R: Which indices are TRUE? (original) (raw)
| which {base} | R Documentation |
|---|
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 fromwhich(x). |
| .dim | dim(.) integer vector. |
| .dimnames | optional list of character dimnames(.). If useNames is true, to be used for constructing dimnames forarrayInd() (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, withlength equal to sum(x), i.e., to the number ofTRUEs in x.
Basically, the result is (1:length(x))[x] in typical cases; more generally, including when x has [NA](../../base/help/NA.html)'s,which(x) is seq_along(x)[!is.na(x) & x] plus[names](../../base/help/names.html) when x has.
If arr.ind == TRUE and x is an [array](../../base/help/array.html) (has a [dim](../../base/help/dim.html) attribute), the result isarrayInd(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 xto logical: only arguments with [typeof](../../base/help/typeof.html) logical are accepted and others give an error.
Author(s)
Werner Stahel and Peter Holzer (ETH Zurich) proposed thearr.ind option.
See Also
[Logic](../../base/help/Logic.html), [which.min](../../base/help/which.min.html) for the index of the minimum or maximum, and [match](../../base/help/match.html) 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)
[Package _base_ version 4.6.0 Index]