php - 为什么 PHP 会将 (int) ((0.1+0.7)*10) 的结果转换为 7，而不是 8？

Question

到底为什么 PHP 会将 (int) ((0.1+0.7)*10) 的结果转换为 7，而不是 8？我知道结果将是 (float) 8 当它转换为 (int) 结果将是 7？为什么会这样？

Answer 1

一切都是关于float。参见 PHP: Floating point numbers :

Warning - Floating point precision

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.

补充阅读:

• What Every Computer Scientist Should Know About Floating-Point Arithmetic
• How dangerous is it to compare floating point values?
• Why can't decimal numbers be represented exactly in binary?