Go template remove the last comma in range loop

Question

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.

Undoable in templates. You must do this either directly in your Go code or expose a function like TrimTrailingComma to your template. — Volker, Mar 08 '17 at 05:11
I think the most elegant solution can be found via this question/answer: https://stackoverflow.com/questions/10747054/special-case-treatment-for-the-last-element-of-a-range-in-google-gos-text-templ — j boschiero, Nov 14 '17 at 18:44
@jboschiero The elegant solution works for slices, not for maps. — Deleplace, Jan 18 '18 at 10:54
@Volker It's possible since Go 1.11. See [answer below](https://stackoverflow.com/a/55211847/1705598). — icza, Mar 17 '19 at 20:59

score 23 · Answer 1 · answered Mar 17 '19 at 20:57

Since Go 1.11 it is now possible to change values of template variables. This gives us the possibility to do this without the need of a custom function (being outside of the template).

The following template does that:

{{$first := true}}
{{range $key, $value := $}}
    {{if $first}}
        {{$first = false}}
    {{else}}
        ,
    {{end}}
    key:{{$key}} value:{{$value}}
{{end}}

Here's the altered working example from the question:

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

Which outputs (try it on the Go Playground):

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

Do you know by any chance which version of Docker I need for it to work? — x-yuri, Jul 01 '19 at 19:34

Charlie Tumahai · Accepted Answer · 2019-09-27T13:15:12.983

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.

As @Volker says, no way to do this in template, maybe this is the best solution — Chris, Mar 08 '17 at 07:46

score 0 · Answer 3 · answered Feb 08 '22 at 21:26

I used the below template to correctly format some JSON using a go template.

It handles a collection of any size or empty.

{{- define "JoinList" -}}
   {{- $lastIndex := math.Sub (len .) 1 -}}
   {{- range $index, $property := . -}}
   {{ if isKind "string" $property.value }}
   "{{ $property.key }}": "{{ $property.value }}"{{ if ne $index $lastIndex }},{{ end }}
   {{- else -}}
   "{{ $property.key }}": {{ $property.value }}{{ if ne $index $lastIndex }},{{ end }}
   {{ end }}
   {{- end -}}
{{- end -}}

Go template remove the last comma in range loop

3 Answers3

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