 2 3 4 5 11 12 13 14 15


which() returns an vector of positions in m1 that correspond to "a" and "c", not the matrix indices (the (x,y) positions). I would like ac.ind to return:

row col
[1,] 1 1
[2,] 2 1
[3,] 3 1
[4,] 4 1
[5,] 5 1
[1,] 1 3
[2,] 2 3
[3,] 3 3
[4,] 4 3
[5,] 5 3


If I do a more simple which() subset, it does return the indices:

#use which to return indices in m2 that correspond to only "a" in m1
>a.ind <- which(arr[,,1] == c("a"), arr.ind = T)

>a.ind
row col
[1,] 1 1
[2,] 2 1
[3,] 3 1
[4,] 4 1
[5,] 5 1


I am using %in% since I want to subset based on two values in m1 ("a" and "c" values). Is there a way to return the indices of an array based on two values in R?

Answer

The issue is that arr[,,1] %in% c("a", "c") returns a vector. One way is to cast this as a matrix with the number of rows equaling the first dimension of arr:

ac.ind <- which(matrix(arr[,,1] %in% c("a", "c"), nrow=dim(arr)[1]), arr.ind = T)
##      row col
## [1,]   1   1
## [2,]   2   1
## [3,]   3   1
## [4,]   4   1
## [5,]   5   1
## [6,]   1   3
## [7,]   2   3
## [8,]   3   3
## [9,]   4   3
##[10,]   5   3