问题
My PHP code:
echo -100.35+100.00;
echo '<br/>';
echo -478.35+478.32+0.03;
Gives output:
-0.34999999999999
-2.9559688030645E-14
I don't understand why, i tried (float) in front of the values but no difference.
回答1:
You should use BC Math Functions for such calculations.
回答2:
Take a look at this manual, it explains that the internal representation is not 100% exact for floats... it has to do with the internal representation of the floats I suspect.
The manual does also refer to these methods and also these for calculations with need of precision.
回答3:
This is how floats work. In PHP, there are integers, or there are floats - no exact decimals or the like.
Floats are simply not EXACT - there isn't too much you can do about it. http://www.php.net/manual/en/language.types.float.php
Note the big warning:
Floating point numbers have limited precision. Although it depends on the system, PHP typically uses the IEEE 754 double precision format, which will give a maximum relative error due to rounding in the order of 1.11e-16. Non elementary arithmetic operations may give larger errors, and, of course, error propagation must be considered when several operations are compounded.
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....
来源：https://stackoverflow.com/questions/22834377/php-calculating-with-negative-numbers-gives-wrong-values