In a template, how can I have a counter?

Question

I require basic incrementation arithmetic and loop for my project to keep the template readable and populate a 9x9 grid with values. The values are stored in a string array and so it is necessary to be able to have control over the index.

Here is my handler with both template functions defined:

func HomeHandler(w http.ResponseWriter, req *http.Request) {
    t := template.New("home.html").Funcs(template.FuncMap{
        "loop": func(n int) []struct{} {
            return make([]struct{}, n)
        },
    }).Funcs(template.FuncMap{
        "inc": func(n int) int {
            return n + 1
        },
    })

    t, err := t.ParseFiles("templates/home.html")

    if err != nil {
        log.Print("template/home error:", err)
    }
    t.ExecuteTemplate(w, "home.html", nil)
}

To create the grid I use the loop function like so:

{{ range loop 3}}
<tbody>
    {{ range loop 3}}
    <tr>
        {{ range loop 9}} 
        <td> <input value="1" type="text" name="value[]" maxlength="1" size="1">
        {{end}}
    {{end}}
{{end}}

However, I'd like to set the value attribute of the input element to the correct value with my data. I believe that I can access the index with the following:

{{index .MyArray 3}}

I will replace "3" with a counter that I'll need to be able to increment correctly.

Unfortunately, it seems like I can't correctly reassign the variable as I can only increment it from 0 to 1 at most.

Here is my template with my counter:

{{$count := 0}}
{{ range loop 3}}
<tbody>
    {{ range loop 3}}
    <tr>
        {{ range loop 9}} 
        {{$count := inc $count}}
        <td> <input value="1" type="text" name="value[]" maxlength="1" size="1">
        {{end}}
    {{end}}
{{end}}

hobbs hobbs · Accepted Answer · 2016-06-04T04:18:02

How about doing it the other way around, since ranging over the actual data is something that templates do easily? You can do without loop and inc and start with

{{ range $index, $value := .MyArray }}
<td><input value="{{ $value }}" type="text" name="value[]" maxlength="1" size="1">
{{ end }}

which will get you all of the inputs you need, but without the surrounding markup, so how do we get that? A little bit of modular arithmetic. Define a template function like so:

"each": func(interval, n int) bool {
    return n % interval == 0
}

then we can pad things out:

{{ range $index, $value := .MyArray }}
{{ if each 27 $index }}<tbody>{{ end }}
{{ if each 9 $index }}<tr>{{ end }}
<td><input value="{{ $value }}" type="text" name="value[]" maxlength="1" size="1">
{{ end }}

In a template, how can I have a counter?

1 Answers