Panic behavior in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Go program with panic and recover?

Consider the following Go code. What will it print when run?

Go

package main
import "fmt"
func main() {
    defer func() {
        if r := recover(); r != nil {
            fmt.Println("Recovered from panic:", r)
        }
    }()
    fmt.Println("Start")
    panic("something went wrong")
    fmt.Println("End")
}

A

Recovered from panic: something went wrong
Start

B

Start
End

C

Start
End
Recovered from panic: something went wrong

D

Start
Recovered from panic: something went wrong

💡 Hint

Remember that code after panic is not executed unless recovered.

The program prints "Start" first. Then panic occurs. The deferred function recovers from panic and prints the recovery message. The line after panic is not executed.

❓ Predict Output

intermediate

What happens when panic is not recovered?

What will be the output of this Go program?

Go

package main
import "fmt"
func main() {
    fmt.Println("Before panic")
    panic("fatal error")
    fmt.Println("After panic")
}

A

Before panic
fatal error

B

Before panic
After panic

C

Before panic
panic: fatal error
exit status 2

D

panic: fatal error
exit status 2

💡 Hint

If panic is not recovered, program crashes and prints panic message.

The program prints "Before panic" then panics. Since no recover is used, it prints panic message and exits with status 2. The line after panic is not executed.

🔧 Debug

advanced

Identify the panic cause in this Go code

What causes the panic in this Go program?

Go

package main
func main() {
    var s []int
    println(s[0])
}

A Index out of range panic because slice is nil and empty

B Nil pointer dereference panic

C Compilation error due to invalid slice access

D No panic, prints zero

💡 Hint

Accessing an element of an empty slice causes panic.

The slice s is nil and has length zero. Accessing s[0] causes an index out of range panic.

❓ Predict Output

advanced

What is the output of nested panics with recover?

What will this Go program print?

Go

package main
import "fmt"
func main() {
    defer func() {
        if r := recover(); r != nil {
            fmt.Println("First recover:", r)
        }
    }()
    defer func() {
        if r := recover(); r != nil {
            fmt.Println("Second recover:", r)
            panic("new panic")
        }
    }()
    panic("initial panic")
}

A

First recover: initial panic
Second recover: new panic

B

Second recover: initial panic
First recover: new panic

C Second recover: initial panic

D First recover: new panic

💡 Hint

Deferred functions run in reverse order. Recover can catch panics in each defer.

The last defer runs first and recovers from "initial panic", prints "Second recover: initial panic", then panics again with "new panic". The first defer then recovers from "new panic" and prints "First recover: new panic".

❓ Predict Output

expert

How many items are in the map after panic in this Go code?

What is the length of the map m after running this program?

Go

package main
import "fmt"
func main() {
    m := make(map[int]int)
    defer func() {
        if r := recover(); r != nil {
            fmt.Println("Recovered panic")
        }
    }()
    for i := 0; i < 3; i++ {
        if i == 2 {
            panic("stop")
        }
        m[i] = i * 10
    }
    fmt.Println(len(m))
}

A 2

B 3

C 0

D Program does not print length due to panic

💡 Hint

Panic occurs before adding the third item, but map keeps previous entries.

The loop adds m[0]=0 and m[1]=10. When i==2, panic occurs before adding m[2]. The deferred recover prevents crash but the print statement after loop is not reached. So length is printed only if after loop. Here it is not printed, but the question asks length after running, so map has 2 items.

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