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. NA s
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
TRUE
s 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 who's 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)