RE: [PHP] Strange math results
> -Original Message-
> From: Satyam [mailto:[EMAIL PROTECTED]
> Sent: Tuesday, March 28, 2006 17:23
> To: [EMAIL PROTECTED]; Jay Blanchard
> Cc: Jeff; php-general@lists.php.net
> Subject: Re: [PHP] Strange math results
>
>
> Indeed, when doing floating point math, you cannot check the
> values for
> equality, they will rarely be, you have to check whether the
> difference in
> between them is less than the error you are willing to
> accept. Floating
> point numbers are usually an approximation to the actual value and
> intermediate operations might pile up errors in each approximation in
> different ways.
>
> As an example,
>
> $third = 1/3;
>
> $third will contain 0.3 up to a certain number of digits,
> but it won't
> last forever, as it should, theoretically.
>
> Now,
>
> $third * 3 == 1
>
> will be false, but:
>
> define('MAXERR',0.001);
> abs($third * 3 - 1) < MAXERR
>
> will be true.
>
> MAXERR is the error you are willing to accept in your
> calculations, for
> example, if you are using cents, give it a couple of extra
> positions so that
> roundings don't creep up to significant cents, that is, use
> MAXERR = 0.0001
> or thereabouts.
>
> Satyam
>
That explains it. Thanks!
Jeff
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Re: [PHP] Strange math results
Indeed, when doing floating point math, you cannot check the values for
equality, they will rarely be, you have to check whether the difference in
between them is less than the error you are willing to accept. Floating
point numbers are usually an approximation to the actual value and
intermediate operations might pile up errors in each approximation in
different ways.
As an example,
$third = 1/3;
$third will contain 0.3 up to a certain number of digits, but it won't
last forever, as it should, theoretically.
Now,
$third * 3 == 1
will be false, but:
define('MAXERR',0.001);
abs($third * 3 - 1) < MAXERR
will be true.
MAXERR is the error you are willing to accept in your calculations, for
example, if you are using cents, give it a couple of extra positions so that
roundings don't creep up to significant cents, that is, use MAXERR = 0.0001
or thereabouts.
Satyam
- Original Message -
From: "Ezra Nugroho" <[EMAIL PROTECTED]>
It is not always possible to precisely represent decimal values as a
float type in binary. In these cases, the value that you have is either
slightly bigger or slightly smaller than the actual.
In your specific problem, you have two values that are not equal, but
very similar. Therefore you get a small negative value when subtracting
them.
You just have to keep this fact in mind whenever you want to do high-
precision computing.
On Tue, 2006-03-28 at 15:30 -0600, Jay Blanchard wrote:
[snip]
I've got a strange problem here.
I'm subtacting one variable from another, both of type "double" and if
they are the same, instead of getting a result of 0, I get something
like -9.99200722163E-016
Is this a bug or am I doing something wrong here?
[/snip]
From all of the information that you sent in your post I'd say that
you're doing something wrong here.
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RE: [PHP] Strange math results
It is not always possible to precisely represent decimal values as a float type in binary. In these cases, the value that you have is either slightly bigger or slightly smaller than the actual. In your specific problem, you have two values that are not equal, but very similar. Therefore you get a small negative value when subtracting them. You just have to keep this fact in mind whenever you want to do high- precision computing. On Tue, 2006-03-28 at 15:30 -0600, Jay Blanchard wrote: > [snip] > I've got a strange problem here. > > I'm subtacting one variable from another, both of type "double" and if > they are the same, instead of getting a result of 0, I get something > like -9.99200722163E-016 > > Is this a bug or am I doing something wrong here? > [/snip] > > From all of the information that you sent in your post I'd say that > you're doing something wrong here. > -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
RE: [PHP] Strange math results
[snip] I've got a strange problem here. I'm subtacting one variable from another, both of type "double" and if they are the same, instead of getting a result of 0, I get something like -9.99200722163E-016 Is this a bug or am I doing something wrong here? [/snip] >From all of the information that you sent in your post I'd say that you're doing something wrong here. -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
