Hi Alex,
Ah. That's what I missed.... the sort direction of the second argument..
Thank you
Lindsay
On Tue, May 23, 2017 at 10:47 PM, Alexander Burger <a...@software-lab.de>
wrote:
> Hi Lindsay,
>
> > My sample code ("ABRA..) is wrong. Sorry about that.
>
> Ah, I see. No problem.
>
>
> > : (rank 'c '((a . a) (b . b) (d . d)) T) # ???
> > -> NIL
> > : (rank 'd '((a . a) (b . b) (d . d)) T) # ???
> > -> NIL
> >
> > : (rank 'd '((a . a) (b . b) (d . d)))
> > -> (d . d)
> > : (rank 'c '((a . a) (b . b) (d . d)))
> > -> (b . b)
> >
> > The latter two work as I would expect.
> > I expected the first two to return '(d . d)....
>
> The reason here is that 'rank' expects a *sorted* list. It traverses the
> list,
> and stops as soon as it can confirm or refute the desired condition.
>
> With a second argument non-NIL, it tries to "Returns the element from lst
> ...
> with a minimal CAR greater or equal to any". However, already the first
> element
> (a . a) is *less* than 'c' or 'd', hence it returns NIL.
>
> If you reverse the list, it works:
>
> : (rank 'c '((d . d) (b . b) (a . a)) T)
> -> (d . d)
>
> : (rank 'b '((d . d) (b . b) (a . a)) T)
> -> (b . b)
>
> It is the purpose of the second argument to tell 'rank' in which direction
> it
> should expect the list to be sorted.
>
> ♪♫ Alex
>
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