Comparing each value in a vector to all other values based on criteria

Question

I have a vector of unique values, but I want to get only the ones meeting a specific criteria with at least one other value. For example, if I have a vector like this one

v <- c(1,2,5,6,8,10,15,16)

And the criteria I want to meet is all values that have at least one other value in the vector with a difference of 1 between them, the result will be

1,2,5,6,15,16

I want to be able to do it with different functions. So for this example let's assume I use a function that gives me the absolute difference between two given number:

AbsDiff <- function(x,y){
           abs(x-y)
           }

So the solution should run the AbsDiff function for each value against all other values, and get the desired result.

Answer 1

Here's a slightly modified version of your function:

m <- outer(v, v, FUN = function(x,y) abs(x-y) == 1)
v[colSums(m) > 0]
# [1]  1  2  5  6 15 16

As you can see, I added a logical check to your function that is TRUE whenever the absolute difference between two values is exactly 1. That makes it easier to extract the relevant indices afterwards.

---

If you want to use your function as it is now, you could do it like so:

m <- outer(v, v, FUN = AbsDiff)
v[colSums(m == 1) > 0]

That means we do the check for ==1 in the second step.