How can I repeat HTML code a specified number of times using Go templates?

Repeating HTML Code with Go Templates

In web development with Go, there are instances where you may need to iterate an HTML line a specified number of times. Let's explore how to achieve this using Go templates.

Using the {{range}} Action

To reiterate a portion of HTML within a Go template, we can utilize the {{range}} action. However, this action requires an iterable object, such as a slice, array, or map.

Passing a Zero-Value Slice

Instead of creating an actual data structure, we can pass a zero-value slice that essentially has no allocated memory. This slice acts as a placeholder for the {{range}} action to iterate over. For instance:

const templ = `<ul>
{{range $idx, $e := .}}
    <li><a href="/?page={{$idx}}">{{$idx}}</a></li>
{{end}}
</ul>`

tmpl := template.Must(template.New("").Parse(templ))
n := 5
if err := tmpl.Execute(os.Stdout, make([]struct{}, n)); err != nil {
    panic(err)
}

Output:

<ul>

    <li><a href="/?page=0">0</a></li>

    <li><a href="/?page=1">1</a></li>

    <li><a href="/?page=2">2</a></li>

    <li><a href="/?page=3">3</a></li>

    <li><a href="/?page=4">4</a></li>

</ul>

Using a Filled Slice

If you need to start the indices at a value other than zero, consider filling the slice with the desired values and then iterating over it.

tmpl := template.Must(template.New("").Parse(templ))
n := 5
values := make([]int, n)
for i := range values {
    values[i] = (i + 1) * 2
}
if err := tmpl.Execute(os.Stdout, values); err != nil {
    panic(err)
}

Output:

<ul>

    <li><a href="/?page=2">2</a></li>

    <li><a href="/?page=4">4</a></li>

    <li><a href="/?page=6">6</a></li>

    <li><a href="/?page=8">8</a></li>

    <li><a href="/?page=10">10</a></li>

</ul>

Using a Zero-Value Slice and Custom Function

An alternative approach is to register a custom function that increments a given number and returns the result. This allows you to use a zero-value slice and call the function within the template to obtain the desired numbers.

func main() {
    tmpl := template.Must(template.New("").Funcs(template.FuncMap{
        "Add": func(i int) int { return i + 1 },
    }).Parse(templ))
    n := 5
    if err := tmpl.Execute(os.Stdout, make([]struct{}, n)); err != nil {
        panic(err)
    }
}

const templ = `<ul>
{{range $idx, $e := .}}{{$idx := (Add $idx)}}
    <li><a href="/?page={{$idx}}">{{$idx}}</a></li>
{{end}}
</ul>`

Output:

<ul>

    <li><a href="/?page=1">1</a></li>

    <li><a href="/?page=2">2</a></li>

    <li><a href="/?page=3">3</a></li>

    <li><a href="/?page=4">4</a></li>

    <li><a href="/?page=5">5</a></li>

</ul>

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