How to Increment Indices in Go Templates: Overcoming the 'Illegal Number Syntax' Error

Arithmetic Operations in Go Templates: Overcoming the Index Increment Challenge

When working with Go templates, it's essential to understand how to perform arithmetic operations. One common use case is adjusting indices to start from 1 instead of 0. However, the out-of-the-box template syntax does not provide direct support for this.

The Problem: Illegal Number Syntax Error

As mentioned in the question, attempting to increment the zero-based index using the expression {{$index 1}} results in a "illegal number syntax: " error. This is because the Go template language does not allow arithmetic operations directly inside template expressions.

The Solution: Custom Functions to the Rescue

To overcome this limitation, we need to create a custom function that performs the necessary arithmetic operation. In this case, a simple increment function will suffice.

Here's an example of how to define and use a custom function named "inc" to increment the index:

funcMap := template.FuncMap{
    "inc": func(i int) int {
        return i + 1
    },
}

We then register this custom function map with the template before parsing the template text:

tmpl, err := template.New("test").Funcs(funcMap).Parse(`{{range $index, $element := .}}`)

Now, we can use the "inc" function to increment the index within the template expression:

{{range $index, $element := .}}
  Number: {{inc $index}}, Text:{{$element}}
{{end}}

By defining a custom function and registering it with the template, we can extend the functionality of Go templates and perform arithmetic operations, such as incrementing the index, as needed.

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