vectorization - R: find nearest index

Question

I have two vectors with a few thousand points, but generalized here:

A <- c(10, 20, 30, 40, 50)
b <- c(13, 17, 20)

How can I get the indicies of A that are nearest to b? The expected outcome would be c(1, 2, 2).

I know that findInterval can only find the first occurrence, and not the nearest, and I'm aware that which.min(abs(b[2] - A)) is getting warmer, but I can't figure out how to vectorize it to work with long vectors of both A and b.

Answer 1

You can just put your code in a sapply. I think this has the same speed as a for loop so isn't technically vectorized though:

sapply(b,function(x)which.min(abs(x - A)))