Is there an R function for finding the index of an element in a vector?

367
votes

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 of x in v.

4
rindexingmatchvectorization
As R is optimized to work with vectors, which(x == v)[[1]] is not so very inefficient. It's one comparison (==) operator applied to all vector elements and one subsetting on the indices (which). That's it. Nothing that should be relevant, as long as you're not running 10.000 repetitions on this function. Other solutions like match and Position may not return as many data as which, but they're not necessarily more efficient. – BurninLeo
My question specified that I would prefer a function that was vectorized over x, and which(x == v)[[1]] is not. – Ryan C. Thompson

4 Answers

517
votes

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.

27
votes

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)

13
votes

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]]
3
votes

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.