String comparison of numbers

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

This issue came up because I was setting some values using the rand() function, then requesting them through a SOAP request where the were received as strings. A numeric comparison between two numers that were seemingly the same failed:

# these two variables are indentical when printed to STDOUT
$var1 = rand(1);
$var2 = $soapres->dataof('//path/to/elem/')->attr->{attr};

if ( $var1 == $var2 ) {
    print "all is well\n";
} else {
    print "got problems?\n";
}
[download]

The comparison invariably failed. However, when I did a string comparison, it succeeded.

# these two variables are indentical when printed to STDOUT
$var1 = rand(1);
$var2 = $soapres->dataof('//path/to/elem/')->attr->{attr};

if ( $var1 eq $var2 ) {
    print "all is well\n";
} else {
    print "got problems?\n";
}
[download]

I'm guessing that this has something to do with the internal representation of the value returned by rand(). I'm guessing that the value is stored internally with a greater precision, so the numeric comparison between that value and the "truncated" value fails.

Of course...this is just my theory.

dsb
This @ISA my( $cool ) %SIG

Comment on String comparison of numbers Select or Download Code

Replies are listed 'Best First'.
Re: String comparison of numbers by philcrow (Priest) on Mar 09, 2006 at 15:21 UTC
When comparing floating point numbers, you should either round them both to the same precision and compare or subtract one from the other and see if the abs of the result is small: `if ( abs( $number1 - $number2 ) < $something_tiny ) { print "all is well.\n"; }` [download] Numerical analysts usually prefer the latter many programmers prefer the former. Phil	[reply] [d/l]
Re^2: String comparison of numbers by dsb (Chaplain) on Mar 10, 2006 at 20:14 UTC
Well, the two arguements need to be exactly identical. So, if that means using a truncated version of the `rand()` value, then that's what I'll do. I'm more curious as to if my theory is correct. dsb This `@ISA my( $cool ) %SIG`	[reply]


go ahead... be a heretic
	PerlMonks