which  R Documentation 
Give the TRUE
indices of a logical object, allowing for array
indices.
which(x, arr.ind = FALSE, useNames = TRUE) arrayInd(ind, .dim, .dimnames = NULL, useNames = FALSE)
x 
a 
arr.ind 
logical; should array indices be returned
when 
ind 
integervalued index vector, as resulting from

.dim 

.dimnames 
optional list of character 
useNames 
logical indicating if the value of 
If arr.ind == FALSE
(the default), an integer vector,
or a double vector if x
is a long 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]
in typical cases;
more generally, including when x
has NA
's,
which(x)
is seq_along(x)[!is.na(x) & x]
plus
names
when x
has.
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.
Unlike most other base R functions this does not coerce x
to logical: only arguments with typeof
logical are
accepted and others give an error.
Werner Stahel and Peter Holzer (ETH Zurich) proposed the
arr.ind
option.
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) ) 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)
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