How to Avoid a Comma After the Last Element in a Go Template Range?

Detecting the Last Item in an Array using Ranges in Go Templates

In Go templates, when iterating over an array using the range keyword, a comma separator is automatically inserted between each element. While this behavior is useful in most cases, it can become a problem when the desired output requires a period rather than a comma after the last element.

Consider the following program:

<code class="go">package main

import (
    "os"
    "text/template"
)

func main() {
    params := map[string]interface{}{
        "items": [3]int{1, 4, 2},
    }
    tpl := "{{range $i, $el := .items}}{{$el}},{{end}}"
    lister, _ := template.New("foo").Parse(tpl)
    lister.Execute(os.Stdout, params)
}</code>

This program outputs the following:

1,4,2,

Note that there is a comma after the last element (2). However, we would prefer the output to be:

1,4,2.

To achieve this, we can modify the template as follows:

<code class="go">tpl := "{{range $i, $el := .items}}{{if $i}},{{end}}{{$el}}{{end}}."</code>

The key difference here is the introduction of the {{if $i}},{{end}} construct. This conditional statement checks if the current loop iteration is not the first one, and if it is, it emits a comma. This ensures that a comma is only inserted between elements in the array, and not after the last element.

By incorporating this change, the program will now output:

1,4,2.

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