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Re^3: Normalizing a range of numbers to a percentage

by BrowserUk (Patriarch)
on Mar 09, 2019 at 04:38 UTC ( #1231084=note: print w/replies, xml ) Need Help??


in reply to Re^2: Normalizing a range of numbers to a percentage
in thread Normalizing a range of numbers to a percentage

the ord() and substr() don't need to do float calculations.

In this$_ * $const first the integer in $_ is promoted to a double (to match the type of $const), then the multiplication is done, and then (to make it useful for the OP though you aren't doing it here) the result needs to be converted (trunc'd) back to an integer. (If you added back that necessity, the difference would be more marked.)

Runtime memoization and look up using a hash (per the Memoize module) would be much slower because each input integer needs to be be converted to a string, then that string must be hashed, then taken modulo the hash size (which must be looked up, then that table entry inspected, and (potentially) a linear search of an array performed, before the value is found.

The lookup essential consists of a direct index and done.

For the OPs purpose, an array lookup would probably be even quicker:

#! perl -slw use strict; my @lookup = map{ int( $_ / 255 * 100 ) } 0 .. 255; print "$_ :: ", $lookup[ $_ ] for 0 .. 255;

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Re^4: Normalizing a range of numbers to a percentage
by Lotus1 (Vicar) on Mar 09, 2019 at 17:08 UTC

    Thanks for the explanation about the float conversions and hash memoization.

    The array lookup is quite a bit faster than the string index. I didn't see in the OP that the result had to be integers. The OP showed floats as the output. Edit: The array lookup method will work equally well with either floats or ints since the int() function is not in the part that does the lookup.

    #!/usr/bin/env perl use warnings; use strict; use Benchmark 'cmpthese'; my $lookup = join '', map{ chr( $_ / 255 * 100 ) } 0 .. 255; my @lookup = map{ int( $_ / 255 * 100 ) } 0 .. 255; my $const = 100 / 255; cmpthese(-2, { lookup => sub { my @output = map { ord( substr $lookup, $_, 1 )} 0 .. 255; }, calc => sub { my @output = map {$_ / 255 * 100} 0 .. 255; }, calc2 => sub { my @output = map {$_ * $const} 0 .. 255; }, arraylookup => sub { my @output = map { $lookup[ $_ ] } 0 .. 255;; }, }); my @output1 = map { ord( substr $lookup, $_, 1 )} 0 .. 5; my @output2 = map {$_ * $const} 0 .. 5; print "@output1\n"; print "@output2\n"; __END__ # Results on my machine (v5.22.1 built for MSWin32-x64-multi-thread): Rate calc calc2 lookup arraylookup calc 11990/s -- -25% -31% -47% calc2 15929/s 33% -- -9% -30% lookup 17496/s 46% 10% -- -23% arraylookup 22838/s 90% 43% 31% -- 0 0 0 1 1 1 0 0.392156862745098 0.784313725490196 1.17647058823529 1.5686274509803 +9 1.96078431372549

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