Golang code to repeat an html code n times

Question

I am working on golang web app. In that I need to iterate an HTML line n number of times.

func index(w http.ResponseWriter, r *http.Request) {
    tmpl := template.Must(template.ParseFiles("templates/index.html"))
    n := 5
    tmpl.Execute(w, n)
}

<ul>
    <li><a href="/?page=1">1</a></li>
    <li><a href="/?page=2">2</a></li>
                    .
                    .
                    .
    <li><a href="/?page=n">n</a></li>
</ul>

How can I implement this?

Answer 1

To repeat something in Go templates, you may use the {{range}} action. But the {{range}} action expects something it can iterate over, e.g. a slice, array or map.

Passing a zero-value slice

So you have to feed that. But an empty slice which requires no memory is sufficient, e.g. make([]struct{}, n).

The template code:

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

Testing it:

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

Output (try it on the Go Playground):

<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

As we can see, indices start from 0. If this is an issue, you may opt to not use the index, but fill the passed slice explicitly with the elements you want. Then the template will look like this:

const templ = `<ul>
{{range .}}
    <li><a href="/?page={{.}}">{{.}}</a></li>
{{end}}
</ul>`

And an example test code that feeds only the odd numbers starting with 2 could look like this:

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 this time (try it on the Go Playground):

<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-valued slice and custom function

If you don't want to bother filling the slice and you only need increasing numbers starting from 1, another option would be to register a function that receives a number, adds 1 to it and returns the result. So you may continue to use the indices of a zero-valued slice, and you can call the custom function to obtain a number equal to index+1:

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 this time (try it on the Go Playground):

<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>