large float's

[yawnmoth]

According to php.net, the largest possible float is ~1.8e308.
wikipedia.org offers more insight, suggesting that is, exactly,
0x7FEFFFFFFFFFFFFF. This sorta begs the question... how can this
number be represented in PHP without resorting to the decimal notation?

<?
echo 0x7FEFFFFFFFFFFFFF;
?>

....doesn't work. Nor does...

<?
echo hexdec('7FEFFFFFFFFFFFFF');
?>

So... any ideas as to what will work?

[Oli Filth]

yawnmoth said the following on 23/02/2006 06:00:

According to php.net, the largest possible float is ~1.8e308.
wikipedia.org offers more insight, suggesting that is, exactly,
0x7FEFFFFFFFFFFFFF. This sorta begs the question... how can this
number be represented in PHP without resorting to the decimal notation?

The maximum floating-point value is platform dependent. For a 64-bit
platform, it's ~1.8e308, but for a 32-bit platform it's ~3.4e38
(assuming you're using the IEEE 754 standard).

This code converts from a hex-representation to a floating-point
representation (for 32-bit):
define("LENGTH", 32);
define("SIGN_BIT", 31);
define("EXPONENT_MSB", 30);
define("EXPONENT_LSB", 23);
define("MANTISSA_MSB", 22);
define("MANTISSA_LSB", 0);

function hex2float($strHex)
{
$dec = hexdec($strHex);
$sign = ($dec & (1 << SIGN_BIT)) != 0;
$exp = (($dec & ((2 << EXPONENT_MSB) - (1 << EXPONENT_LSB))) >>
EXPONENT_LSB)
- (1 << (EXPONENT_MSB - EXPONENT_LSB))
- (MANTISSA_MSB - MANTISSA_LSB);
$man = (($dec & ((2 << MANTISSA_MSB) - (1 << MANTISSA_LSB))) >>
MANTISSA_LSB)
+ (2 << (MANTISSA_MSB - MANTISSA_LSB));
$float = floatval($man * pow(2, $exp) * ($sign ? -1 : 1));
}
For a 64-bit version, you'll need to alter the constants; you can get
the required values from
http://stevehollasch.com/cgindex/coding/ieeefloat.html.

Note that this code doesn't deal with special cases, i.e. NaN, infinity,
etc.

--
Oli

[Chung Leong]

Oli Filth wrote:

The maximum floating-point value is platform dependent. For a 64-bit
platform, it's ~1.8e308, but for a 32-bit platform it's ~3.4e38
(assuming you're using the IEEE 754 standard).

Actually the bit-ness of a CPU only refers to its integer handling
capability. Nearly all FPUs--even the ancient 8087--can handle both
single and double precision floating points.

Float points in PHP are defined as doubles, which in C are always
64-bit IIRC.