Is there any standard library to convert float64 t

Question

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

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

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

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

Answer 1

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

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.

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).

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)
}

Testing it:

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))
}

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