Go Template get a variable dynamically

Question

I have yml parameters that look like

nodes: one, two
instanceMinOne: 1
instanceMaxOne: 2
instanceMinTwo: 4
instanceMaxTwo: 6

Is there a way with go templating to dynamically read for example instanceMinOne where the variable name is composed of instanceMin + the dynamic value coming from the nodes list ?

something like (this obviously doesn't work but just give the idea of what I'd like to achieve)

{{ - range $nodeName := (split .Parameters.nodes) } } } }
   instance-min: {{ .Parameters.instanceMin$nodeName }}
   instance-max: {{ .Parameters.instanceMan$nodeName }}
{{ - end }}

Answer 1

To achieve what you want, you have to solve 2 tasks:

• string concatenation
• indexing with a dynamic value

For concatenation you may use the builtin print function, e.g.

{{ $key := print "instanceMin" $nodeName }}

For indexing, use the builtin index function:

instance-min: {{ index $.Parameters $key }}

(Note: The {{range}} action changes the dot, so inside it you need $ to reference outside of the loop variable.)

Or in one line:

instance-min: {{ index $.Parameters (print "instanceMin" $nodeName) }}

See a runnable demo:

func main() {
    t := template.Must(template.New("").Parse(src))

    params := map[string]any{
        "Parameters": map[string]any{
            "nodes":          []string{"One", "Two"},
            "instanceMinOne": 1,
            "instanceMaxOne": 2,
            "instanceMinTwo": 4,
            "instanceMaxTwo": 6,
        },
    }

    if err := t.Execute(os.Stdout, params); err != nil {
        panic(err)
    }
}

const src = `{{- range $idx, $nodeName := .Parameters.nodes }}
   instance-min: {{ index $.Parameters (print "instanceMin" $nodeName) }}
   instance-max: {{ index $.Parameters (print "instanceMax" $nodeName) }}
{{- end }}`

This will output (try it on the Go Playground):

instance-min: 1
instance-max: 2
instance-min: 4
instance-max: 6