圆数到最近的“第n”基于第一个非零 [英] Round number to nearest "nth" based on first non zero

问题描述

例如：

  x = 0.002341 
 rounded = 0.002 
 
x = 0.000048123 
 rounded = 0.00005 
  

对于基数> 0的情况，应该执行

  x = 1.000234 
 rounded = 1.0002 
  

我知道我可以使用 Double（round（1000 * x）/ 1000）如果我知道数字，但我希望它可以工作任何数字。有一个快速的功能吗？

解决方案

你可以用对数有一点乐趣来解决这个问题：

  func roundFirst（x：Double） - >双{
如果x == 0 {
 return x; 
} 
 let mul：Double = pow（10，floor（log10（abs（x））））
 return round（x / mul）* mul 
} 
  

log10（abs（x））的非小数部分给你一个正数或负数的十个数字的倒数，你用作乘数。 floor 删除分数， pow（10，...）给你一个乘数，用于你的四舍五入

我在操场上尝试了几个数字。这是我所得到的：

  println（roundFirst（0.002341））// 0.002 
 println（roundFirst 0.000048123））// 5e-05 
 println（roundFirst（0.0））// 0.0 
 println（roundFirst（2.6））// 3.0 
 println（roundFirst（123.0））// 100 
 println（roundFirst（-0.002341））// -0.002 
 println（roundFirst（-0.000048123））// -5e-05 
 println（roundFirst（-2.6））// -3.0 
 println（roundFirst（-123.0））// -100 
  

I want to round a Double to the nearest non zero number that follows the decimal.

For example:

x = 0.002341
rounded = 0.002

x = 0.000048123
rounded = 0.00005

For cases where the base number is > 0, it should perform as such

x = 1.000234
rounded = 1.0002

I know I can use Double(round(1000*x)/1000) if I know the number of digits, but I want it to work for any number. Is there a swift function that does this?

解决方案

You can have a little fun with logarithms to solve this:

func roundFirst(x:Double) -> Double {
    if x == 0 {
        return x;
    }
    let mul : Double = pow(10, floor(log10(abs(x))))
    return round(x/mul)*mul
}

The non-fractional part of log10(abs(x)) gives you a positive or negative power of ten of the inverse of the number which you use as your multiplier. floor drops the fraction, and pow(10,...) gives you a multiplier to use in your rounding trick.

I tried this in the playground with a few numbers. Here is what I've got:

println(roundFirst(0.002341))     // 0.002
println(roundFirst(0.000048123))  // 5e-05
println(roundFirst(0.0))          // 0.0
println(roundFirst(2.6))          // 3.0
println(roundFirst(123.0))        // 100
println(roundFirst(-0.002341))    // -0.002
println(roundFirst(-0.000048123)) // -5e-05
println(roundFirst(-2.6))         // -3.0
println(roundFirst(-123.0))       // -100

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