Returning errors in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the common way to return errors in Go functions?

In Go, functions often return an error as the last return value. If there is no error, the error value is nil.

beginner

How do you check if a function returned an error in Go?

You compare the returned error value to nil. If it is not nil, an error occurred and should be handled.

intermediate

What does the following Go code return?

func divide(a, b int) (int, error) {
  if b == 0 {
    return 0, fmt.Errorf("cannot divide by zero")
  }
  return a / b, nil
}

This function returns the result of a divided by b and nil error if b is not zero. If b is zero, it returns 0 and an error with the message "cannot divide by zero".

intermediate

Why is it useful to return errors instead of panicking in Go?

Returning errors allows the caller to decide how to handle the problem, making the program more robust and easier to maintain.

beginner

What package is commonly used to create error values in Go?

The fmt package is commonly used with fmt.Errorf to create formatted error messages.

In Go, what does a function usually return to indicate an error?

A An error value as the last return value

B A boolean true or false

C A panic message

D A string describing the error

Go functions typically return an error type as the last return value to indicate if an error occurred.

How do you check if a Go function returned an error?

A Check if error is true

B Check if error equals zero

C Check if error is nil

D Check if error is empty string

In Go, an error is nil if no error occurred. If it is not nil, an error happened.

What does fmt.Errorf do in Go?

A Creates a formatted error value

B Prints an error message

C Stops the program

D Returns a boolean

fmt.Errorf creates an error value with a formatted message.

What should you do if a Go function returns a non-nil error?

A Return nil

B Ignore it

C Always panic

D Handle the error appropriately

You should handle errors to keep your program stable and predictable.

Which of these is a correct function signature for returning an error in Go?

A func example() error int

B func example() (int, error)

C func example() error

D func example() int error

The correct syntax places error as the last return type: (int, error).

Explain how to return and check errors in a Go function.

Think about how Go functions signal problems and how callers respond.

Describe why returning errors is preferred over panicking in Go programs.

Consider the difference between stopping a program and letting it recover.

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