all.equal, relative vs absolute
The all.equal()
function is useful to comparing whether two numeric objects are equivalent,
but it has a weird quirk.
> all.equal(4, 5)
[1] "Mean relative difference: 0.25"
You might expect that the difference should be 1. But note the “relative
difference.” The default for all.equal is to report the
difference for a call of all.equal(x, y) as |x - y|/x. This also
means that all.equal(x, y) is not going to report the same
difference as all.equal(y, x).
The scale= argument makes all.equal instead report
absolute difference:
> all.equal(4, 5, scale = 1)
[1] "Mean absolute difference: 1"
This comes in especially with the tolerance= argument. Let’s say
you want to consider two values “equal” if they’re less than .5 apart.
> all.equal(4, 4.4, tolerance = .5)
[1] TRUE
> all.equal(4, 5, tolerance = .5)
[1] TRUE
That second one is clearly wrong. However, if we set the
scale= argument to 1, the tolerance is applied on the absolute
scale instead.
> all.equal(4, 5, tolerance = .5, scale = 1)
[1] "Mean absolute difference: 1"This work is licensed under
CC BY-NC 4.0