仅迭代Go模板中数组的前n个项目

Question

I have a vector with n elements. I'm using it to render items in a template. But I need to render only the 5 first elements. Please note that is possible to have less than 5 elements in vector, in this case will render all elements. Is there is a way to do it in template?

{{range .Categorias}}
    <li class="nav-item">
        {{.Nome}}
    </li>
{{end}}

Answer 1

Easiest would be to only pass 5 elements, so you don't need any logic in the template.

You can also do this in template, if you also store the index in the {{range}} action. Then you can use an {{if}} action to check the index, and only render the body of the {{if}} if the index is less than 5:

{{range $i, $e := .Categorias}}{{if lt $i 5}}
    <li class="nav-item">
        {{.Nome}}
    </li>
{{end}}{{end}}

Here's a simple example demonstrating it:

func main() {
    t := template.Must(template.New("").Parse(src))
    numbers := []int{0, 1, 2, 3, 4, 5, 6, 7, 8}
    if err := t.Execute(os.Stdout, numbers); err != nil {
        panic(err)
    }
}

const src = `{{range $i, $e := .}}{{if lt $i 5}}{{.}} {{end}}{{end}}`

Output (try it on the Go Playground):

0 1 2 3 4 

Go 1.10 template extension

Note that in Go 1.10 there will be new {{break}} and {{continue}} actions which will provide an alternative, better solution for this.

It will look something like this:

{{range $i, $e := .Categorias}}
    <li class="nav-item">
        {{.Nome}}
    </li>
{{if eq $i 4}}{{break}}{{end}}{{end}}

This new {{break}} action will provide a superior solution as the above {{range}} action will only iterate over 5 elements at most (while the other solution without {{break}} has to iterate over all elements, just elements with index >= 5 are not rendered).