#49715 [Bgs]: float to string conversions

ID: 49715 User updated by: terrafrost at gmail dot com Reported By: terrafrost at gmail dot com Status: Bogus Bug Type: Scripting Engine problem Operating System: Windows XP PHP Version: 5.2.11 New Comment: Maybe you should reread my bug report instead of giving knee jerk reactions. Do you even know what a mantissa is? If you hit Ctrl + F5 on the webpage you cited, you'll come across that term. Anyway, if the mantissa has 53 bits of precision, why is PHP only showing 48 bits? Oh - but because the precision is "limited", I guess that's not really a problem, is it? You know what else has "limited precision"? 32-bit integers. Even though those support numbers from &#8722;2,147,483,648 to 2,147,483,647, I guess it's not really a problem if PHP's echo statement was only capable of showing numbers ranging from, say, -128 to 127. After all, why waste an opportunity to arbitrarily and artificially reduce precision for no real reason what-so-ever! And if anyone complains, just remind them that they, too, have a "limited precision"! Previous Comments: ------------------------------------------------------------------------ [2009-10-01 16:14:13] jani@php.net Floating point values have a limited precision. Hence a value might not have the same string representation after any processing. That also includes writing a floating point value in your script and directly printing it without any mathematical operations. If you would like to know more about "floats" and what IEEE 754 is, read this: http://docs.sun.com/source/806-3568/ncg_goldberg.html Thank you for your interest in PHP. ------------------------------------------------------------------------ [2009-09-29 20:52:56] terrafrost at gmail dot com Description: ------------ Displaying a float in base-10 when fractional values are involved is difficult, as per <http://php.net/float>. Even if fractional values aren't involved, displaying the float might still be problematic, if the number you're trying to display is to big to be stored in the mantissa of a floating point number. Problem is, floating point numbers aren't always displayed correctly, even if the mantissa can hold the number in question. For example, if you have a double precision floating point number, your mantissa has 53 bits. The following corroborates this: <?php $test = 1; $bits = 0; while (fmod($test, 2) == 1) { $test = 2 * $test + 1; $bits++; } echo $bits; ?> That, for me, displays 53. The problem occurs with anything utilizing 48 or more mantissa bits: <?php $bits = 0; while (true) { if (bcpow(2, $bits) != (string) pow(2, $bits)) { break; } $bits++; } echo $bits; ?> Reproduce code: --------------- <?php echo pow(2, 52); echo "\r\n"; echo 4503599627370496; echo "\r\n"; echo pow(2, 52) == 4503599627370496 ? 'equal' : 'not equal'; echo "\r\n"; echo pow(2, 52) == 4503599627370500 ? 'equal' : 'not equal'; ?> Expected result: ---------------- 4503599627370496 4503599627370496 equal not equal Actual result: -------------- 4503599627370500 4503599627370500 equal not equal ------------------------------------------------------------------------ -- Edit this bug report at http://bugs.php.net/?id=49715&edit=1