How to use fmt.Errorf to create custom error messages with formatted output in Go?

To use fmt.Errorf to create custom error messages with formatted output in Go, you can use the following syntax:

err := fmt.Errorf("Error message with formatted output: %s", variable)

Here's a step-by-step explanation:

Use the fmt.Errorf function to create a new error, with the error message as the first argument.
Format the error message using the %s verb (or any other appropriate verb) in the usual manner.
Pass the variables you want to include in the formatted output as additional arguments. These variables will replace the corresponding format verbs in the error message.

Here's an example:

package main

import (
	"fmt"
	"strings"
)

func main() {
	input := "Hello, World!"
	expectedOutput := "welcome to Go."

	if !strings.Contains(input, expectedOutput) {
		err := fmt.Errorf("Expected output '%s' not found in input: %s", expectedOutput, input)
		fmt.Println(err)
	}
}

In the example above, the fmt.Errorf function is used to create a custom error message that includes the expected output and the actual input. The err variable will contain the formatted error message.

Note: fmt.Errorf returns an error type, so you can assign the result to an error variable or return it from a function.