which {base}

R Documentation

Which indices are TRUE?

Description

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

Usage

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

Arguments

x	a logical vector or array. NAs are allowed and omitted (treated as if FALSE).
arr.ind	logical; should array indices be returned when x is an array? Anything other than a single true value is treated as false.
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.

Value

If arr.ind == FALSE (the default), an integer vector, or a double vector if x is a long vector, with length equal to sum(x), i.e., to the number of TRUEs in x.

Basically, the result is (1:length(x))[x] in typical cases; more generally, including when x has NA's, which(x) is seq_along(x)[!is.na(x) & x] plus names when x has.

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.

Note

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

Author(s)

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

See Also

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.

Examples

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)

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