On Wed, Aug 4, 2010 at 2:53 AM, Jacob Todd <jaketodd...@gmail.com> wrote:
> In K&R, chapter 6, section 6, there is a funtion called hash that hashes a
> string, which will be stored in a linked list. The function in question is on
> page 144, but here it is for those of you who don't have K&R handy.
>
> /* hash: form hash value for string s */
> unsigned
> hash(char *s)
> {
> unsigned hashval;
>
> for(hashval = 0; *s != '\0'; s++)
> hashval = *s + 31 * hashval;
> return hashval % HASHSIZE;
> }
>
> So what is the purpose of the magic 31? The only thing that I think might be a
> reference to what it may be for is the paragraph prior, which states
>
> The hashing function, ..., adds each character value in the string to a
> scrambled combination of the previous ones and returns the remainder
> modulo the array size.
>
> Does the magic 31 have to do with scrambling?
>
It's also worth remembering that K & R was written at a time many
decades ago when performance aspects of computer architecture were a
lot, lot simpler. Apparently they have
#define HASHSIZE 101
which given that there's no really efficient way of computing % for
arbitrary numbers is going to be quite slow (particularly if the
strings are short), which is why hashes for modern machines use table
sizes that avoid needing a mod. (There are other things that are slow
on modern architectures that modern hash functions avoid.) I'd use K&R
for the C syntax and some of the higher level ideas of programming,
but not try to understand good hashing technology from there.
--
cheers, dave tweed__________________________
computer vision reasearcher: david.tw...@gmail.com
"while having code so boring anyone can maintain it, use Python." --
attempted insult seen on slashdot
