关于php：错误的计算？

Wrong Calculation?

本问题已经有最佳答案，请猛点这里访问。

Possible Duplicate:
Is double Multiplication Broken in .NET?
PHP unexpected result of float to int type cast

号

1	echo (int) ( (0.1+0.7) * 10);

给我答案7，但不是8。

1	echo (int) ( (0.1+0.8) * 10);

号

给出正确答案9

怎么了？有人能解释一下吗

桑克斯-导航

相关讨论

尝试

1	echo (float) ( (0.1+0.7) * 10);

这是正常的——在处理浮点数时有一种叫做精度的东西。它在大多数现代语言中都存在。有关详细信息，请参阅：http://www.mredkj.com/javascript/nfbasic2.html。

((0.1+0.7) * 10)可能与7.9999999和(int) 7.9999999 = 7类似。

另一方面，((0.1+0.8) * 10)可能是9.00000001和(int)9.00000001 = 9。

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....

So never trust floating number results to the last digit, and do not
compare floating point numbers directly for equality. If higher
precision is necessary, the arbitrary precision math functions and gmp
functions are available.

号

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