Find the largest element in a vector less than values in another vector in R

Question

I have a sorted vector x and another vector y (not necessarily the same length). For each entry of y, I want to find the index of the largest entry of x less than or equal to the entry of y.

For example if

x <- c(2,4,8,9,12) 
y <- c(5,10)

I want to return indices of x for each entry of y:

• 2 (because 4 is the largest entry of x less than 5) and
• 4.

I can do this easily by looping over y, but I want to know if there is a way to vectorize this. So can I vectorize:

for (k in 1:length(y)){
    max(which(x < y[k]))
}

Answer 1

Using Vectorize:

x <- c(2,4,8,9,12) 
y <- c(5,10)

largest_less_than<-function(x,y){
  which(x == max(x[x < y]))
}

largest_less_than <- Vectorize(largest_less_than, vectorize.args = 'y')

largest_less_than(x = x, y = y)

Answer 2

Assuming x is sorted, the findInterval function will work:

findInterval(y,x)
# 2 4