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.

Practice

1.

What does a Go function return when there is no error?

easy

A. It returns nil as the error value.

B. It returns 0 as the error value.

C. It returns "no error" as the error value.

D. It returns an empty string "" as the error value.

Solution

1. Step 1: Understand Go error convention

   In Go, functions return an error type to indicate problems. If no problem occurs, the error is nil.

2. Step 2: Identify the correct no-error return value

   The standard way to show no error is to return nil, not zero, empty string, or text.

3. Final Answer:

   It returns nil as the error value. -> Option A

4. Quick Check:

   No error means error = nil [OK]

Hint: No error always returns nil, never zero or empty string [OK]

Common Mistakes:

• Returning 0 instead of nil for no error
• Returning empty string instead of nil
• Returning a string message instead of nil

2.

Which of the following is the correct way to declare a Go function that returns an int and an error?

func example() ???

easy

A. func example() (int, error)

B. func example() int, error

C. func example() int error

D. func example() (int error)

Solution

1. Step 1: Recall Go function return syntax

   Go functions return multiple values by listing types in parentheses separated by commas.

2. Step 2: Match correct syntax

   The correct syntax for returning an int and error is (int, error). Other options miss commas or parentheses.

3. Final Answer:

   func example() (int, error) -> Option A

4. Quick Check:

   Multiple returns use parentheses and commas [OK]

Hint: Use parentheses and commas for multiple return types [OK]

Common Mistakes:

• Omitting parentheses for multiple returns
• Missing comma between return types
• Writing return types without parentheses

3.

What will be the output of this Go program?

package main
import (
  "errors"
  "fmt"
)
func divide(a, b int) (int, error) {
  if b == 0 {
    return 0, errors.New("division by zero")
  }
  return a / b, nil
}
func main() {
  result, err := divide(10, 0)
  if err != nil {
    fmt.Println(err)
  } else {
    fmt.Println(result)
  }
}

medium

A. runtime error

B. division by zero

C. 10

D. 0

Solution

1. Step 1: Analyze divide function behavior

   If divisor b is zero, function returns error with message "division by zero" and result 0.

2. Step 2: Check main function output

   Main calls divide(10, 0), gets error != nil, so prints the error message.

3. Final Answer:

   division by zero -> Option B

4. Quick Check:

   Error returned prints message [OK]

Hint: If error is not nil, print error message [OK]

Common Mistakes:

• Printing result even when error is not nil
• Expecting 0 as output instead of error message
• Assuming runtime panic on division by zero

4.

Identify the error in this Go function that returns an error:

func readFile(name string) (string, error) {
  data := ""
  file, err := os.Open(name)
  if err != nil {
    return data, err
  }
  defer file.Close()
  // read file content
  return data, nil
}

medium

A. Function should return only error, not string.

B. Return statement should return only data.

C. Defer statement should be before error check.

D. Missing import of os package.

Solution

1. Step 1: Check for required imports

   The function uses os.Open but does not import os package, causing a compile error.

2. Step 2: Verify other code parts

   Return types and defer placement are correct; only missing import is the problem.

3. Final Answer:

   Missing import of os package. -> Option D

4. Quick Check:

   Using os.Open requires importing os [OK]

Hint: Check imports when using standard library functions [OK]

Common Mistakes:

• Forgetting to import packages used
• Placing defer before error check
• Returning wrong number of values

5.

You want to write a Go function parseNumber that takes a string and returns an int and an error. It should return an error if the string is empty or not a valid number. Which implementation correctly returns errors?

func parseNumber(s string) (int, error) {
  if s == "" {
    return 0, errors.New("empty input")
  }
  n, err := strconv.Atoi(s)
  if err != nil {
    return 0, err
  }
  return n, nil
}

hard

A. Returns error only for invalid number, not empty string.

B. Returns nil error even if input is empty.

C. Correctly returns errors for empty and invalid input.

D. Returns zero and nil error for all inputs.

Solution

1. Step 1: Check empty string error handling

   The function returns error "empty input" if string is empty, which is correct.

2. Step 2: Check invalid number error handling

   Uses strconv.Atoi to convert string; returns error if conversion fails.

3. Step 3: Confirm success return

   If no error, returns parsed number and nil error.

4. Final Answer:

   Correctly returns errors for empty and invalid input. -> Option C

5. Quick Check:

   Empty and invalid inputs return errors [OK]

Hint: Return error immediately on invalid input [OK]

Common Mistakes:

• Not checking empty string before conversion
• Ignoring error from strconv.Atoi
• Returning nil error on invalid input