Returning errors in Go - Mini Project: Build & Apply

Returning errors

📖 Scenario: You are writing a simple program to check if a number is positive. If the number is not positive, you want to return an error message.

🎯 Goal: Build a Go program that defines a function to check if a number is positive and returns an error if it is not. Then call this function and handle the error properly.

📋 What You'll Learn

Create a function that returns an error when the input number is not positive

Use the built-in errors.New function to create the error

Call the function with a negative number to test error handling

Print the error message if an error is returned

💡 Why This Matters

🌍 Real World

Returning errors is important in real programs to handle unexpected situations like invalid input or failed operations.

💼 Career

Understanding how to return and handle errors is a key skill for Go developers working on reliable and maintainable software.

1

Create a function to check if a number is positive

Write a function called checkPositive that takes an int parameter named num and returns an error. The function should return nil for now.

Go

func checkPositive(num int) error {
    // Your code here
    return nil
}

Hint

Define the function with the correct name, parameter, and return type. Return nil to indicate no error.

2

Add error return for non-positive numbers

Modify the checkPositive function to return an error using errors.New with the message "number is not positive" if num is less than or equal to zero. Otherwise, return nil. Remember to import the errors package.

Go

package main

import "errors"

func checkPositive(num int) error {
    // Your code here
}

Hint

Use an if statement to check the number and return the error with errors.New.

3

Call the function and handle the error

In the main function, call checkPositive with the value -5 and store the result in a variable called err. Use an if statement to check if err is not nil.

Go

package main

import "errors"

func checkPositive(num int) error {
    if num <= 0 {
        return errors.New("number is not positive")
    }
    return nil
}

func main() {
    // Your code here
}

Hint

Call the function with -5 and check if the returned error is not nil.

4

Print the error message

Inside the if err != nil block, use fmt.Println(err) to print the error message. Remember to import the fmt package.

Go

package main

import (
    "errors"
    // Your code here
)

func checkPositive(num int) error {
    if num <= 0 {
        return errors.New("number is not positive")
    }
    return nil
}

func main() {
    err := checkPositive(-5)
    if err != nil {
        // Your code here
    }
}

Hint

Use fmt.Println(err) to show the error message on the screen.

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