Returning errors in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Go code with error handling?

Consider the following Go code that tries to open a file and returns an error if it fails. What will be printed when the file does not exist?

Go

package main

import (
	"fmt"
	"os"
)

func openFile(filename string) error {
	_, err := os.Open(filename)
	return err
}

func main() {
	err := openFile("nonexistent.txt")
	if err != nil {
		fmt.Println("Error occurred:", err)
	} else {
		fmt.Println("File opened successfully")
	}
}

A File opened successfully

B Error occurred: open nonexistent.txt: no such file or directory

C Error occurred: <nil>

D Compilation error: missing return statement

💡 Hint

Think about what happens when os.Open tries to open a file that does not exist.

The os.Open function returns an error if the file does not exist. The error is returned by openFile and checked in main. Since the file "nonexistent.txt" does not exist, the error is not nil, so the error message is printed.

❓ Predict Output

intermediate

What error does this Go function return?

Look at this Go function that returns an error. What will be the output when calling checkValue(5)?

Go

package main

import (
	"errors"
	"fmt"
)

func checkValue(x int) error {
	if x < 10 {
		return errors.New("value too small")
	}
	return nil
}

func main() {
	err := checkValue(5)
	if err != nil {
		fmt.Println(err.Error())
	} else {
		fmt.Println("Value is fine")
	}
}

A Compilation error: undefined errors package

B Value is fine

C value too small

D nil

💡 Hint

Check the condition inside checkValue and what error is returned.

The function returns an error with the message "value too small" if the input is less than 10. Since 5 is less than 10, the error is returned and printed.

🔧 Debug

advanced

Which option causes a compile-time error due to wrong error return?

Identify which code snippet will cause a compile-time error because it returns a wrong type instead of an error.

A func doSomething() error { return "error string" }

B func doSomething() error { return nil }

C func doSomething() error { return fmt.Errorf("fail") }

D func doSomething() error { return errors.New("fail") }

💡 Hint

Remember that the function must return a value of type error.

Option A returns a string instead of an error type, causing a compile-time type mismatch error. The other options correctly return error or nil.

❓ Predict Output

advanced

What is the output of this Go code using wrapped errors?

Consider this Go code that wraps an error. What will be printed?

Go

package main

import (
	"errors"
	"fmt"
)

func inner() error {
	return errors.New("inner error")
}

func outer() error {
	return fmt.Errorf("outer error: %w", inner())
}

func main() {
	err := outer()
	fmt.Println(err.Error())
}

A outer error

B inner error

C Compilation error: unknown verb %w

D outer error: inner error

💡 Hint

Look at how %w is used in fmt.Errorf.

The %w verb wraps the inner error inside the outer error message. So the printed message includes both errors.

❓ Predict Output

expert

What is the output of this Go code using custom error type and error interface?

Analyze this Go code with a custom error type implementing the error interface. What will be printed?

Go

package main

import (
	"fmt"
)

type MyError struct {
	Code int
	Msg  string
}

func (e *MyError) Error() string {
	return fmt.Sprintf("Error %d: %s", e.Code, e.Msg)
}

func doTask(fail bool) error {
	if fail {
		return &MyError{Code: 404, Msg: "Not Found"}
	}
	return nil
}

func main() {
	err := doTask(true)
	if err != nil {
		switch e := err.(type) {
		case *MyError:
			fmt.Println("Custom error detected:", e.Error())
		default:
			fmt.Println("Other error:", err.Error())
		}
	} else {
		fmt.Println("Task succeeded")
	}
}

A Custom error detected: Error 404: Not Found

B Other error: Error 404: Not Found

C Task succeeded

D Compilation error: cannot use &MyError literal as error

💡 Hint

Check how the type assertion works with the custom error type.

The error returned is of type *MyError. The type switch detects this and prints the custom message with code and text.