是否有任何标准库可以将 float64 转换为具有最大有效位数的固定宽度的字符串? [英] Is there any standard library to convert float64 to string with fix width with maximum number of significant digits?

问题描述

假设要在 12 个固定宽度的表格中打印，我们需要打印 float64 数字:

Imagine for printing in a 12 fixed width table we need printing float64 numbers:

fmt.Printf("%12.6g
", 9.405090880450127e+119) //"9.40509e+119"
fmt.Printf("%12.6g
", 0.1234567890123)        //"    0.123457"
fmt.Printf("%12.6g
", 123456789012.0)         //" 1.23457e+11"

我们更喜欢 0.1234567890 而不是0.123457"，我们会丢失 6 个有效数字.
我们更喜欢 123456789012 而不是1.23457e+11"，我们会丢失 6 个有效数字.

We prefer 0.1234567890 to " 0.123457" we lose 6 significant digits.
We prefer 123456789012 to " 1.23457e+11" we lose 6 significant digits.

是否有任何标准库可以将 float64 转换为 string 并具有最大有效位数的固定宽度?提前致谢.

Is there any standard library to convert float64 to string with fix width with maximum number of significant digits? Thanks in Advance.

推荐答案

基本上你有 2 种输出格式:科学记数法或正则形式.这两种格式之间的转折点是1e12.

Basically you have 2 output formats: either a scientific notation or a regular form. The turning point between those 2 formats is 1e12.

所以如果 x >= 1e12 则可以分支.在这两个分支中，您可以使用 0 小数位进行格式化以查看数字的长度，因此您可以计算 12 宽度适合多少小数位，因此您可以使用计算出的精度构造最终格式字符串.

So you can branch if x >= 1e12. In both branches you may do a formatting with 0 fraction digits to see how long the number will be, so you can calculate how many fraction digits will fit in for 12 width, and so you can construct the final format string, using the calculated precision.

科学计数法中也需要预校验(%g)，因为指数的宽度可能不同(例如e+1、e+10, e+100).

The pre-check is required in the scientific notation too (%g), because the width of exponent may vary (e.g. e+1, e+10, e+100).

这是一个示例实现.这是为了让您入门，但这并不意味着处理所有情况，也不是最有效的解决方案(但相对简单并且可以完成工作):

Here is an example implementation. This is to get you started, but it does not mean to handle all cases, and it is not the most efficient solution (but relatively simple and does the job):

// format12 formats x to be 12 chars long.
func format12(x float64) string {
    if x >= 1e12 {
        // Check to see how many fraction digits fit in:
        s := fmt.Sprintf("%.g", x)
        format := fmt.Sprintf("%%12.%dg", 12-len(s))
        return fmt.Sprintf(format, x)
    }

    // Check to see how many fraction digits fit in:
    s := fmt.Sprintf("%.0f", x)
    if len(s) == 12 {
        return s
    }
    format := fmt.Sprintf("%%%d.%df", len(s), 12-len(s)-1)
    return fmt.Sprintf(format, x)
}

测试它:

fs := []float64{0, 1234.567890123, 0.1234567890123, 123456789012.0, 1234567890123.0,
    9.405090880450127e+9, 9.405090880450127e+19, 9.405090880450127e+119}

for _, f := range fs {
    fmt.Println(format12(f))
}

输出(在 Go Playground 上试试):

Output (try it on the Go Playground):

0.0000000000
0.1234567890
1234.5678901
123456789012
1.234568e+12
9405090880.5
9.405091e+19
9.40509e+119

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