Is there an R function for finding the index of an

Question

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.

Answer 1

R has overloaded the double equals == operator with a method of finding the index of a needle in a vector haystack. It yields a logical vector, containing TRUE values for each match in the haystack.

Example:

haystack <- c(1, 2, 4, 3, 4)
needle <- 4
indices <- needle == haystack
indices
[1] 3  5
haystack[indices]
[1] 4  4

It works if both are vectors, and can be expanded to use multiple vectors as well.

Answer 2

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.

Answer 3

A small note about the efficiency of abovementioned methods:

 library(microbenchmark)

  microbenchmark(
    which("Feb" == month.abb)[[1]],
    which("Feb" %in% month.abb))

  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]]

Answer 4

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)