which {base}R Documentation

Which indices are TRUE?


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


which(x, arr.ind = FALSE)


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?


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 a matrix whose rows each are the indices of one element of x; see Examples below.


Werner Stahel and Peter Holzer holzer@stat.math.ethz.ch, for the array case.

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.


which(LETTERS == "R")
which(ll <- c(TRUE,FALSE,TRUE,NA,FALSE,FALSE,TRUE))#> 1 3 7
names(ll) <- letters[seq(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 2.5.0 Index]