Overview

Below is the syntax of the panic function

func panic(v interface{})

It takes empty interface as an argument. It does not provides any why to format the error string. However there is a workaround. Sprintf function of fmt package can be used to format the error message before passing it to the panic function. Let’s see a program for that

Example

package main

import (
	"fmt"
)

func main() {
	a := []string{"a", "b"}
	checkAndPrint(a, 2)
	fmt.Println("Exiting normally")
}

func checkAndPrint(a []string, index int) {
	if index > (len(a) - 1) {
		errorString := fmt.Sprintf("Out of bounds access for slice. Index passed: %d", index)
		panic(errorString)
	}
	fmt.Println(a[index])
}

Output

panic: Out of bounds access for slice. Index passed: 2

goroutine 1 [running]:
main.checkAndPrint(0xc00009af58, 0x2, 0x2, 0x2)
        main.go:17 +0x157
main.main()
        main.go:10 +0x81
exit status 2

In the above program we have a function checkAndPrint which checks and prints slice element at an index passed in the argument. If the index passed is greater than the length of the array then the program panics. Notice how we format the error string before passing it to the panic in the checkAndPrint function

errorString := fmt.Sprintf("Out of bounds access for slice. Index passed: %d", index)

The program also outputs the correct formatted message

panic: Out of bounds access for slice. Index passed: 2