Index value for matrix in R?

filtering, matrix, r

Solution

Just looked at the help for `which()` after posting this and found the answer: the arr.ind parameter.

which(a==23, arr.ind=TRUE)
     row col
[1,]   3   5

Problem

Is there a function to get an index (row number and column number) for a matrix? Suppose that I have a simple matrix: ``` a <- matrix(1:50, nrow=5) ``` Is there an easy way to get back something like c(3, 5) for the number "23", for instance? In this case, saying `which(a==23)` just returns 23. This seems to work but I'm sure that there's a better way: ``` matrix.index <- function(a, value) { idx <- which(data.frame(a)==value) col.num <- ceiling(idx/nrow(a)) row.num <- idx - (col.num-1) * nrow(a) return(c(row.num, col.num)) } > matrix.index(a, 23) [1] 3 5 > matrix.index(a, 50) [1] 5 10 ```

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