Re: sorted?

On Mon, Dec 9, 2024 at 1:37 PM <[email protected]> wrote:

> On Mon, Dec 09, 2024 at 01:20:48PM +0100, Mikael Djurfeldt wrote:
> > On Mon, Dec 9, 2024 at 12:43 PM <[email protected]> wrote:
> >
> > > On Mon, Dec 09, 2024 at 11:37:33AM +0000, Ricardo G. Herdt wrote:
> > > > Hi Jeremy,
> > > >
> > > > Am 09.12.2024 11:21 schrieb Jeremy Korwin-Zmijowski:
> > > > > The reference says :
> > > > >
> > > > >    Scheme Procedure: *sorted?* items less
> > > > >    C Function: *scm_sorted_p* (items, less)
> > > > >
> > > > >        Return |#t| if items is a list or vector such that, for each
> > > > >        element x and the next element y of items, |(less y x)|
> returns
> > > > >        |#f|. Otherwise return |#f|.
> > > > >
> > > > > I think the description should be :
> > > > >
> > > > >    Return |#t| if items is a list or vector such that, for each
> element
> > > > >    x and the next element y of items, |(less y x)| returns |#t|.
> > > > >    Otherwise return |#f|.
> > > >
> > > > Actually no, since less is applied to y and x in that order. This way
> > > > (sorted? '(1 1) <) correctly returns #t as your experiments show.
> > >
> > > I don't get it. (< 1 1) is /always/ #f, regardless of the order of the
> > > ones?
> > >
> > > I'm as confused as Jeremy is.
> > >
> >
> > sorted? checks whether *a list* has its elements sorted. It does not
> apply
> > a predicate to all successive pairs like some fold operation. Two
> > consecutive equal elements are in sorted order since it wouldn't matter
> if
> > you switched them. The less operation is used to sort the list (or check
> > it).
>
> Thanks for your explanation. I must admit that I'm still confused. Perhaps
> I'll have to have a look at the implementation.
>
> I'm sometimes slow :-)
>

No problem---I'm too.

Think about it this way:

How would you sort this list of numbers: 7 1 3 8 2 1 4 ?

It's 1 1 2 3 4 7 8, right? That is what we want (sort '(7 1 3 8 2 1 4)) to
output (+ the parentheses of course).

Now, `sorted?' returns true if its input is what `sort' would have produced
as output, otherwise false.