How to Customize Index Output in Go Templates: Can You Do Arithmetic in Templates?

Customizing Index Output in Go Templates: Arithmetic in Templates

In Go templates, the range action allows convenient iteration through arrays, providing both the element and its zero-based index. However, for certain scenarios, you may need to modify the displayed indices to start from a non-zero value.

Why the Initial Approach Fails

Initially, the attempt to perform arithmetic operations within the template, such as "{{$index 1}}", results in an "illegal number syntax" error. This is because the template language does not inherently support arithmetic operations.

The Solution: Custom Functions

To achieve the desired behavior, it is necessary to define a custom function that performs the arithmetic operation outside the template. In this case, we create a function called "inc" that increments the given index by 1.

Here's the modified Go template code that incorporates the "inc" function:

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

The custom function is defined as follows:

func inc(i int) int {
  return i + 1
}

Usage

To use the custom function, it must be registered with the template's FuncMap. The example provided initializes a FuncMap and adds the "inc" function to it, allowing the template to access it.

In this scenario, the template iterates through a slice of strings, displaying the modified indices and corresponding elements.

Conclusion

By creating custom functions, you can extend the capabilities of Go templates to perform operations that are not natively supported within the template language syntax. This allows for greater flexibility and enables complex operations such as arithmetic manipulation within templates.

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