Re: sorted?

[Mikael Djurfeldt]

On Mon, Dec 9, 2024 at 1:58 PM Mikael Djurfeldt <mikael@djurfeldt.com>
wrote:

> On Mon, Dec 9, 2024 at 1:37 PM <tomas@tuxteam.de> wrote:
>
>> On Mon, Dec 09, 2024 at 01:20:48PM +0100, Mikael Djurfeldt wrote:
>> > On Mon, Dec 9, 2024 at 12:43 PM <tomas@tuxteam.de> 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.
>
> Don't confused by the "less" that you pass in as second argument to
`sort'. That is something you are *required* to pass to sort for it to
function as expected. It is something that `sort' uses.