> -----Original Message-----
> From: Bryan R Harris [mailto:[EMAIL PROTECTED]]
> Sent: Monday, June 24, 2002 2:58 PM
> To: [EMAIL PROTECTED]
> Subject: RE: when to use hashes...
>
>
>
> > Use a hash when you want to access data by a key.
> >
> >> (like "if you have over 500,000 items, use a 2-d
> >> array instead of a hash")
> >
> > Perl doesn't have 2-d arrays.
>
> Sure it does, it just takes a little more work to implement them. =)
Nah, really. It doesn't.
>
>
> > If you have 500,000 of something, that gets in to
> > the realm of "a lot". A database may be the best
> > answer. Or not. It depends...
>
> So about how many keys do you have to have before you start
> questioning
> whether you should be using a hash? Is 15,000 keys too many?
>
> I've got lots of data files with 15000+ x,y,z values. The
> x,y values are
> the same, but the z's are different between the files. I
> want to output
> only data points where ALL the z's in all files for a given
> x,y are greater
> than 0. Should I load the x,y values as hash keys, and the
> max z values as
> the "value"?
Well, maybe that's the right thing to do. How are the files organized?
Are the x,y pairs in the same sequence in each file:
File 1 File 2
------ ------
10,15,8 10,15,-2
-14,22,3 -14,22,612
19,47,52 19,47,18
If so, you could just read a line from file 1 and then read a line
from file 2, knowing that the x,y from file 2 corresponds to the x,y
from file 1. Then compare the Z's and print if they fall within your
limits. No need for a data structure at all.
OTOH, if the lines in File 2 are unpredictable with respect to those
in file 1, then yes, you'll most likely want a hash.
$hash{$x}{$y} = $z;
Now when I read a line from file 2, I can find file 1's Z value by
a hash lookup: $hash{$x}{$y};
If the values for x and y are all non-negative integers less than some
value (2^32 would be an absolute upper limit, I supposed), you could
use array indexes instead of hashing:
$arr[$x][$y] = $z;
But this kind of thing is *not* the typical situation where folks
are being encouraged to use hashes instead of searching through
arrays... Which is why, as always, *It Depends*
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