A Go function to render a number to a string based on the following user-specified criteria: thousands separator, decimal separator, decimal precision. I didn't feel it was worth to publish a library just for this piece of code, hence the snippet. Feel free to reuse as you wish.

render_number.go

/*

Author: https://github.com/gorhill
Source: https://gist.github.com/gorhill/5285193

A Go function to render a number to a string based on
the following user-specified criteria:

* thousands separator
* decimal separator
* decimal precision

Usage: s := RenderFloat(format, n)

The format parameter tells how to render the number n.

http://play.golang.org/p/LXc1Ddm1lJ

Examples of format strings, given n = 12345.6789:

"#,###.##" => "12,345.67"
"#,###." => "12,345"
"#,###" => "12345,678"
"#\u202F###,##" => "12 345,67"
"#.###,###### => 12.345,678900
"" (aka default format) => 12,345.67

The highest precision allowed is 9 digits after the decimal symbol.
There is also a version for integer number, RenderInteger(),
which is convenient for calls within template.

I didn't feel it was worth to publish a library just for this piece
of code, hence the snippet. Feel free to reuse as you wish.

*/

import (
	"math"
	"strconv"
)

var renderFloatPrecisionMultipliers = [10]float64{
    1,
    10,
    100,
    1000,
    10000,
    100000,
    1000000,
    10000000,
    100000000,
    1000000000,
}

var renderFloatPrecisionRounders = [10]float64{
    0.5,
    0.05,
    0.005,
    0.0005,
    0.00005,
    0.000005,
    0.0000005,
    0.00000005,
    0.000000005,
    0.0000000005,
}

func RenderFloat(format string, n float64) string {
    // Special cases:
    //   NaN = "NaN"
    //   +Inf = "+Infinity"
    //   -Inf = "-Infinity"
    if math.IsNaN(n) {
        return "NaN"
    }
    if n > math.MaxFloat64 {
        return "Infinity"
    }
    if n < -math.MaxFloat64 {
        return "-Infinity"
    }

    // default format
    precision := 2
    decimalStr := "."
    thousandStr := ","
    positiveStr := ""
    negativeStr := "-"

    if len(format) > 0 {
        // If there is an explicit format directive,
        // then default values are these:
        precision = 9
        thousandStr = ""

        // collect indices of meaningful formatting directives
        formatDirectiveChars := []rune(format)
        formatDirectiveIndices := make([]int, 0)
        for i, char := range formatDirectiveChars {
            if char != '#' && char != '0' {
                formatDirectiveIndices = append(formatDirectiveIndices, i)
            }
        }

        if len(formatDirectiveIndices) > 0 {
            // Directive at index 0:
            //   Must be a '+'
            //   Raise an error if not the case
            // index: 0123456789
            //        +0.000,000
            //        +000,000.0
            //        +0000.00
            //        +0000
            if formatDirectiveIndices[0] == 0 {
                if formatDirectiveChars[formatDirectiveIndices[0]] != '+' {
                    panic("RenderFloat(): invalid positive sign directive")
                }
                positiveStr = "+"
                formatDirectiveIndices = formatDirectiveIndices[1:]
            }

            // Two directives:
            //   First is thousands separator
            //   Raise an error if not followed by 3-digit
            // 0123456789
            // 0.000,000
            // 000,000.00
            if len(formatDirectiveIndices) == 2 {
                if (formatDirectiveIndices[1] - formatDirectiveIndices[0]) != 4 {
                    panic("RenderFloat(): thousands separator directive must be followed by 3 digit-specifiers")
                }
                thousandStr = string(formatDirectiveChars[formatDirectiveIndices[0]])
                formatDirectiveIndices = formatDirectiveIndices[1:]
            }

            // One directive:
            //   Directive is decimal separator
            //   The number of digit-specifier following the separator indicates wanted precision
            // 0123456789
            // 0.00
            // 000,0000
            if len(formatDirectiveIndices) == 1 {
                decimalStr = string(formatDirectiveChars[formatDirectiveIndices[0]])
                precision = len(formatDirectiveChars) - formatDirectiveIndices[0] - 1
            }
        }
    }

    // generate sign part
    var signStr string
    if n >= 0.000000001 {
        signStr = positiveStr
    } else if n <= -0.000000001 {
        signStr = negativeStr
        n = -n
    } else {
        signStr = ""
        n = 0.0
    }

    // split number into integer and fractional parts
    intf, fracf := math.Modf(n + renderFloatPrecisionRounders[precision])

    // generate integer part string
    intStr := strconv.Itoa(int(intf))

    // add thousand separator if required
    if len(thousandStr) > 0 {
        for i := len(intStr); i > 3; {
            i -= 3
            intStr = intStr[:i] + thousandStr + intStr[i:]
        }
    }

    // no fractional part, we can leave now
    if precision == 0 {
        return signStr + intStr
    }

    // generate fractional part
    fracStr := strconv.Itoa(int(fracf * renderFloatPrecisionMultipliers[precision]))
    // may need padding
    if len(fracStr) < precision {
        fracStr = "000000000000000"[:precision-len(fracStr)] + fracStr
    }

    return signStr + intStr + decimalStr + fracStr
}

func RenderInteger(format string, n int) string {
    return RenderFloat(format, float64(n))
}

~LICENSE

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
                   Version 2, December 2004

Copyright (C) 2004 Sam Hocevar <[email protected]>

Everyone is permitted to copy and distribute verbatim or modified
copies of this license document, and changing it is allowed as long
as the name is changed.

           DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
  TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

 0. You just DO WHAT THE FUCK YOU WANT TO.

On my Raspberry Pi which runs a 32-bit version of Raspbian Linux (even though it has a 64-bit CPU) using go version go1.14.2 linux/arm, I had to change this line. Otherwise RenderInteger("#,###.", n) would fail when n > 2^32.

https://gist.github.com/gorhill/5285193#file-render_number-go-L165

from:

intStr := strconv.Itoa(int64(intf))

to:

intStr := strconv.FormatInt(int64(intf), 10)