Is there an R function for finding the index of an element in a vector?
In R, I have an element x
and a vector v
. I want to find the first index of an element in v
that is equal to x
. I know that one way to do this is: which(x == v)[[1]]
, but that seems excessively inefficient. Is there a more direct way to do it?
For bonus points, is there a function that works if x
is a vector? That is, it should return a vector of indices indicating the position of each element开发者_Python百科 of x
in v
.
The function match
works on vectors:
x <- sample(1:10)
x
# [1] 4 5 9 3 8 1 6 10 7 2
match(c(4,8),x)
# [1] 1 5
match
only returns the first encounter of a match, as you requested. It returns the position in the second argument of the values in the first argument.
For multiple matching, %in%
is the way to go:
x <- sample(1:4,10,replace=TRUE)
x
# [1] 3 4 3 3 2 3 1 1 2 2
which(x %in% c(2,4))
# [1] 2 5 9 10
%in%
returns a logical vector as long as the first argument, with a TRUE
if that value can be found in the second argument and a FALSE
otherwise.
the function Position
in funprog {base} also does the job. It allows you to pass an arbitrary function, and returns the first or last match.
Position(f, x, right = FALSE, nomatch = NA_integer)
A small note about the efficiency of abovementioned methods:
library(microbenchmark)
microbenchmark(
which("Feb" == month.abb)[[1]],
which(month.abb %in% "Feb"))
Unit: nanoseconds
min lq mean median uq max neval
891 979.0 1098.00 1031 1135.5 3693 100
1052 1175.5 1339.74 1235 1390.0 7399 100
So, the best one is
which("Feb" == month.abb)[[1]]
Yes, we can find the index of an element in a vector as follows:
> a <- c(3, 2, -7, -3, 5, 2)
> b <- (a==-7) # this will output a TRUE/FALSE vector
> c <- which(a==-7) # this will give you numerical value
> a
[1] 3 2 -7 -3 5 2
> b
[1] FALSE FALSE TRUE FALSE FALSE FALSE
> c
[1] 3
This is one of the most efficient methods of finding the index of an element in a vector.
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