Find first greater element with higher index

I have two vectors, `A` and `B`. For every element in `A` I want to find the index of the first element in `B` that is greater and has higher index. The length of `A` and `B` are the same. So for vectors: ``` A <- c(10, 5, 3, 4, 7) B <- c(4, 8, 11, 1, 5) ``` I want a result vector: ``` R <- c(3, 3, 5, 5, NA) ``` Of course I can do it with two loops, but it's very slow, and I don't know how to use apply() in this situation, when the indices matter. My data set has vectors of length 20000, so the speed is really important in this case. A few bonus questions: What if I have a sequence of numbers (like `seq = 2:10`), and I want to find the first number in `B` that is higher than a+s for every a of A and every s of seq. Like with question 1), but I want to know the first greater, and the first lower value, and create a matrix, which stores which one was first. So for example I have a of `A`, and 10 from seq. I want to find the first value of `B`, which is higher than a+10, or lower than a-10, and then store it's index and value.