How do I really do this?

How do I really do this?

[Bruce Korb]

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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?

[Marco Parrone]

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Bruce Korb on Wed, 03 Mar 2004 15:52:15 -0800 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.

Why don't you create the empty hash-table first with `make-hash-table'?

Then you add entries using `hashq-set!' and alike, and you read values
using `hashq-ref' and alike.

guile> (make-hash-table 10)
#(() () () () () () () () () ())
guile> (define myhash (make-hash-table 10))
guile> (hashq-set! myhash 'myvarname "myvarvalue")
"myvarvalue"
guile> myhash
#(() () () () ((myvarname . "myvarvalue")) () () () () ())
guile> (hashq-ref myhash 'myvarname)
"myvarvalue"

> I've tried:
>
>   (define (string->symbol (function-returning-string))
>           (value-function))
>
> I thought that to be really obvious, but I was wrong.

Some ways are:

guile> (eval-string "(define abc 1)")
guile> abc
1
guile> (eval `(define ,(string->symbol "mytestvarname") ,1) (interaction-environment))
guile> mytestvarname
1
guile> 

> Meanwhile, can someone send me a hint?

Another way would be to use an associative list, a list of key and
value pairs, and get values using (cdr (assq KEY ASSOCLIST)).

guile> (define myassoc (list (cons 'key1 "val1") (cons 'key2 "val2")))
guile> myassoc
((key1 . "val1") (key2 . "val2"))
guile> (assq 'key1 myassoc)
(key1 . "val1")
guile> (cdr (assq 'key1 myassoc))
"val1"
guile> 

> If either of these mecanisms are documented somewhere,
> I'd gladly read it.

The guile info manual is great :)

-- 
Marco Parrone (marc0) <marc0@autistici.org> [0x45070AD6]

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Re: How do I really do this?

[Christopher Cramer]

On Wed, Mar 03, 2004 at 03:52:15PM -0800, Bruce Korb wrote:
> I've tried:
> 
>   (define (string->symbol (function-returning-string))
>           (value-function))

You need to use eval to do something like this.

guile> (eval `(define ,(string->symbol "foo") 'bar) (current-module))
guile> foo
bar

Not sure why you'd want to avoid explicitly using a hash table. That's
pretty much what define does anyway.

-- 
Christopher Cramer <crayc@pyro.net> <http://www.pyro.net/~crayc/>
In politics you have to understand not where the voters are when a poll
is taken, but where they are likely to end up on Election Day.
-- Rep. Tom Davis, former NRCC Chairman


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Re: How do I really do this?

[Thien-Thi Nguyen]

   From: Bruce Korb <bkorb@veritas.com>
   Date: Wed, 03 Mar 2004 15:52:15 -0800

   Meanwhile, can someone send me a hint?

an alist w/ strings as keys works w/ `equal?', `member' and `assoc-ref'.

thi


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