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.
转载于:https://stackoverflow.com/questions/5577727/is-there-an-r-function-for-finding-the-index-of-an-element-in-a-vector
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)
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.
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]]