Find array index of elements of a matrix that match a value in a vector of candidate values

Question

I have been googeling and stackoverflowing this for a while but I cant seem to find the right answer.

I have a matrix that contains different characters strings like "a" , or "gamma" , or even numbers coerced to characters.

How do I get the array indices of matrix m if an element of m matches a value in a vector of candiate values (note that these values could be any character string). Here is what I tried. I though which(m %in% ...) would do it but it doesnt return what I expected.

m <- matrix(c(0, "a", "gamma", 0, 0.5, 0, 0, 0, 0), ncol = 3)
m
#>      [,1]    [,2]  [,3]
#> [1,] "0"     "0"   "0" 
#> [2,] "a"     "0.5" "0" 
#> [3,] "gamma" "0"   "0"

which(m == "a", arr.ind = TRUE) # as expected
#>      row col
#> [1,]   2   1

which(m == "a" | m == "gamma", arr.ind = TRUE) # also obvious
#>      row col
#> [1,]   2   1
#> [2,]   3   1

candidates <- c("a", "gamma", "b")
which(m %in% candidates, arr.ind = TRUE) # not what I expected
#> [1] 2 3

^{Created on 2019-09-11 by the reprex package (v0.3.0)}

• The result I want is the row and column index of elements in m that match a value in candiates .
• I prefer a base R solution if possible.

Any help?

Answer 1

The problem is that %in% doesn't preserve the dimensions of the input. You can write your own function that would do that. For example

`%matin%` <- function(x, table) {
  stopifnot(is.array(x))
  r <- x %in% table
  dim(r) <- dim(x)
  r
}

candidates <- c("a", "gamma", "b")
which(m %matin% candidates, arr.ind = TRUE)
#      row col
# [1,]   2   1
# [2,]   3   1

Answer 2

MrFlick's solution is probably one of the best you'll get but if you must stick with in-built functions in base R, here's one way -

which(matrix(m %in% candidates, dim(m)), arr.ind = T)

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

Another way with lapply and Reduce but above should be faster -

which(Reduce("|", lapply(candidates, function(x) m == x)), arr.ind = T)

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