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.

Sweated

Undoable in templates. You must do this either directly in your Go code or expose a function like TrimTrailingComma to your template. – Dey

thank you, I will find other way to solve it – Sweated

I think the most elegant solution can be found via this question/answer: #10747554 – Tantivy

@jboschiero The elegant solution works for slices, not for maps. – Coakley

@Dey It's possible since Go 1.11. See answer below. – Advise

Answer 1

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.

Masaccio

As @Dey says, no way to do this in template, maybe this is the best solution – Sweated

Answer 2

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

Advise

Do you know by any chance which version of Docker I need for it to work? – Leavy

great, this simplifies a lot when there are nested loops. – Couvade

Answer 3

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 -}}

Coordinate