四舍五入有效数字 [英] Rounding with significant digits

问题描述

我有一个值为0.00004876544的浮点数。

例如，0.00004876544会读取0.000049。

如何获得显示在第一个重要数字后的小数点后两位？解决方案我没有通过编译器来检查它，但是这里是算法的基本问题（从答案转换到）：

<$（$）
$ b <$ （float）round：（float）num toSignificantFigures：（int）n {
if（num == 0）{
return 0;
}

double d = ceil（log10（num <0？-num：num））;
int power = n - （int）d;

double magnitude = pow（10，power）;
长移=圆（数量级）;
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$ b $ p
$ b

重要的是要记住Objective-C是C的一个超集，所以在C中有效的任何东西在Objective-C中也是有效的。此方法使用 math.h 中定义的C函数。

In Xcode /Objective-C for the iPhone.

I have a float with the value 0.00004876544. How would I get it to display to two decimal places after the first significant number?

For example, 0.00004876544 would read 0.000049.

解决方案

I didn't run this through a compiler to double-check it, but here's the basic jist of the algorithm (converted from the answer to this question):

-(float) round:(float)num toSignificantFigures:(int)n {
    if(num == 0) {
        return 0;
    }

    double d = ceil(log10(num < 0 ? -num: num));
    int power = n - (int) d;

    double magnitude = pow(10, power);
    long shifted = round(num*magnitude);
    return shifted/magnitude;
}

The important thing to remember is that Objective-C is a superset of C, so anything that is valid in C is also valid in Objective-C. This method uses C functions defined in math.h.

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