ID: 27744
Updated by: [EMAIL PROTECTED]
Reported By: t dot steve at ariadne-quatra dot com
Status: Bogus
Bug Type: Math related
Operating System: *
PHP Version: *
New Comment:
'Exact' is not the same thing as 'no fractional part'. For numbers
sufficiently large, the least significant digits can represent hundreds
or thousands or greater, but this doesn't mean those digits are
accurate.
Some numbers *can* be represented exactly, but the fact that they can't
be distinguished from numbers that can *not* be represented exactly is
the whole point in fp arithmetic. Multiplying by a large number cannot
restore the accuracy that was lost by storing the (exact) number in a
double.
BTW PHP already uses doubles internally for its float type.
Previous Comments:
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[2004-03-31 11:06:06] garbo_doe at hotmail dot com
I'm sorry I keep posting :) (Maybe I'm just stupid. This will be my
last post, maybe I'm plain wrong.)
>Reducing precision during the calculation would magnify >rounding
errors greatly when doing a series of operations >on the floating point
values.
Yes. But for subtraction, the difference can never have "more decimals"
(precision) than either its minuend or subtrahend, anything else
introduces more error in multiple operations. Or?
In C++, if you substract a float (750.00f) from a float (749.99f), it
will result in 0.0100098. *However*, if you do the same with double,
insted of floats, it would result in 0.01 (I think). The first one,
float substraction, is probably faster, the second is more "accurate".
(I don't think that type of speed optimization is relevant to a
php-script?)
Another option is ((a * 100) - (b * 100)) / 100;
"100" would have to be modified to the "greatest" precision of either a
or b. This will not "magnify rounding errors", because it will be
exact.
I'm only talking about subtraction.
What I'm looking for is somthing like "AUTO_PRECISION = yes" and it
would do something like:
$old = ini_set('precision', max_decimals(750, 749.99));
echo (string)(750 - 749.99);
ini_set('precision',$old);
or use a C++ double instead of float (?).
Thanks for your patience! :)
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[2004-03-31 10:55:26] t dot steve at ariadne-quatra dot com
Guys, guys, guys... :)
I want to make one thing clear: No offense was meant, and I still think
PHP is great, and that I learned something new - even though I think
this is probably not the correct forum for that. :)
I never imagined this would turn out to be such a long discussion - it
seems I accidentally touched on something that others came across too.
Just as pont of interest: We also use PHP for our (travel agency..)
intra+extranet admin system, and among other things, we use
PHP-generated forms to order hotel rooms for our clients. At times we
get forms requesting a hotel room for "3.9999999.. nights...". :)) Now
I know why.
Thanks again for all the reactions, and keep up the excellent work! :)
------------------------------------------------------------------------
[2004-03-31 10:28:25] [EMAIL PROTECTED]
For a computer there is simply no such thing as exactly 0.23, it is as
simple as that. When displaying floating point numbers, either use
printf() and specify the number of significant digits you want or use
PHP's precision setting.
As for this suggestion:
> Maybe a magicless solution would be: don't use greater
> precision then either the minuend or subtrahend (the one
> with greatest precision).
Reducing precision during the calculation would magnify rounding errors
greatly when doing a series of operations on the floating point values.
You want to apply lower precision at the end at display time, not
during the calculation. And at display time we don't know what the
precicion of the operands were that led to this value which is why you
need to explicitly express what precision you want values displayed at.
------------------------------------------------------------------------
[2004-03-31 06:41:40] garbo_doe at hotmail dot com
echo (string)(75000.00 - 74999.00); returns 1, not 0.9999999999. Is
this a bug then? ;)
"Guys, how in the world is PHP supposed to magically guess what
precision you want results displayed in."
Maybe a magicless solution would be: don't use greater precision then
either the minuend or subtrahend (the one with greatest precision).
Thanks for you answers and for PHP - it's great! :-)
(I hope I'm not too annoying continuing this discussion...I sense some
irritation, "we have already explained", on this matter :-)
------------------------------------------------------------------------
[2004-03-31 03:12:23] [EMAIL PROTECTED]
Since you don't believe us:
http://docs.sun.com/source/806-3568/ncg_goldberg.html
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the rest of the comments, please view the bug report online at
http://bugs.php.net/27744
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