Returning errors in Go - Step-by-Step Execution

Concept Flow - Returning errors

Start function

↓

Perform operation

↓

Error occurred?

No→Return result, nil

Yes↓

Return zero value, error

↓

Function ends

The function starts, does some work, checks if an error happened, and returns either the result with no error or a zero value with an error.

Execution Sample

Go

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

This function divides two numbers and returns an error if dividing by zero.

Execution Table

Step	Input (a,b)	Condition (b==0)	Action	Return values
1	a=10, b=2	false	Calculate 10/2	5, nil
2	a=10, b=0	true	Return error	0, error("cannot divide by zero")

💡 Function returns after either successful division or error detection.

Variable Tracker

Variable	Start	After Step 1	After Step 2
a	10	10	10
b	2 or 0	2	0
error	nil	nil	error("cannot divide by zero")
result	undefined	5	0

Key Moments - 2 Insights

Why do we return 0 along with the error instead of just the error?

In Go, functions must return all declared return values. When an error occurs, we return the zero value for the result (here 0) and the error. See execution_table row 2.

What does 'nil' mean when returned as the error?

'nil' means no error happened. In execution_table row 1, error is nil, so the division was successful.

Visual Quiz - 3 Questions

Look at the execution_table, what is returned when b=0?

A 5, nil

B 0, error("cannot divide by zero")

C 10, nil

D 0, nil

💡 Hint

Check execution_table row 2 for b=0 case.

At which step is the condition b==0 false?

A Both steps

B Step 2

C Step 1

D Neither step

💡 Hint

Look at the Condition column in execution_table.

If we change the return to 'return a / b, error' without checking b==0, what happens?

A Program panics at runtime if b=0

B Returns zero and nil error

C Always returns error

D Compiles but never runs

💡 Hint

Think about division by zero in Go and error handling in execution_table.

Concept Snapshot

func functionName(...) (resultType, error) {
  if errorCondition {
    return zeroValue, error
  }
  return result, nil
}

- Return zero value with error on failure
- Return result with nil error on success
- Always return both values

Full Transcript

This example shows how a Go function returns errors. The function divide takes two integers and returns an integer result and an error. It checks if the divisor is zero. If yes, it returns zero and an error message. If no, it returns the division result and nil error. The execution table shows two cases: dividing 10 by 2 returns 5 and no error; dividing 10 by 0 returns zero and an error. Variables a and b keep their input values. The error variable is nil when no error occurs and holds an error message otherwise. Returning zero with an error is required because Go functions must return all declared values. Nil means no error. If we skip the error check and divide by zero, the program will crash at runtime.

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