Get iterator index in go template (consul-template) - loops

Question

I'm trying to get a simple index that I can append to output of a Go template snippet using consul-template. Looked around a bit and couldn't figure out the simple solution. Basically, given this input
backend web_back
balance roundrobin
{{range service "web-busybox" "passing"}}
server {{ .Name }} {{ .Address }}:80 check
{{ end }}
I would like to see web-busybox-n 10.1.1.1:80 check
Where n is the current index in the range loop. Is this possible with range and maps?

Answer 1

There is no iteration number when ranging over maps (only a value and an optional key). You can achieve what you want with custom functions.
One possible solution that uses an inc() function to increment an index variable in each iteration:
func main() {
t := template.Must(template.New("").Funcs(template.FuncMap{
"inc": func(i int) int { return i + 1 },
}).Parse(src))
m := map[string]string{
"one": "first",
"two": "second",
"three": "third",
}
fmt.Println(t.Execute(os.Stdout, m))
}
const src = `{{$idx := 0}}
{{range $key, $value := .}}
index: {{$idx}} key: {{ $key }} value: {{ $value }}
{{$idx = (inc $idx)}}
{{end}}`
This outputs (try it on the Go Payground) (compacted output):
index: 0 key: one value: first
index: 1 key: three value: third
index: 2 key: two value: second
See similar / related questions:
Go template remove the last comma in range loop
Join range block in go template
Golang code to repeat an html code n times

Answer 2

The example below looks for all servers providing the pmm service, but will only create the command to register with the first pmm server found (when $index == 0)
{{- range $index, $service := service "pmm" -}}
{{- if eq $index 0 -}}
sudo pmm-admin config --server {{ $service.Address }}
{{- end -}}
{{- end -}}