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?

ind

integer-valued index vector, as resulting from which(x).

.dim

dim(.) integer vector

.dimnames

optional list of character dimnames(.), of which only .dimnames[[1]] is used.

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 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.

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) )
which(m %% 3 == 0)
which(m %% 3 == 0, 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(m %% 3 == 0, arr.ind=FALSE)
which(m %% 3 == 0, arr.ind=TRUE)

vm <- c(m)
dim(vm) <- length(vm) #-- funny thing with  length(dim(...)) == 1
which(vm %% 3 == 0, arr.ind=TRUE)
[Package base version 2.15.1 Index]