Results depending on evaluation order

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

Hi Monks,

I was trying the code from Okay! What!?!?!?

my $value1 = 14.4;
print "value1 = $value1\n";

my $value2 = 10 + 14.4 - 10;
print "value2 = $value2\n";

if ( $value1 == $value2 ) {
    print "value 1 equals value 2\n";
} else {
    print "value 1 does not equal value2\n";
}
[download]

Output:

value1 = 14.4
value2 = 14.4
value 1 does not equal value2
[download]

and from My floating point comparison does not work. Why ?

$number = 1.80;
$premium = $number * ( 1 + 10/100 );
$expected = 1.98;
print "Number 1 : $premium \n";
print "Number 2 : $expected \n";
print "Not" if $expected != $premium;
print "Equal !! ";
[download]

Output:

Number 1 : 1.98
Number 2 : 1.98
NotEqual !!
[download]

when I came up with the following behavior (tested on Perl 5.10.1 and on 5.18.2):

Swapping operands on the first sample code:

my $value2 = 10 + 14.4 - 10;          # wrong evaluated
[download]

my $value2 =  14.4 + 10 - 10;         # wrong evaluated
[download]

my $value2 = 10 - 10 + 14.4;          # right evaluated
[download]

Using eval:

my $value2 = eval (10 + 14.4 - 10);   # right evaluated
[download]

Using eval on the second sample code:

$number = 1.80;
$premium = eval ($number * ( 1 + 10/100 ));
$expected = 1.98;
print "Number 1 : $premium \n";
print "Number 2 : $expected \n";
print "Not" if $expected != $premium;
print "Equal !! ";
[download]

Output:

Number 1 : 1.98
Number 2 : 1.98
Equal !!
[download]

So, I got odd results depending on the order of the operands and right results on evaluating.

How can that be?

Comment on Results depending on evaluation order Select or Download Code

Replies are listed 'Best First'.
Re: Results depending on evaluation order by LanX (Saint) on Apr 23, 2014 at 16:38 UTC
left to right evaluation: `10 - 10 + 14.4 = (10 - 10) + 14.4 = 0 + 14.4 = 14.4` I already explained that the mantissa needs to be shifted to loose precision. Cheers Rolf ( addicted to the Perl Programming Language)	[reply] [d/l]
Re^2: Results depending on evaluation order by fishy (Friar) on Apr 23, 2014 at 16:50 UTC
Interesting. Thanks Rolf! What about eval? Why it got right evaluated?	[reply]
Re^3: Results depending on evaluation order by moritz (Cardinal) on Apr 23, 2014 at 17:51 UTC
eval expects a string, so the number is converted to a string, and the stringification rounds a tiny bit, so it accidentally produces the right result.	[reply]
Re^4: Results depending on evaluation order by choroba (Cardinal) on Apr 23, 2014 at 20:54 UTC
Re: Results depending on evaluation order by InfiniteSilence (Curate) on Apr 23, 2014 at 18:47 UTC
There is a difference between what is represented and what is actually being stored, `DB<11> p sprintf("%.8f", $expected); 1.98000000 DB<10> p sprintf("%.8f", $premium); 1.98000000 DB<12> p ($expected == $premium) DB<13>` [download] When you subtract the two the difference is exposed: `DB<16> p ($expected - $premium) -2.22044604925031e-16` [download] But it is very small. Let's lop off the unnecessary information: `DB<13> $pa = 1.98 DB<14> $pb = 1.98 DB<15> p ($pb == $pb) 1 DB<17> p ($pb - $pa) 0 DB<18> DB<22> $exp = sprintf("%.8f", $expected) DB<23> $pre = sprintf("%.8f", $premium) DB<24> p ($pre - $exp) 0` [download] So you see if you set limits on what Perl stores you should get more accurate comparisons. Celebrate Intellectual Diversity	[reply] [d/l] [select]


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