iPhone / Obj C：为什么转换浮点数为int：（int）float * 100不起作用？ [英] iPhone/Obj C: Why does convert float to int: (int) float * 100 does not work?

问题描述

在我的代码中，我使用float来进行货币计算，但舍入已经产生了不希望的结果，所以我试图将其全部转换为int。在我的init函数中，尽可能少地改变了基础结构，我做了这个：

pre code $ - （id）initWithPrice :(浮）p;
{
[self setPrice：（int）（p * 100）];





我在主要部分乘以100 b / c，给出的值如.xx至2位小数。我不正常地注意到，对于浮点数1.18，int整数为117.有没有人知道它是这样做的？浮动离开它为1.18。我希望它是118美元的int equiv以美分。

谢谢。

解决方案

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浮点总是有点不准确。使用IEEE浮点编码，可以精确地表示两个幂（例如4,2,1,0.5,0.25,0.125,0.0625，...），但是像0.1这样的数字总是一个近似值（只​​要把它表示为一个和（int）强制转换会截断任何进来的数据，所以如果p * 100由于这个不精确性而解析为117.9999995，那么将会变成1.17而不是1.18。

更好的解决方法是在p * 100上使用类似roundf的东西。更好的办法是，如果你可以上游，并在整个程序中使用整数完全转换为定点数学。

In my code, I am using float to do currency calculation but the rounding has yielded undesired results so I am trying to convert it all to int. With as little change to the infrastructure as possible, in my init functions, I did this:

-(id)initWithPrice:(float)p;
{
[self setPrice:(int)(p*100)];
}

I multiply by 100 b/c in the main section, the values are given as .xx to 2 decimals. I abnormally I notice is that for float 1.18, the int rounds it to 117. Does anyone know it does that? The float leaves it as 1.18. I expect it to be 118 for the int equiv in cents.

Thanks.

解决方案

Floating point is always a little imprecise. With IEEE floating point encoding, powers of two can be represented exactly (like 4,2,1,0.5,0.25,0.125,0.0625,...) , but numbers like 0.1 are always an approximation (just try representing it as a sum of powers of 2).

Your (int) cast will truncate whatever comes in, so if p*100 is resolving to 117.9999995 due to this imprecision , that will become 1.17 instead of 1.18.

Better solution is to use something like roundf on p*100. Even better would be if you can go upstream and fully convert to fixed-point math using integers in the entire program.

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