ameyer has asked for the wisdom of the Perl Monks concerning the following question:

I am attempting to Perlize a simple arithmetic expression that returns the pressure of CO2 gas in PSI needed to carbonate beer with a given number of gas volumes at a given temperature.

Volumes and Temperature are both known. So we have an expressions akin to: 

X = 1 + 2^3 + 4*5 - 6*7^8

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The full expression is: 

Pressures=-16.6999-(0.0101059*Temperature)+(0.00116512*(Temperature^2)
+)+(0.1733554*(Temperature)*(Volumes))+(4.24267*(Volumes))-(0.0684226*
+(Volumes^2))

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This works in Excel yielding a pressure of 12.27462975 for a temperature of 40 and 2.5 volumes of CO2. This does not work in Perl.. 

$temp = 40;
$vol = 2.5;
$pressure=(-16.6999)-(0.0101059*$temp)+(0.00116512*($temp^2))+(0.17335
+4*$temp*$vol)+(4.24267*$vol)-(0.0684226*($vol^2));

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Here I get a pressure of 10.88687404 which is definitely wrong. (I am matching values on a known table for testing).

Is this an issue with floating point precision? I couldn't find a floating point module... Please help.

Janitored by Arunbear - added code tags, and changed title from 'Problems with floating point arithmetic. Please Help!'

Replies are listed 'Best First'.
Re: Convert arithmetic expression from Excel to Perl syntax
by kvale (Monsignor) on Nov 04, 2004 at 18:02 UTC
    Floating point in perl is fine, but the exponentiation operator in perl is **, not ^. 
    $temp = 40;
$vol = 2.5;
$pressure=(-16.6999)-(0.0101059*$temp)+(0.00116512*($temp**2))+
   (0.173354*$temp*$vol)+(4.24267*$vol)-(0.0684226*($vol**2));

    [download]

    -Mark

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[d/l]
Re: Convert arithmetic expression from Excel to Perl syntax
by ikegami (Patriarch) on Nov 04, 2004 at 18:00 UTC
    ^ doesn't mean "to the power of" in Perl, it means "exclusively ored with". Change $temp^2 to $temp*$temp and $vol^2 to $vol*$vol.

    Update: As other pointed out, Perl does have a power operator: ** (although it's not quite as fast, as you can see in Benchmarks I posted below).

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[d/l]
[select]
      You can also di $temp**2 or $vol**2. Exactly the same, but faster.
[reply]
        You're right that ** works too, but you're wrong about it being faster (at least on my system). 
        # srand(0) is used to make sure both
# subs are using the same input set.

sub m1 {
   unless ($i1) { srand(0); $i1=1; }
   $n = rand;
   $p = $n ** 2;
}

sub m2 {
   unless ($i2) { srand(0); $i2=1; }
   $n = rand;
   $p = $n * $n;
}


use Benchmark;
Benchmark::cmpthese(0, {
   '$n**2' => \&m1,
   '$n*$n' => \&m2,
});


__END__

           Rate $n**2 $n*$n
$n**2  722292/s    --  -29%
$n*$n 1013884/s   40%    --

        [download]
[reply]
[d/l]
Re: Convert arithmetic expression from Excel to Perl syntax
by ameyer (Initiate) on Nov 04, 2004 at 18:08 UTC
    Thanks! That did it.
[reply]

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