[Rd] compairing doubles

[Mark van der Loo]

how about

is_evenly_spaced <- function(x,...) all.equal(diff(sort(x)),...)

(use ellipsis to set tolerance if necessary)


Op vr 31 aug. 2018 om 15:46 schreef Emil Bode <emil.bode using dans.knaw.nl>:

> Agreed that's it's rounding error, and all.equal would be the way to go.
> I wouldn't call it a bug, it's simply part of working with floating point
> numbers, any language has the same issue.
>
> And while we're at it, I think the function can be a lot shorter:
> .is_continous_evenly_spaced <- function(n){
>   length(n)>1 && isTRUE(all.equal(n[order(n)], seq(from=min(n), to=max(n),
> length.out = length(n))))
> }
>
> Cheers, Emil
>
>     El vie., 31 ago. 2018 a las 15:10, Felix Ernst
>     (<felix.gm.ernst using outlook.com>) escribió:
>     >
>     > Dear all,
>     >
>     > I a bit unsure, whether this qualifies as a bug, but it is definitly
> a strange behaviour. That why I wanted to discuss it.
>     >
>     > With the following function, I want to test for evenly space
> numbers, starting from anywhere.
>     >
>     > .is_continous_evenly_spaced <- function(n){
>     >   if(length(n) < 2) return(FALSE)
>     >   n <- n[order(n)]
>     >   n <- n - min(n)
>     >   step <- n[2] - n[1]
>     >   test <- seq(from = min(n), to = max(n), by = step)
>     >   if(length(n) == length(test) &&
>     >      all(n == test)){
>     >     return(TRUE)
>     >   }
>     >   return(FALSE)
>     > }
>     >
>     > > .is_continous_evenly_spaced(c(1,2,3,4))
>     > [1] TRUE
>     > > .is_continous_evenly_spaced(c(1,3,4,5))
>     > [1] FALSE
>     > > .is_continous_evenly_spaced(c(1,1.1,1.2,1.3))
>     > [1] FALSE
>     >
>     > I expect the result for 1 and 2, but not for 3. Upon Investigation
> it turns out, that n == test is TRUE for every pair, but not for the pair
> of 0.2.
>     >
>     > The types reported are always double, however n[2] == 0.1 reports
> FALSE as well.
>     >
>     > The whole problem is solved by switching from all(n == test) to
> all(as.character(n) == as.character(test)). However that is weird, isn’t it?
>     >
>     > Does this work as intended? Thanks for any help, advise and
> suggestions in advance.
>
>     I guess this has something to do with how the sequence is built and
>     the inherent error of floating point arithmetic. In fact, if you
>     return test minus n, you'll get:
>
>     [1] 0.000000e+00 0.000000e+00 2.220446e-16 0.000000e+00
>
>     and the error gets bigger when you continue the sequence; e.g., this
>     is for c(1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7):
>
>     [1] 0.000000e+00 0.000000e+00 2.220446e-16 2.220446e-16 4.440892e-16
>     [6] 4.440892e-16 4.440892e-16 0.000000e+00
>
>     So, independently of this is considered a bug or not, instead of
>
>     length(n) == length(test) && all(n == test)
>
>     I would use the following condition:
>
>     isTRUE(all.equal(n, test))
>
>     Iñaki
>
>     >
>     > Best regards,
>     > Felix
>     >
>     >
>     >         [[alternative HTML version deleted]]
>     >
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>
>
>
>     --
>     Iñaki Ucar
>
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