How do I really do this?

How do I really do this?

[Bruce Korb]

I would like to be able to associate a value with a string.
hash functions look perfect, except I can't use them unless
I have a hash table and there isn't any clear way I've found
to create hash table entries without having a hash table.
I've tried:

  (define (string->symbol (function-returning-string))
          (value-function))

I thought that to be really obvious, but I was wrong.
If either of these mecanisms are documented somewhere,
I'd gladly read it.  Meanwhile, can someone send me a hint?
Thank you. - Bruce


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Re: How do I really do this?

[Neil Jerram]

Bruce Korb <bkorb@veritas.com> writes:

> I would like to be able to associate a value with a string.

What level of uniqueness do you want?  If two strings with the same
contents are read twice, should the value associated with one
automatically be associated with the other.

> hash functions look perfect, except I can't use them unless
> I have a hash table and there isn't any clear way I've found
> to create hash table entries without having a hash table

You what?  What does it mean to create a hash table entry without a
hash table?

> I've tried:
> 
>   (define (string->symbol (function-returning-string))
>           (value-function))

This is bizarre and makes no sense.  The syntax is wrong, and I can't
guess what you mean.
 
> I thought that to be really obvious, but I was wrong.
> If either of these mecanisms are documented somewhere,
> I'd gladly read it.  Meanwhile, can someone send me a hint?
> Thank you. - Bruce

No, although I'd like to!  Please ask again with more clarity.

        Neil


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Re: How do I really do this?

[Bruce Korb]

Neil Jerram wrote:
> 
> Bruce Korb <bkorb@veritas.com> writes:
> 
> > I would like to be able to associate a value with a string.

> > hash functions look perfect, except I can't use them unless
> > I have a hash table and there isn't any clear way I've found
> > to create hash table entries without having a hash table
> 
> You what?  What does it mean to create a hash table entry without a
> hash table?

How do I get a hash table started?  I've grepped throught he
ice-9 code and I'm sure something's in there.  But looking at
code around 'hash-create' and 'hashq-create' or even just "hash"
didn't yield anything that worked.  I guess I just need something
really explicit, like:

   (define my-hash-table (hash-table-create))

except that doesn't work.  What does?

> > I've tried:
> >
> >   (define (string->symbol (function-returning-string))
> >           (value-function))
> 
> This is bizarre and makes no sense.  The syntax is wrong, and I can't
> guess what you mean.

OK.  This works:

   (define sym-name (value-function))

So, why doesn't this:

   (define (string->symbol "sym-name") (val-func))

If the "sym-name" in the first expression is a symbol, then
why isn't the string->symbol function yielding a symbol?

> > I thought that to be really obvious, but I was wrong.
> > If either of these mecanisms are documented somewhere,
> > I'd gladly read it.  Meanwhile, can someone send me a hint?
> > Thank you. - Bruce
> 
> No, although I'd like to!  Please ask again with more clarity.

:-)  Thank you, Neil.

Regards, Bruce


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Re: How do I really do this?

[Neil Jerram]

Bruce Korb <bkorb@veritas.com> writes:

> How do I get a hash table started?  I've grepped throught he
> ice-9 code and I'm sure something's in there.  But looking at
> code around 'hash-create' and 'hashq-create' or even just "hash"
> didn't yield anything that worked.  I guess I just need something
> really explicit, like:
> 
>    (define my-hash-table (hash-table-create))
> 
> except that doesn't work.  What does?

The procedure you want is `make-hash-table':

guile> (help make-hash-table)
`make-hash-table' is a primitive procedure in the (guile) module.

 - Scheme Procedure: make-hash-table [n]
     Make a hash table with optional minimum number of buckets N

Note also that you might have found this using `apropos':

guile> (apropos "hash")
(guile): hashq	#<primitive-procedure hashq>
(guile): hashv	#<primitive-procedure hashv>
(guile): hash	#<primitive-procedure hash>
(guile): make-hash-table	#<primitive-procedure make-hash-table>
(guile): make-weak-key-hash-table	#<primitive-procedure make-weak-key-hash-table>
(guile): make-weak-value-hash-table	#<primitive-procedure make-weak-value-hash-table>
(guile): make-doubly-weak-hash-table	#<primitive-procedure make-doubly-weak-hash-table>
(guile): hash-table?	#<primitive-procedure hash-table?>
(guile): weak-key-hash-table?	#<primitive-procedure weak-key-hash-table?>
(guile): weak-value-hash-table?	#<primitive-procedure weak-value-hash-table?>
(guile): doubly-weak-hash-table?	#<primitive-procedure doubly-weak-hash-table?>
(guile): hash-clear!	#<primitive-procedure hash-clear!>
(guile): hashq-get-handle	#<primitive-procedure hashq-get-handle>
(guile): hashq-create-handle!	#<primitive-procedure hashq-create-handle!>
(guile): hashq-ref	#<primitive-procedure hashq-ref>
(guile): hashq-set!	#<primitive-procedure hashq-set!>
(guile): hashq-remove!	#<primitive-procedure hashq-remove!>
(guile): hashv-get-handle	#<primitive-procedure hashv-get-handle>
(guile): hashv-create-handle!	#<primitive-procedure hashv-create-handle!>
(guile): hashv-ref	#<primitive-procedure hashv-ref>
(guile): hashv-set!	#<primitive-procedure hashv-set!>
(guile): hashv-remove!	#<primitive-procedure hashv-remove!>
(guile): hash-get-handle	#<primitive-procedure hash-get-handle>
(guile): hash-create-handle!	#<primitive-procedure hash-create-handle!>
(guile): hash-ref	#<primitive-procedure hash-ref>
(guile): hash-set!	#<primitive-procedure hash-set!>
(guile): hash-remove!	#<primitive-procedure hash-remove!>
(guile): hashx-get-handle	#<primitive-procedure hashx-get-handle>
(guile): hashx-create-handle!	#<primitive-procedure hashx-create-handle!>
(guile): hashx-ref	#<primitive-procedure hashx-ref>
(guile): hashx-set!	#<primitive-procedure hashx-set!>
(guile): hash-fold	#<primitive-procedure hash-fold>
(guile): hash-for-each	#<primitive-procedure hash-for-each>
(guile): hash-for-each-handle	#<primitive-procedure hash-for-each-handle>
(guile): hash-map->list	#<primitive-procedure hash-map->list>
(guile): symbol-hash	#<primitive-procedure symbol-hash>
(guile): read-hash-procedures
(guile): read-hash-extend	#<primitive-procedure read-hash-extend>
(guile): source-whash

> OK.  This works:
> 
>    (define sym-name (value-function))
> 
> So, why doesn't this:
> 
>    (define (string->symbol "sym-name") (val-func))

Because this is the syntax for defining a procedure, and needs to be
one of:

(define (string->symbol) ...)
(define (string->symbol ARG1 ...) ...)
(define (string->symbol . ARGS) ...)

where all the ARGs must be symbols (not strings).

Also, string->symbol is already defined, so why are you trying to
redefine it?

Perhaps you're not trying to redefine it?  In which case, I still
don't understand what you are trying to do.

        Neil


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Re: How do I really do this?

[Mikael Djurfeldt]

Bruce Korb <bkorb@veritas.com> writes:

> I would like to be able to associate a value with a string.
> hash functions look perfect, except I can't use them unless
> I have a hash table and there isn't any clear way I've found
> to create hash table entries without having a hash table.
> I've tried:
>
>   (define (string->symbol (function-returning-string))
>           (value-function))
>
> I thought that to be really obvious, but I was wrong.
> If either of these mecanisms are documented somewhere,
> I'd gladly read it.  Meanwhile, can someone send me a hint?

(define t (make-hash-table))
(hash-set! t (function-returning-string) (value-function))

Then you can lookup values again with (hash-ref t <string>).

This is all documented in the Guile refence manual in the info node
'Hash Tables'.

You don't gain anything by trying to piggyback on 'define', because
'define' uses exactly the same hash table internally as that returned
by (make-hash-table).

M


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Re: How do I really do this?

[Bruce Korb]

Mikael Djurfeldt wrote:

> (define t (make-hash-table))
> (hash-set! t (function-returning-string) (value-function))
> 
> Then you can lookup values again with (hash-ref t <string>).
> 
> This is all documented in the Guile refence manual in the info node
> 'Hash Tables'.

The only Guile .info stuff I have in my distribution is:

> Scheme Programming
> * data-rep: (data-rep).  Data Representation in Guile -- how to use
>                          Guile objects in your C code.

which is why I was having so much trouble.  :(  I was looking up
the hash stuff in the libguile C sources and the "make-hash-table"
is an ice-9 defined wrapper around the primitive "make-vector".
That made it too difficult to find.  Thank you both for your help!
(You, Mike, and Neil, too!)

Regards, Bruce


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Re: How do I really do this?

[Paul Jarc]

Bruce Korb <bkorb@veritas.com> wrote:
> I would like to be able to associate a value with a string.
> hash functions look perfect, except I can't use them unless
> I have a hash table and there isn't any clear way I've found
> to create hash table entries without having a hash table.

Why do you want to avoid creating a hash table?


paul


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Re: How do I really do this?

[Andreas Rottmann]

Neil Jerram <neil@ossau.uklinux.net> writes:

> Bruce Korb <bkorb@veritas.com> writes:
>
>> OK.  This works:
>> 
>>    (define sym-name (value-function))
>> 
>> So, why doesn't this:
>> 
>>    (define (string->symbol "sym-name") (val-func))
>
> Because this is the syntax for defining a procedure, and needs to be
> one of:
>
> (define (string->symbol) ...)
> (define (string->symbol ARG1 ...) ...)
> (define (string->symbol . ARGS) ...)
>
> where all the ARGs must be symbols (not strings).
>
> Also, string->symbol is already defined, so why are you trying to
> redefine it?
>
> Perhaps you're not trying to redefine it?  In which case, I still
> don't understand what you are trying to do.
>
I think he maybe wants to define a variable named after a
string. </wildguess>

Andy
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Re: How do I really do this?

[Carl Witty]

On Sat, 2004-03-06 at 10:00, Neil Jerram wrote:
> Bruce Korb <bkorb@veritas.com> writes:
> > OK.  This works:
> > 
> >    (define sym-name (value-function))
> > 
> > So, why doesn't this:
> > 
> >    (define (string->symbol "sym-name") (val-func))
> 
> Because this is the syntax for defining a procedure, and needs to be
> one of:
> 
> (define (string->symbol) ...)
> (define (string->symbol ARG1 ...) ...)
> (define (string->symbol . ARGS) ...)
> 
> where all the ARGs must be symbols (not strings).
> 
> Also, string->symbol is already defined, so why are you trying to
> redefine it?
> 
> Perhaps you're not trying to redefine it?  In which case, I still
> don't understand what you are trying to do.

I think he's trying to get at something more like
  (eval `(define ,(string->symbol "sym-name") ,(val-func)))

(Bruce, hopefully this will help you get an answer to your question. 
I'd answer it myself, but it's been too long since I've dealt with
Scheme or Guile...)

Carl Witty



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Re: How do I really do this?

[Marius Vollmer]

Mikael Djurfeldt <djurfeldt@nada.kth.se> writes:

> You don't gain anything by trying to piggyback on 'define', because
> 'define' uses exactly the same hash table internally as that returned
> by (make-hash-table).

This might be confusing.  'make-hash-table' always creates a new hash
table, it doesn't reuse the one that is also used by 'define'.  What
Mikael is saying is that the hash table that is used to store bindings
created with 'define' is also created by 'make-hash-table' and is not
some super special data structure.

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Re: How do I really do this?

[Paul Jarc]

Neil Jerram <neil@ossau.uklinux.net> wrote:
> Bruce Korb <bkorb@veritas.com> writes:
>> OK.  This works:
>>
>>    (define sym-name (value-function))
>>
>> So, why doesn't this:
>>
>>    (define (string->symbol "sym-name") (val-func))
...
> Perhaps you're not trying to redefine it?  In which case, I still
> don't understand what you are trying to do.

I think he was trying to define sym-name, just as in the first case -
as if "define" were a normal procedure.  Of course, make-hash-table is
the way to go.


paul


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Re: How do I really do this?

[Bruce Korb]

Carl Witty wrote:

> I think he's trying to get at something more like
>   (eval `(define ,(string->symbol "sym-name") ,(val-func)))

Even reading the docs didn't get me to that construct.
Yes!  That _was_ what I was trying to do.  Of course, I'm going
to use hash tables for what I was trying to do, but it is also
very nice to be clued into the "," operator :-).  Thank you!

Regards, Bruce


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Re: How do I really do this?

[Bruce Korb]

Marius Vollmer wrote:
> 
> Mikael Djurfeldt <djurfeldt@nada.kth.se> writes:
> 
> > You don't gain anything by trying to piggyback on 'define', because
> > 'define' uses exactly the same hash table internally as that returned
> > by (make-hash-table).
> 
> This might be confusing.  ....


Thank you everyone  :-)  I have it working now.  I even now know
how to use:

   (eval '(define ,(...) ,(...) ))

That's really cool  :-)  It's just that that syntax is a little,
well, obscure (to me).  Anyway, I am using hash tables with the
(make-hash-table) function.  Thank you all again.

Regards, Bruce


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