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- Subject
- Posted on
- ROUND
- 12-21-2006
posted on
December 21, 2006, 4:49 pm
December 21, 2006, 4:49 pm
echo "round 20.545 -".round(20.545,2)."<br>";
echo "round 20.555 -".round(20.555,2)."<br>";
echo "number_format 20.545 -".number_format(20.545, 2, ',',
'.')."<br>";
echo "number_format 20.555 -".number_format(20.555, 2, ',',
'.')."<br>";
PHP Version 4.3.0 / FreeBSD
round 20.545 -20.55
round 20.555 -20.56
number_format 20.545 -20,55
number_format 20.555 -20,55
PHP Version 4.4.4 / CENTOS
round 20.545 -20.55
round 20.555 -20.55
number_format 20.545 -20,55
number_format 20.555 -20,55
PHP Version 5.1.4 / Windows NT
round 20.545 -20.55
round 20.555 -20.56
number_format 20.545 -20,55
number_format 20.555 -20,56
I know that the round always goes for the odd but this is not
consistence with the SO running PHP
Thanks
Re: ROUND
My only observation about the examples you chose is that the fractional
parts are infinitely repeating "radiximals" in binary.
0.545 = 545/1000 = 109/200 (irreducible)
0.555 = 555/1000 = 111/200 (irreducible)
Both of the numbers above can't be expressed as precise binary floating
point numbers because the denominator after reduction has prime factors that
are not "2".
PHP.INI may be a factor, but it may also be that something at a lower level
is occurring (i.e. machine arithmetic, how certain rounding settings on
floating-point processors or libraries are set).
In other words, you've chosen antagonistic examples.
Why don't you try 7/8 (0.875), 15/16 (0.9275), or 31/32 (0.96875) as the
fractional part of the numbers and see if you can get the same behavior.
I'm guessing that you may not.
Any fraction with a reasonable denomiator (< 2^16) that is a power of 2
would be a reasonable test case.
Note that these are all exactly representable by a typical machine.
Just a guess.
Re: ROUND
PHP code
echo "round(10.045,2) - ".round(10.045,2)."<br />";
echo "round(20.045,2) - ".round(20.045,2)."<br />";
echo "round(30.045,2) - ".round(30.045,2)."<br />";
echo "round(40.045,2) - ".round(40.045,2)."<br />";
This is very strange:
round(10.045,2) - 10.04
round(20.045,2) - 20.05
round(30.045,2) - 30.05
round(40.045,2) - 40.05
echo "round(10.045,2) - ".round(10.045,2)."<br />";
echo "round(20.045,2) - ".round(20.045,2)."<br />";
echo "round(30.045,2) - ".round(30.045,2)."<br />";
echo "round(40.045,2) - ".round(40.045,2)."<br />";
This is very strange:
round(10.045,2) - 10.04
round(20.045,2) - 20.05
round(30.045,2) - 30.05
round(40.045,2) - 40.05
Re: ROUND
kkmigas@gmail.com wrote:
> Has i told you on the other newsgroup
>
> This 2 servers have the same hardware configuration
>
> On Windows 2003
> http://efeito.org/info.php
>
> On Linux
> http://efeito.net/info.php
>
(Top posting fixed)
And as I and others have told you - when using floating point, results
when using expressions like these are unpredictable.
It's just like when using base 10:
round((1.0/3.0) + (1.0/6.0))
can equal zero, because neither 1.0/3.0 nor 1.0/1.6 can be expressed as
an exact value in base 10. They are repeating decimals.
Sure, they *should* come out to 0.5. But their actual value will be
0.499999999 to however many decimal places could be handled.
*Some* libraries will "fix" this by adding a small value (i.e.
0.000000001) to the results to "fudge" the value. But no such fudge
factor is required, nor is it always good.
You are always best to use integers if you need exact values, or add
your own fudge factor to the results.
It's been that way for the almost 40 years I've been programming, and
it's not going to change now. That's why many processors now have a
"packed decimal" or a "binary coded decimal" type internally to give
exact results.
--
==================
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attglobal.net
==================
> Has i told you on the other newsgroup
>
> This 2 servers have the same hardware configuration
>
> On Windows 2003
> http://efeito.org/info.php
>
> On Linux
> http://efeito.net/info.php
>
(Top posting fixed)
And as I and others have told you - when using floating point, results
when using expressions like these are unpredictable.
It's just like when using base 10:
round((1.0/3.0) + (1.0/6.0))
can equal zero, because neither 1.0/3.0 nor 1.0/1.6 can be expressed as
an exact value in base 10. They are repeating decimals.
Sure, they *should* come out to 0.5. But their actual value will be
0.499999999 to however many decimal places could be handled.
*Some* libraries will "fix" this by adding a small value (i.e.
0.000000001) to the results to "fudge" the value. But no such fudge
factor is required, nor is it always good.
You are always best to use integers if you need exact values, or add
your own fudge factor to the results.
It's been that way for the almost 40 years I've been programming, and
it's not going to change now. That's why many processors now have a
"packed decimal" or a "binary coded decimal" type internally to give
exact results.
--
==================
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attglobal.net
==================
Re: ROUND
Why are you posting this code and quoting my post?
.045 = 45/1000 = 9/200 = denominator not a power of 2.
Like your earlier examples, these numbers are not exactly expressable as
base-2 decimals.
It would be interesting to determine the breakpoint between
where the rounding changes. I'm going to guess that 15.045 is one way and
16.045 is the other.
Dav.e
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