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which {base}

Which indices are TRUE?
Package: 
base
Version: 
R 3.0.1

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.

Values

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.

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)

Author(s)

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

Documentation reproduced from R 3.0.1. License: GPL-2.