Find index of value in a sorted vector in R

问题

I have an ordered vector of unique integers in R and I want to find the index of the element closest to but less than or equal to some value. For example, for the vector 4 8 15 16 23 42 and the search value 17, I would like the function to return 4, the index of 16. In Python, I would use bisect module. Is there anything similar in R?

回答1:

You can use binsearch from the gtools package to get the log(n) behavior of a binary search:

library(gtools)
x <- c(4, 8, 15, 16, 23, 42)
binsearch(function(y) x[y]-17, range=c(1, length(x)))
# $call
# binsearch(fun = function(y) x[y] - 17, range = c(1, length(x)))
# 
# $numiter
# [1] 3
# 
# $flag
# [1] "Between Elements"
# 
# $where
# [1] 4 5
# 
# $value
# [1] -1  6

We can see the sublinear scaling of the number of iterations with a larger example:

set.seed(144)
x <- sort(runif(1000000))
binsearch(function(y) x[y]-0.5, range=c(1, length(x)))
# $call
# binsearch(fun = function(y) x[y] - 0.5, range = c(1, length(x)))
# 
# $numiter
# [1] 21
# 
# $flag
# [1] "Between Elements"
# 
# $where
# [1] 500577 500578
# 
# $value
# [1] -1.990702e-07  5.827751e-07

回答2:

Base R provides findInterval which implements a binary search:

findInterval(17, c(4, 8, 15, 16, 23, 42))

---

@Khashaa already mentioned this in a comment.

来源：https://stackoverflow.com/questions/31526464/find-index-of-value-in-a-sorted-vector-in-r