Panic behavior in Go

Introduction

Panic is used in Go to stop the normal flow of a program when something unexpected happens. It helps to catch serious errors quickly.

When a program encounters a situation it cannot recover from, like a missing file.

When a function receives invalid input that should never happen.

During development, to catch bugs by stopping the program immediately.

When a critical resource is unavailable and continuing would cause wrong results.

Syntax

Go

panic("error message")

The panic function takes a message or value that explains the error.

When panic is called, the program stops normal execution and starts to unwind the stack.

Examples

This stops the program and shows the message "something went wrong".

Go

panic("something went wrong")

This function panics if the number is negative.

Go

func checkNumber(n int) {
    if n < 0 {
        panic("negative number not allowed")
    }
}

Sample Program

This program prints a start message, then panics with an error message. The line after panic does not run.

Go

package main

import "fmt"

func main() {
    fmt.Println("Start program")
    panic("unexpected error occurred")
    fmt.Println("This line will not run")
}

Important Notes

Panic should be used only for serious errors, not for normal error handling.

You can recover from panic using the recover function inside a deferred function.

Summary

Panic stops the program immediately with an error message.

Use panic for unexpected, unrecoverable errors.

Normal errors should be handled with error values, not panic.

Practice

1. What happens when a panic is called in a Go program?

easy

A. The program ignores the panic and continues running.

B. The program restarts automatically.

C. The program stops immediately and prints an error message.

D. The program logs the error but continues execution.

Solution

1. Step 1: Understand panic behavior

   In Go, calling panic immediately stops normal execution and begins panicking.

2. Step 2: Effect on program flow

   The program prints a stack trace and error message, then terminates unless recovered.

3. Final Answer:

   The program stops immediately and prints an error message. -> Option C

4. Quick Check:

   Panic stops program = B [OK]

Hint: Panic always stops the program immediately [OK]

Common Mistakes:

• Thinking panic allows program to continue
• Confusing panic with error return
• Assuming panic restarts the program

2. Which of the following is the correct syntax to cause a panic with the message "error occurred"?

easy

A. panic("error occurred";)

B. panic("error occurred")

C. panic: "error occurred"

D. panic = "error occurred"

Solution

1. Step 1: Recall panic syntax

   Panic is a built-in function called with parentheses and a string argument.

2. Step 2: Check each option

   panic("error occurred") uses correct function call syntax. Others have invalid assignment, colon, or extra semicolon.

3. Final Answer:

   panic("error occurred") -> Option B

4. Quick Check:

   Panic is a function call = D [OK]

Hint: Panic is called like a function with parentheses [OK]

Common Mistakes:

• Using assignment instead of function call
• Adding extra semicolons inside panic call
• Using colon instead of parentheses

3. What will be the output of this Go program?

package main
import "fmt"
func main() {
    fmt.Println("Start")
    panic("something went wrong")
    fmt.Println("End")
}

medium

A. Start
End

B. something went wrong
panic stack trace

C. Start

D. Start
something went wrong
panic stack trace

Solution

1. Step 1: Trace program execution

   The program prints "Start" first, then calls panic with a message.

2. Step 2: Effect of panic on following code

   After panic, the program stops immediately, so "End" is not printed. Panic message and stack trace appear.

3. Final Answer:

   Start something went wrong panic stack trace -> Option D

4. Quick Check:

   Panic stops after printing Start = C [OK]

Hint: Panic prints message and stops; code after panic not run [OK]

Common Mistakes:

• Expecting code after panic to run
• Ignoring panic message output
• Confusing panic output with normal print

4. Identify the error in this Go code snippet:

func divide(a, b int) int {
    if b == 0 {
        panic("division by zero")
    }
    return a / b
}

func main() {
    result := divide(10, 0)
    fmt.Println(result)
}

medium

A. Missing import of "fmt" package

B. Cannot panic inside a function

C. Division by zero is allowed, no panic needed

D. Function divide should return error instead of int

Solution

1. Step 1: Check code for syntax errors

   The code uses fmt.Println but does not import "fmt" package.

2. Step 2: Validate panic usage

   Panic is allowed inside functions; division by zero should panic to avoid runtime error.

3. Final Answer:

   Missing import of "fmt" package -> Option A

4. Quick Check:

   Missing import causes compile error = A [OK]

Hint: Check imports when using fmt functions [OK]

Common Mistakes:

• Thinking panic is not allowed in functions
• Ignoring missing imports
• Assuming division by zero returns zero

5. You want to write a Go function that safely divides two integers and returns the result. If the divisor is zero, it should panic with "division by zero". Which of these implementations correctly uses panic and recover to handle the error and return zero instead of crashing the program?

func safeDivide(a, b int) int {
    // Your code here
}

func main() {
    fmt.Println(safeDivide(10, 0))
}

hard

A. func safeDivide(a, b int) int {
defer func() {
if r := recover(); r != nil {
fmt.Println("Recovered from panic")
}
}()
if b == 0 {
panic("division by zero")
}
return a / b
}

B. func safeDivide(a, b int) int {
if b == 0 {
panic("division by zero")
}
return a / b
}

C. func safeDivide(a, b int) int {
defer func() {
if r := recover(); r != nil {
return 0
}
}()
if b == 0 {
panic("division by zero")
}
return a / b
}

D. func safeDivide(a, b int) int {
if b == 0 {
return 0
}
return a / b
}

Solution

1. Step 1: Understand panic and recover usage

   Recover must be called inside a deferred function to catch panic and prevent program crash.

2. Step 2: Analyze each option

   func safeDivide(a, b int) int { defer func() { if r := recover(); r != nil { fmt.Println("Recovered from panic") } }() if b == 0 { panic("division by zero") } return a / b } uses defer with recover to catch panic and prints a message, then continues. It returns the division result or panics if divisor is zero. func safeDivide(a, b int) int { defer func() { if r := recover(); r != nil { return 0 } }() if b == 0 { panic("division by zero") } return a / b } tries to return inside defer, which does not affect the outer function's return. func safeDivide(a, b int) int { if b == 0 { panic("division by zero") } return a / b } panics without recovery. func safeDivide(a, b int) int { if b == 0 { return 0 } return a / b } returns 0 without panic.

3. Final Answer:

   Option A correctly uses panic and recover to handle error and avoid crash. -> Option A

4. Quick Check:

   Defer recover catches panic = A [OK]

Hint: Use defer with recover to catch panic safely [OK]

Common Mistakes:

• Returning inside defer does not change function return
• Not using recover causes program crash
• Ignoring panic and returning zero silently