| which {base} | R Documentation |
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.
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(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)