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Re^6: [OT] The statistics of hashing. (odd)

by tye (Cardinal)
on Apr 03, 2012 at 15:43 UTC ( #963269=note: print w/ replies, xml ) Need Help??


in reply to Re^5: [OT] The statistics of hashing.
in thread [OT] The statistics of hashing.

I suspect you need to start $toggle reversed for odd values (or for even values, though you note that you verified against the two even cases). Though, I would have expected that particular mistake to lead to negative values (but you don't show most of the code so I didn't go much further in looking). The numbers are supposed to be f(...)**$P where f() is positive (and monotonic), so, yes, odd values of $P should give you results between the results for $P-1 and $P+1, not like the results you posted.

Sorry for the wrapping.

You might want to put the big tables into READMORE tags so that they don't impact the rendering of the whole thread (though that should only happen for people who have code wrapping badly configured).

- tye        


Comment on Re^6: [OT] The statistics of hashing. (odd)
Re^7: [OT] The statistics of hashing. (odd)
by BrowserUk (Pope) on Apr 03, 2012 at 16:07 UTC
    I suspect you need to start $toggle reversed for odd values

    Indeed. Using my $toggle = $P & 1 ? 1 : -1;, the output makes much more sense:

    you don't show most of the code so I didn't go much further in looking

    The entire code looks like this:

    #! perl -slw use strict; sub PTn { my @row; for( 1 .. shift ) { push @row, 1; $row [$_] += $row [$_ - 1] for reverse 1 .. @row - 2; } return @row; } sub expN { my( $P, $N, $X ) = @_; my $xDIVn = $X / $N; my @coefs = PTn( $P+1 ); my $rv = 0; my $toggle = $P & 1 ? 1 : -1; for my $p ( reverse 1 .. $P ) { $rv += $toggle * $coefs[ $p ] * $N / $p * exp( -$p * $xDIVn ); $toggle *= -1; } return $rv + $X; } our $H //= 10; our $B //= 32; print "Using N x 2**$B vectors:"; for my $inserts ( map{ 2**$_*3/4, 2**$_ } 4 .. $B ) { printf "%10d : ", $inserts; for my $h ( 1 .. $H ) { printf "%6d ", expN( $h, 2**$B, $inserts ) - expN( $h, 2**$B, +0 ); } print ''; }

    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.

    The start of some sanity?

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