which: Which indices are TRUE?
In robertzk/monadicbase: The R Base Package

Description Usage Arguments Value Note Author(s) See Also Examples

Give the TRUE indices of a logical object, allowing for array indices.

1 2	which(x, arr.ind = FALSE, useNames = TRUE) arrayInd(ind, .dim, .dimnames = NULL, useNames = FALSE)

`x`	a `logical` vector or array. `NA`s are allowed and omitted (treated as if `FALSE`).
`arr.ind`	logical; should array indices be returned when `x` is an array?
`ind`	integer-valued index vector, as resulting from `which(x)`.
`.dim`	`dim(.)` integer vector
`.dimnames`	optional list of character `dimnames(.)`. If `useNames` is true, to be used for constructing dimnames for `arrayInd()` (and hence, `which(*, arr.ind=TRUE)`). If `names(.dimnames)` is not empty, these are used as column names. `.dimnames[[1]]` is used as row names.
`useNames`	logical indicating if the value of `arrayInd()` should have (non-null) dimnames at all.

If arr.ind == FALSE (the default), an integer vector with length equal to sum(x), i.e., to the number of TRUEs in x; Basically, the result is (1:length(x))[x].

If arr.ind == TRUE and x is an array (has a dim attribute), the result is arrayInd(which(x), dim(x), dimnames(x)), namely a matrix whose rows each are the indices of one element of x; see Examples below.

Unlike most other base R functions this does not coerce to x to logical: only arguments with typeof logical are accepted and others give an error.

Werner Stahel and Peter Holzer (ETH Zurich) proposed the arr.ind option.

Logic, which.min for the index of the minimum or maximum, and match for the first index of an element in a vector, i.e., for a scalar a, match(a, x) is equivalent to min(which(x == a)) but much more efficient.

which(LETTERS == "R")
which(ll <- c(TRUE, FALSE, TRUE, NA, FALSE, FALSE, TRUE)) #> 1 3 7
names(ll) <- letters[seq(ll)]
which(ll)
which((1:12)%%2 == 0) # which are even?
which(1:10 > 3, arr.ind = TRUE)

( m <- matrix(1:12, 3, 4) )
div.3 <- m %% 3 == 0
which(div.3)
which(div.3, arr.ind = TRUE)
rownames(m) <- paste("Case", 1:3, sep = "_")
which(m %% 5 == 0, arr.ind = TRUE)

dim(m) <- c(2, 2, 3); m
which(div.3, arr.ind = FALSE)
which(div.3, arr.ind = TRUE)

vm <- c(m)
dim(vm) <- length(vm) #-- funny thing with  length(dim(...)) == 1
which(div.3, arr.ind = TRUE)