# [plt-scheme] Re: User data structures and equality

 From: Noel Welsh (noelwelsh at yahoo.com) Date: Tue Apr 10 16:39:28 EDT 2007 Previous message: [plt-scheme] Re: User data structures and equality Next message: [plt-scheme] Re: User data structures and equality Messages sorted by: [date] [thread] [subject] [author]

```I must be missing something.  If D is finite you can assign a unique integer to each element in D, and that's your hash (or your vector index).  If you know the structure of D (and presumably you do, as you know equality) can also assign unique integers.  Consider a data type like:

number-list := null | cons number list

assign 0 = null, 1 = cons

Then (cons 1 (cons 2 null))  = 11120
(cons 2 null) = 120
etc.  These are unique.  If the elements of the list weren't numbers you'd have to do a bit more work, but can still generate unique numbers.

Anyway, that's what comes to mind.  Hope it's useful.

N.

----- Original Message ----
From: Paulo J. Matos <pocm at soton.ac.uk>
To: PLT-list Mailing <plt-scheme at list.cs.brown.edu>
Sent: Tuesday, April 10, 2007 9:26:12 PM
Subject: [plt-scheme] Re: User data structures and equality

On 4/10/07, Paulo J. Matos <pocm at soton.ac.uk> wrote:
> Hello all,
>
> I want to define a bijective function f: D -> N, where D is a finite
> collection of user data structures which have a known equality
> procedure that given two elements of D know if they are equal or not.
> Implementing this with alists is trivial, however, computing f and
> f^-1 is done in linear time. The 'good' way would be hash-tables but I
> can't since I have no hash-function for the data structures, only the
> equality procedure [there is no obvious ordering between the user
> structures since I don't really know what they are].
>

Agrh, I always forget something... Another way (better) would be to
use a simple vector. Computing f^-1 is constant, f  is linear but I'm
still hoping for a better solution.

> Any suggestions?
>
> Cheers,
> --
> Paulo Jorge Matos - pocm at soton.ac.uk
> http://www.personal.soton.ac.uk/pocm
> PhD Student @ ECS
> University of Southampton, UK
>

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```

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