unique_by: Unique with auxiliary function
In funprog: Functional Programming

Remove duplicate elements, given a transformation.

1	unique_by(x, f, first = TRUE)

`x`	a vector or a list.
`f`	a function to apply to each element of `x`. This function must produce comparable results.
`first`	if several elements are identical after being transformed by `f`, keep the first. Otherwise, keep the last.

An object of the same type as x. Only elements that are unique after being transformed by f are kept.

1 2 3 4	unique_by(-3:2, abs) unique_by(-3:2, abs, first = FALSE) unique_by(c(1, 2, 4, 5, 6), function(x) x %% 3) unique_by(list(1:2, 2:3, 2:4), length)