Re^2: False positive on inequality comparison

in reply to Re: False positive on inequality comparison
in thread False positive on inequality comparison

That's something I would never have thought of. But it worked perfectly. String comparison found no difference -- even when a numeric comparison did find a difference. Thanks for the idea.

Comment on Re^2: False positive on inequality comparison

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Re^3: False positive on inequality comparison by BrowserUk (Patriarch) on Apr 06, 2009 at 19:31 UTC
Using string comparison on floats is dangerous, unless you don't care if the floats are actually different! As this shows in the last line, Perl's default stringification for floats will only show you 6 decimal places of precision, anything beyond that accuracy and you are just pretending the are the same. `$first = unpack 'd', pack 'b64', '1001110001010011100010111001000010000101000001010110011000000010';; $second = unpack 'd', pack 'b64', '1010110001010011100010111001000010000101000001010110011000000010';; printf "%f %f :> %17.17f\n", $first, $second, $first - $second;; 181.019658 181.019658 :> 0.00000000000011369` [download] 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. "Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."	[reply] [d/l]

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Re^3: False positive on inequality comparison
by BrowserUk (Patriarch) on Apr 06, 2009 at 19:31 UTC

Using string comparison on floats is dangerous, unless you don't care if the floats are actually different!

As this shows in the last line, Perl's default stringification for floats will only show you 6 decimal places of precision, anything beyond that accuracy and you are just pretending the are the same.

$first = unpack 'd', pack 'b64', 
'1001110001010011100010111001000010000101000001010110011000000010';;

$second = unpack 'd', pack 'b64', 
'1010110001010011100010111001000010000101000001010110011000000010';;

printf "%f %f :> %17.17f\n", $first, $second, $first - $second;;
181.019658 181.019658 :> 0.00000000000011369
[download]

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.

"Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."

[reply]
[d/l]

In Section Seekers of Perl Wisdom