which
Which indices are TRUE?
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
logicalvector or array.NAs are allowed and omitted (treated as ifFALSE). - arr.ind
- logical; should array indices be returned
when
xis 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.
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
library(base)
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(vm %% 3 == 0, arr.ind = TRUE)
Community examples
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