几种语言的int（（0.1 + 0.7）* 10）= 7。怎么预防这个？ [英] int((0.1+0.7)*10) = 7 in several languages. How to prevent this?

问题描述

最近我遇到了几种语言的错误/功能。我对它是如何引起的有一个非常基本的了解（我想要一些详细的解释），但是当我想到多年来我必须犯下的所有错误时，问题是如何确定 嘿，这可能会导致一个荒谬的bug，我最好使用任意精确函数 ，其他语言确实有这个bug（而那些没有，为什么）。另外，为什么0.1 + 0.7会这样做，即0.1 + 0.3没有，还有其他众所周知的例子吗？

Recently I came across a bug/feature in several languages. I have a very basic knowledge about how it's caused (and I'd like some detailed explanation), but when I think of all the bugs I must have made over the years, the question is how can I determine "Hey, this might cause a riddiculous bug, I'd better use arbitrary precision functions", what other languages do have this bug (and those who don't, why). Also, why 0.1+0.7 does this and i.e. 0.1+0.3 doesn't, are there any other well-known examples?

PHP

//the first one actually doesn't make any sense to me,
//why 7 after typecast if it's represented internally as 8?
debug_zval_dump((0.1+0.7)*10); //double(8) refcount(1)
debug_zval_dump((int)((0.1+0.7)*10)); //long(7) refcount(1)
debug_zval_dump((float)((0.1+0.7)*10)); //double(8) refcount(1)

Python：

>>> ((0.1+0.7)*10)
7.9999999999999991
>>> int((0.1+0.7)*10)
7

Javascript：

Javascript:

alert((0.1+0.7)*10); //7.999999999999999
alert(parseInt((0.7+0.1)*10)); //7

Ruby：

>> ((0.1+0.7)*10).to_i                                                  
=> 7                                                                    
>>((0.1+0.7)*10)                                                       
=> 7.999999999999999                                                    

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