Count occurence of multiple numbers in vector one by one

Question

I have two vectors

a <- c(1, 5, 2, 1, 2, 3, 3, 4, 5, 1, 2)
b <- (1, 2, 3, 4, 5, 6)

I want to know how many times each element in b occur

Answer 1

Here is a base R option, using sapply with which:

a <- c(1, 5, 2, 1, 2, 3, 3, 4, 5, 1, 2)
b <- c(1, 2, 3, 4, 5, 6)

sapply(b, function(x) length(which(a == x)))
[1] 3 3 2 1 2 0

Demo