Re: Hash tables

[Robert Miyashiro <robert at mercedsystems dot com>]

Sorry, I misread your email.  The (values) confuses me a bit.  Do you mean
to use a (begin) there instead?


On 1/16/08 10:08 AM, "Robert Miyashiro" <robert@mercedsystems.com> wrote:

> The key 53gqi _does_ have a value, (cdr '("53gqi")) => '().  So, your
> default doesn't get used.  The default is only used if there is no mapping
> for the key at all:
> 
> alist->hash-table alist [ equal? [ hash [ size-hint]]] → hash-table
> 
>     Takes an association list alist and creates a hash table hash-table
> which maps the car of every element in alist to the cdr of corresponding
> elements in alist. The equal?, hash, and size-hint parameters are
> interpreted as in make-hash-table. If some key occurs multiple times in
> alist, the value in the first association will take precedence over later
> ones. (Note: the choice of using cdr (instead of cadr) for values tries to
> strike balance between the two approaches: using cadr would render this
> procedure unusable for cdr alists, but not vice versa.)
> 
> 
> 
> hash-table-ref hash-table key [ thunk ] → value
> 
>     This procedure returns the value associated to key in hash-table. If no
> value is associated to key and thunk is given, it is called with no
> arguments and its value is returned; if thunk is not given, an error is
> signalled. Given a good hash function, this operation should have an
> (amortised) complexity of O(1) with respect to the number of associations in
> hash-table.
> 
> Function: hash-table-ref/default hash-table key default → value
> 
>     Evaluates to the same value as (hash-table-ref hash-table key (lambda ()
> default)). Given a good hash function, this operation should have an
> (amortised) complexity of O(1) with respect to the number of associations in
> hash-table.
> 
> 
> 'robert
> 
> 
> On 1/16/08 10:01 AM, "Daniel Terhorst" <daniel.s.terhorst@gmail.com> wrote:
> 
>> I ran into some unexpected behavior with hash tables today, rather by
>> coincidence, it seems. If I create a hash-table with two keys, "53gqi"
>> and "6vxk4", I become unable to delete the first from the table. If I
>> change the keys in any way or try to remove the second one first, it
>> works as expected, so this seems to be due to some kind of collision.
>> I wouldn't expect that to cause delete to no-op, however, so I suspect
>> it's just a bug that hasn't been detected until now.
>> 
>> 
>> 
>> ;; -*- scheme -*-
>> 
>> (require 'srfi-69)
>> 
>> (let ((H (alist->hash-table '(("53gqi")
>>                               ("6vxk4"))))
>>       (key "53gqi"))
>>   (values (html:p (hash-table-ref H key))
>>           (hash-table-delete! H key)
>>           (html:p (hash-table-ref/default H key "GONE =)"))))
>> 
>> #\newline
>> 
>> 
>> 
>> ---
>> 
>> Expected:
>> ()
>> GONE =)
>> 
>> 
>> Actual:
>> ()
>> ()
>