将float截断为2个小数位的最简单方法? [英] Easiest way to truncate float to 2 decimal places?
问题描述
在Swift中，有没有一种方法可以将浮点数截断为2个小数，以便您可以用它执行进一步的计算?我见过的所有线程都涉及到强制转换为字符串，我无法弄清楚如何在数学上使用它.
In Swift, is there a way to truncate a float to 2 decimals, such that you can perform further calculations with it? All of the threads I've seen deal with casting to a string, which I can't figure out how to then use mathematically.
我尝试使用扩展程序(在此论坛上找到)，认为我可以在截断后将其转换为浮点数，但最终以另一个未截断的浮点数开始了我的工作.我需要将返回值设置为四分之一步(即6.50、6.75、5.25等)，最终得到的结果应该是6.990022....
I tried using an extension (found on this forum), figuring I could cast back to float after the truncation, but I end up where I started, with another, non-truncated float. I need my return value to be in quarter steps (i.e. 6.50, 6.75, 5.25, etc), and what I'm ending up with, are results like 6.990022....
必须有一种简单的方法来执行此操作，但是我碰壁了.预先感谢...
There has to be a simple way to do this, but I'm hitting a wall. Thanks in advance...
问题出在这里:
func roundToNearestQuarter(#power : Float) -> String {
var errorToLowerQuarterRaw : Float = power % 0.25 // 0.210000038146973
var errorToLowerQuarterString = errorToLowerQuarterStepRaw.string(2) // "0.21"
var errorToLowerQuarter = NSString(string: errorToLowerQuaterStepString).floatValue // 0.209999993443489
// more code
}
roundToNearestQuater(6.71)
推荐答案
您不能将Float或Double精确舍入到2个十进制数字 .
原因是这些数据类型使用二进制浮点表示形式，
并且不能精确表示0.1或0.01之类的数字.
例如参见
You cannot round a Float or Double to 2 decimal digits exactly.
The reason is that these data types use a binary floating point representation,
and cannot represent numbers like 0.1 or 0.01 exactly.
See for example
- Why Are Floating Point Numbers Inaccurate?
- What Every Computer Scientist Should Know About Floating-Point Arithmetic
但是你说:
我需要我的返回值以四分之一步为单位(即6.50、6.75、5.25等)，
I need my return value to be in quarter steps (i.e. 6.50, 6.75, 5.25, etc),
以及完全可能，因为0.25 = 2 -2 可以精确地表示为浮点数.
and that is exactly possible because 0.25 = 2-2 can be represented exactly as a floating point number.
round()函数将浮点数四舍五入为最接近的整数值.
要四舍五入到最接近的四分之一，您只需使用因子4缩放"计算即可:
The round() function rounds a floating point number to the nearest integral value.
To round to the nearest quarter, you just have to "scale" the calculation with the factor 4:
func roundToNearestQuarter(num : Float) -> Float {
return round(num * 4.0)/4.0
}
roundToNearestQuarter(6.71) // 6.75
roundToNearestQuarter(6.6) // 6.5
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