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

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

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 2

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)