Turn matrix into a paired list using apply in R?
apply, matrix, r
Solution
You don't necessarily need to use an `apply()` loop. Here's an idea in base R that uses `which()` to find the relevant indices, and then does some simple extraction and combining.
wM <- which(M > 0, arr.ind = TRUE)
matrix(sort(paste(rownames(wM), colnames(M)[wM[,2]], sep = ",")))
# [,1]
# [1,] "A,B"
# [2,] "A,C"
# [3,] "B,C"
# [4,] "B,D"
# [5,] "C,A"
# [6,] "D,B"
# [7,] "D,C"
Alternatively, you could use `row` and `col` in place of `which`.
M0 <- M > 0
paste(rownames(M)[row(M)[M0]], colnames(M)[col(M)[M0]], sep = ",")
# [1] "C,A" "A,B" "D,B" "A,C" "B,C" "D,C" "B,D"
Problem
I have a matrix similar to the one below but larger. ``` vec <- c(0, 1, 1, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 1, 1, 0) M <- matrix(vec, nrow = 4, byrow = TRUE) colnames(M) <- rownames(M) <- LETTERS[1:4] A, B, C, D A, 0, 1, 1, 0 B, 0, 0, 1, 1 C, 1, 0, 0, 0 D, 0, 1, 1, 0 ``` Ultimately, I would like a matrix or a data.frame where each time a call in a row has a value, I end up with a rowname,column name pair. So, given the above matrix, I end up with the matrix or data frame below: ``` A,B A,C B,C B,D C,A D,B D,C ``` I can do this with loops and with expand.grid, but this seems like something I should be able to do with the apply functions. For example, I can get part way there like this: ``` tmp <- apply(M, 1, function(x) colnames(M)[which(x > 0)]) ``` which gives me ``` $A [1] "B" "C" $B [1] "C" "D" $C [1] "A" $D [1] "B" "C" ``` If that is easy, then can I also include the number in the cell (for cells over 0) as a third column?