which

R Base 3.6.2

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

Syntax

which(x, arr.ind = FALSE, useNames = TRUE)
arrayInd(ind, .dim, .dimnames = NULL, useNames = FALSE)

Arguments

Parameter	Description
`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?
`ind`	integer-valued index vector, as resulting from which(x).
`.dim`	dim(.) integer vector
`.dimnames`	optional list of character dimnames(.). If useNames is true, to be used for constructing dimnames for arrayInd() (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.

Return Value

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 TRUEs 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 elem

Examples

Example

# NOT RUN {
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)
# }
# NOT RUN {
<!-- %dont -->
# }

which

Syntax

Arguments

Return Value

Examples

See Also