php - 来自微时的圆形 float

Question

PHP 版本 5.3.6

例子

<?php
$timeStart = microtime(true);
// some code ...    
$timeExecution = microtime(true) - $timeStart;
$time = round($timeExecution, 2);
file_put_contents('h:/round.txt', $timeExecution . ' = ' . $time . "\n", FILE_APPEND);
?>

文件 round.txt 包含以下几行:

131.3048491477966 = 131.3

8.340715885162354 = 8.34

8.198318004608154 = 8.199999999999999

这怎么可能？

Answer 1

http://php.net/manual/en/language.types.float.php

Additionally, rational numbers that are exactly representable as floating point numbers in base 10, like 0.1 or 0.7, do not have an exact representation as floating point numbers in base 2, which is used internally, no matter the size of the mantissa. Hence, they cannot be converted into their internal binary counterparts without a small loss of precision. This can lead to confusing results: for example, floor((0.1+0.7)*10) will usually return 7 instead of the expected 8, since the internal representation will be something like 7.9999999999999991118....