php maths issues

[edzillion]

Hi

I am having some issues with multiplying doubles. I presume php isn't the best for maths, but I want to use it to keep things simple.

Basically:

If I multiply one double by another like so:

 

function format_price($price){
$doubresult = $price * 31.1034768;
return $doubresult;
}

 

format_price(15.7389); = 466.552152

format_price(15.7103); = 466.552152

 

I presume this is because of the way that php rounds down when multiplying numbers with a large amount of decimal places.

Any way of doing this accurately?

 

Thanks

Ed

 

 

 

 

 

 

 

 

 

[tinker]

it works better in php than my calc (on linux):

 

489.53451100752 (php)  or (489.534511008 calc)

488.64495157104 (php)  or (488.644951571 calc)

 

But you can typecast like this...

function format_price($price){
//$doubresult = doubleval($price * 31.1034768);
$doubresult = (double)$price * (double)31.1034768;
return $doubresult;
}
print format_price(15.7389)."<br>";
print format_price(15.7103)."<br>";

[hawkenterprises]

Just to further note

 

Everything is in PHP is a string, kinda like in ASP everything is a DIM until you do something that is TYPE Specific, like math functions or type casting.

 

$IamString = '42';
$IamString *= 24.5; // Now I'm an float (there are no doubles just the names for historical reasons in PHP)
$IamString .= 'two for tuesday';  // Now I'm a string again

 

So just remember PHP is type-less language unless explicitly defined.

 

[corbin]

Sorry for the offtopic-ness.

 

So if you do:

 

$num = 5;
echo var_dump($num);

 

Why does it say int?  Everything isn't a string, conversions just happen as needed.  For example, if you look into the incrementing code of the php, ($i++, $i += x), you'll see that at some point it checks if the new number is greater than the signed integer max (2^31-1) (I don't remember exactly how it checks, but I would imagine it would check if i is 1 less than the max since checking after it rolled over would cause issues lol), and if it is, it will convert it to a float.

 

In the same sense:

 

$i = 5;
$i2 = 10;
//both numbers are stored as integers
$new = $i*i2;
echo "{$i} * {$i2} = {$new}";
//or
echo $i . ' * ' . $i2 . ' = ' . $new;
//The integers are temporarily converted to strings so that they can be concatenated
//same sense:
$si = '5';
$si2 = '10';
$snew = $si*$si2; //$si and $si2 are converted to a numeric data type, in this case an integer so that they can be multiplied
echo "{$si} * {$si2} = {$snew}";
//in this case, si and si2 are already strings, but snew has to be temporarily converted to a string.

 

Therefore, I think it is incorrect to say PHP is a type-less language, but rather it should be said that PHP transitions between types as needed without explicit coder instruction.

 

Then again, I could be entirely wrong, and PHP does store everything in a string; that's just how I've gathered that it works.

 

 

 

On topic:

 

PHP has a certain setting for how far it keeps numerical accuracy (I think it defaults to 14), so any decimal places after that are just chopped off I think.

 

Example:

 

C:\Users\Corbin>php -r "echo (double)5.00000000000005;"
5
C:\Users\Corbin>php -r "echo (double)5.0000000000005;"
5.0000000000005

 

(It's worth noting that without the five, it handles the first one fine...)

 

So, once you get past a certain number of decimal places, it's very difficult to do mathematical operations accurately with the built-in php operators.

 

"I presume php isn't the best for maths"

 

In general, no computer language is good for very minute numbers.  C++, C (The 'engine' behind PHP is in C if I remember correctly), Java, and I'm sure other language would run into the exact same problem.

 

Every datatype has it's limitations math wise.  A signed integer's happens to be 2^31-1, while the double's max is gigantic (http://en.wikipedia.org/wiki/Double_precision) (The decimal max is dependant on certain factors, but in PHP it seems to always be the configuration directive).

 

(P.S. for number_format(15.7389), Windows calc (calc.exe) gives me 489.53451100752)

[hawkenterprises]

Sorry for the offtopic-ness.

So if you do:

$num = 5;

echo var_dump($num);

Why does it say int?  Everything isn't a string, conversions just happen as needed.  For example, if you look into the incrementing code of the php, ($i++, $i += x), you'll see that at some point it checks if the new number is greater than the signed integer max (2^31-1) (I don't remember exactly how it checks, but I would imagine it would check if i is 1 less than the max since checking after it rolled over would cause issues lol), and if it is, it will convert it to a float.

 

There is a difference between these two statements

 

$num = 5;

 

and

 

$num = '5';

 

Also it goes more as to how PHP stores the information that is really what I was trying to illustrate.  The fact is $num is typeless you are declaring type in the above to statements. 

[Orio]

Of course theres a difference. Types are juggled automatically though. If everything was typeless, there wouldn't be functions like is_string(), is_double(), is_bool() etc'.

 

Orio.

[corbin]

Sorry for the offtopic-ness.

So if you do:

$num = 5;

echo var_dump($num);

Why does it say int?  Everything isn't a string, conversions just happen as needed.  For example, if you look into the incrementing code of the php, ($i++, $i += x), you'll see that at some point it checks if the new number is greater than the signed integer max (2^31-1) (I don't remember exactly how it checks, but I would imagine it would check if i is 1 less than the max since checking after it rolled over would cause issues lol), and if it is, it will convert it to a float.

 

There is a difference between these two statements

 

$num = 5;

 

and

 

$num = '5';

 

Also it goes more as to how PHP stores the information that is really what I was trying to illustrate.  The fact is $num is typeless you are declaring type in the above to statements. 

 

No, $num is not typless unless you delare the type.  In the world of computers, there's no such thing as typeless.

 

"There is a difference between these two statements"

 

That's what I was saying!

 

"Everything is in PHP is a string, kinda like in ASP everything is a DIM until you do something that is TYPE Specific, like math functions or type casting."

 

That's what I was trying to correct, because that is flat out wrong.

 

"So just remember PHP is type-less language unless explicitly defined."

 

Also flat out wrong.