如何解释 which() output 中的数组索引？

Question

I'm very new to R, so this is very much a beginner's question.

I'm trying to read/interpret the returned matrix from a which(x, arr.ind = TRUE) command.

The data I'm using is:

qux <- array(data = c(10,5,1,4,7,4,3,3,1,3,4,3,1,7,8,3,7,3), dim = c(3, 2, 3))

I run which() while filtering for values lower than 3 and tell the which() function to return array indices, using arr.ind = TRUE .

> which(qux<3, arr.ind = TRUE)

     dim1 dim2 dim3
[1,]    3    1    1
[2,]    3    1    2
[3,]    1    1    3

I would assume the numbers in the left most column indicate the rows (of which there are 3 in the array), the other three columns to the right indicate the dimensions (of which there are also 3), and the numbers that make up the matrix are the column numbers.

However, this can't be true since there are only two columns in all the dimensions of the array, whereas there's three 3's in the matrix.

Please help me understand this elementary part of R.

Kind regards.

Answer 1

Your qux is a three dimensional array with exactly three values that are 1, the rest are greater than three.

The output you see gives the positions for those values that are less than three

which(qux<3, arr.ind = TRUE)
#      dim1 dim2 dim3
# [1,]    3    1    1
# [2,]    3    1    2
# [3,]    1    1    3

and

qux[3,1,1]
# [1] 1
qux[3,1,2]
# [1] 1
qux[1,1,3]
# [1] 1

so each row of the which() matrix gives you the index for each of the three dimensions in your original array.