understanding application of function: x > y - z & x < y + z

Question

I am not particularly great with math and am trying to understand some R code. There is a function called "compareforequality" that looks like this:

compareforequality <- function(val1, val2, epsilon)
{
    val1 = as.numeric(val1);
    val2 = as.numeric(val2);
    equal = val1 > (val2 - epsilon) & val1 < val2 + epsilon;
    equal
}

where val1 and val2 are vector of numbers that signify timepoints (usually integers between -10 and 1000 that identify days in a time series), and epsilon is set to 1e-10. I can see that it will return true/false if the values are the same/different, but what is the application of a function like this instead of using something like identical(). What effect does the value of epsilon have on the comparison?

Thanks,

Answer 1

The point is not for them to be exactly equal, it's to compare for rough equality, as in " val1 is within epsilon of val2 ".

The classic example of the usefulness of something like this is probably floating point numbers, where (for instance) 0.1 + 0.2 != 0.3 , but 0.1 + 0.2 is within epsilon of 0.3 for some small epsilon , which is quite often enough.