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Re: Re: Re: Flip Flop III - Musical Buckets

by bobione (Pilgrim)
on Apr 25, 2001 at 18:04 UTC ( #75454=note: print w/replies, xml ) Need Help??


in reply to Re: Re: Flip Flop III - Musical Buckets
in thread Flip Flop III - Musical Buckets

It's supposed to help me ? :)
Why %_ have this index order ???
In the first line, why the 9 come between the 2 and the 3 ? ---> 1239345678 , Why this position ?
After what on the second line, the 8 take place one number before the 9 ---> 128394567.
On third line 7 go one number before the 8 ---> 172839456...
It could be the rule but I am not sure at all !

BobiOne
Can Perl remain obfuscated for me ? ... No

  • Comment on Re: Re: Re: Flip Flop III - Musical Buckets

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Re (tilly) 4: Flip Flop III - Musical Buckets
by tilly (Archbishop) on Apr 25, 2001 at 21:17 UTC
    As it was my explanation in chatter that inspired this obfuscation, here is a quick explanation.

    A hashing function is a function that takes input (usually a string) and gives back a number in some range.

    A hash lookup algorithm takes keys, passes the keys to a hashing function, and then uses that number to select a bucket, then stores the key/value in the bucket in some way. Hopefully the keys are pretty much randomly distributed so each bucket has few things in it. Details vary, there are different hashing algorithms you can use, different ways to store things in buckets, often there is some dynamic decision on how many buckets to use, etc.

    Perl's hashes are called hashes because they use a hash lookup algorithm to store data. That is why lookups are fast. But when you ask for them back (with keys or values) it just walks the buckets, and returns things out of the buckets. The order you get back depends on all sorts of things, starting with the hashing function and number of buckets...

    As a test, anyone who can understand Re: Re: Shift, Pop, Unshift and Push with Impunity! probably understood this explanation perfectly.

    Anyways, the order that stuff comes back is entirely determined by internal details, looks random, but isn't. So if you figure out what order keys will give for (for instance) a series of numbers, then just put the values in matched to where you want them to go and voila! You get output sorted from values just right for no apparent reason! :-)

Re: Re: Re: Re: Flip Flop III - Musical Buckets
by c-era (Curate) on Apr 25, 2001 at 19:23 UTC
    Hash keys are not stored in order. They are stored by the fastest way to retrive the keys/values. If you want to know the exact reason why it is in that order, you will either need to look at the perl source code and figure it out, or talk with someone that <bold>REALLY</bold> knows the internals of perl. Also, you will notice that order is different in different versions of perl.
Re: Re: Re: Re: Flip Flop III - Musical Buckets
by larryk (Friar) on Apr 27, 2001 at 16:20 UTC
    I think it's got something to do with the order in which Perl stores hash keys.

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