Panic behavior in Go - Mini Project: Build & Apply

Panic behavior

📖 Scenario: Imagine you are writing a simple Go program that processes user input. Sometimes, the input might cause unexpected errors. Go uses panic to stop the program immediately when something goes wrong.

🎯 Goal: You will create a Go program that demonstrates how panic works by intentionally causing a panic and then recovering from it using defer and recover.

📋 What You'll Learn

Create a function called causePanic that triggers a panic with the message "Something went wrong!".

Create a function called handlePanic that uses defer and recover to catch the panic and print "Recovered from panic: " followed by the panic message.

Call handlePanic and then causePanic inside the main function.

Print "Program continues after panic recovery." after calling causePanic.

💡 Why This Matters

🌍 Real World

In real Go programs, panic and recover help handle unexpected errors like file read failures or invalid data, allowing programs to clean up and continue safely.

💼 Career

Understanding panic behavior is important for Go developers to write robust, fault-tolerant applications that can handle errors without crashing unexpectedly.

1

Create the causePanic function

Write a function called causePanic that triggers a panic with the message "Something went wrong!" using panic("Something went wrong!").

Go

package main

func causePanic() {
    // Your code here
}

func main() {

}

Hint

Use the panic function inside causePanic to stop the program with the given message.

2

Create the handlePanic function with defer and recover

Write a function called handlePanic that uses defer to call an anonymous function. Inside the anonymous function, use recover() to catch the panic. If a panic is caught, print "Recovered from panic: " followed by the panic message using println.

Go

package main

func causePanic() {
    panic("Something went wrong!")
}

func handlePanic() {
    // Your code here
}

func main() {

}

Hint

Use defer to delay the anonymous function until handlePanic returns. Inside it, use recover() to catch the panic value.

3

Call handlePanic and causePanic inside main

Inside the main function, first call handlePanic() and then call causePanic().

Go

package main

func causePanic() {
    panic("Something went wrong!")
}

func handlePanic() {
    defer func() {
        if r := recover(); r != nil {
            println("Recovered from panic: ", r)
        }
    }()
}

func main() {
    // Your code here
}

Hint

Call handlePanic() before causePanic() in main to catch the panic.

4

Print a message after panic recovery

After calling causePanic() in main, add a println statement that prints "Program continues after panic recovery.".

Go

package main

func causePanic() {
    panic("Something went wrong!")
}

func handlePanic() {
    defer func() {
        if r := recover(); r != nil {
            println("Recovered from panic: ", r)
        }
    }()
}

func main() {
    handlePanic()
    causePanic()
    // Your code here
}

Hint

Use println("Program continues after panic recovery.") after causePanic() in main.

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