Go template remove the last comma in range loop

Issue

I have code like this:

package main

import (
    "text/template"
    "os"
)

func main() {
    type Map map[string]string
    m := Map {
        "a": "b",
        "c": "d",
    }
    const temp = `{{range $key, $value := $}}key:{{$key}} value:{{$value}},{{end}}`
    t := template.Must(template.New("example").Parse(temp))
    t.Execute(os.Stdout, m)
}

it will output :

key:a value:b,key:c value:d,

but I want something like this:

key:a value:b,key:c value:d

I don’t need the last comma, how to remove it. I found a solution for looping an array here: https://groups.google.com/d/msg/golang-nuts/XBScetK-guk/Bh7ZFz6R3wQJ , but I can’t get index for a map.

Solution

Here’s how to write comma separated key-value pairs using a template function.

Declare a function that returns a function that increments and returns a counter:

func counter() func() int {
    i := -1
    return func() int {
        i++
        return i
    }
}

Add this function to the template:

t := template.Must(template.New("example").Funcs(template.FuncMap{"counter": counter}).Parse(temp))

Use it in the template like this:

    {{$c := counter}}{{range $key, $value := $}}{{if call $c}}, {{end}}key:{{$key}} value:{{$value}}{{end}}

This template writes the separators before the key-value pairs instead after the pairs.

The counter is created before the loop and incremented on each iteration through the loop. The separator is not written the first time through the loop.

Run it in the playground.

The logic in the template can be simplified by moving the if statement to Go code:

func separator(s string) func() string {
    i := -1
    return func() string {
        i++
        if i == 0 {
            return ""
        }
        return s
    }
}

Add the function to the template:

t := template.Must(template.New("example").Funcs(template.FuncMap{"separator": separator}).Parse(temp))

Use it like this:

{{$s := separator ", "}}{{range $key, $value := $}}{{call $s}}key:{{$key}} value:{{$value}}{{end}}

Run it on the playground.

Answered By – Bayta Darell

Answer Checked By – Cary Denson (GoLangFix Admin)

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