Returning errors in Go

Introduction

Returning errors helps your program tell when something went wrong. It lets you handle problems safely and clearly.

When a function might fail and you want to tell the caller what happened.

When reading a file that might not exist or be readable.

When dividing numbers and the divisor could be zero.

When connecting to a database that might be down.

When parsing user input that might be invalid.

Syntax

Go

func functionName(params) (returnType, error) {
    if problem {
        return zeroValue, errors.New("error message")
    }
    return result, nil
}

The function returns two values: the result and an error.

If there is no error, return nil for the error.

Examples

This function returns an error if you try to divide by zero.

Go

func divide(a, b float64) (float64, error) {
    if b == 0 {
        return 0, errors.New("cannot divide by zero")
    }
    return a / b, nil
}

This function returns an error if the name is empty.

Go

func greet(name string) (string, error) {
    if name == "" {
        return "", errors.New("name cannot be empty")
    }
    return "Hello, " + name, nil
}

Sample Program

This program tries to divide two numbers. It prints the result if no error happens. If there is an error, it prints the error message.

Go

package main

import (
    "errors"
    "fmt"
)

func divide(a, b float64) (float64, error) {
    if b == 0 {
        return 0, errors.New("cannot divide by zero")
    }
    return a / b, nil
}

func main() {
    result, err := divide(10, 2)
    if err != nil {
        fmt.Println("Error:", err)
    } else {
        fmt.Println("Result:", result)
    }

    result, err = divide(5, 0)
    if err != nil {
        fmt.Println("Error:", err)
    } else {
        fmt.Println("Result:", result)
    }
}

Important Notes

Always check if the error is nil before using the result.

Use errors.New to create simple error messages.

Returning errors helps keep your program safe and clear.

Summary

Functions can return an error to show if something went wrong.

Return nil for error when everything is okay.

Always check errors before using the returned data.

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