How to Format a Go float64 Value into a Fixed-Width String with Maximum Significant Digits?

How to Convert Float64 to a Fixed-Width String with Maximum Significant Digits in Go

In Go, when printing float64 values within a fixed width table, it's often desirable to preserve as many significant digits as possible. However, the default formatting can result in lost precision due to the automatic switch to scientific notation for large numbers.

The Need for a Custom Formatting Solution

For cases where both scientific notation and regular form are unacceptable, a customized formatting approach is required. The primary focus should be on calculating the optimal precision that fits within the specified width while retaining maximum significant digits.

Proposed Solution

The following implementation offers a straightforward solution:

<br>// format12 formats x to be 12 chars long.<br>func format12(x float64) string {</p>
<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">if x >= 1e12 {
    s := fmt.Sprintf("%.g", x)
    format := fmt.Sprintf("%%12.%dg", 12-len(s))
    return fmt.Sprintf(format, x)
}

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)

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}

This solution employs a two-step process:

1. Determining the Precision:

   • For values greater than or equal to 1e12, a scientific notation with an appropriate precision is used.
   • For smaller values, a regular form with a sufficient precision is calculated based on the length of the integer part.

2. Creating the Formatting String:

   • Based on the calculated precision, a custom formatting string is constructed to ensure the desired fixed width.

Sample Usage and Output

To demonstrate its functionality, let's consider a few sample values:

<br>fs := []float64{0, 1234.567890123, 0.1234567890123, 123456789012.0, 1234567890123.0,</p>
<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">9.405090880450127e+9, 9.405090880450127e+19, 9.405090880450127e+119}

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