Panic behavior in Go - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to cause a panic with the message "error occurred".

Go

func main() {
    panic([1])
}

Drag options to blanks, or click blank then click option'

A"error occurred"

Berror occurred

Cpanic("error occurred")

Dfmt.Println("error occurred")

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the string message

Calling panic inside panic argument

Using print functions instead of panic

The panic function takes a value to display as the panic message. It must be a string literal here.

2fill in blank

medium

Complete the code to recover from a panic inside the deferred function.

Go

func main() {
    defer func() {
        if r := [1](); r != nil {
            println("Recovered from panic")
        }
    }()
    panic("fail")
}

Drag options to blanks, or click blank then click option'

Arecover

Bclose

Cdefer

Dpanic

Attempts:

3 left

💡 Hint

Common Mistakes

Using panic instead of recover

Calling recover outside a deferred function

Not checking if recover returned nil

The recover function is used inside a deferred function to catch a panic and prevent the program from crashing.

3fill in blank

hard

Fix the error in the code to properly recover from a panic and print the panic message.

Go

func main() {
    defer func() {
        if r := [1](); r != nil {
            println("Panic message:", r)
        }
    }()
    panic("something went wrong")
}

Drag options to blanks, or click blank then click option'

Afmt.Println

Brecover

Cpanic

Dclose

Attempts:

3 left

💡 Hint

Common Mistakes

Using panic instead of recover

Trying to recover outside defer

Not checking if recover returned nil

Using recover() inside a deferred function allows catching the panic and accessing its message.

4fill in blank

hard

Fill both blanks to create a map of strings to their lengths, but only include words longer than 3 characters.

Go

func main() {
    words := []string{"go", "code", "panic", "fun"}
    lengths := map[string]int{
        [1]: len([2]) for _, word := range words if len(word) > 3
    }
    fmt.Println(lengths)
}

Drag options to blanks, or click blank then click option'

Aword

Bwords

Clen(word)

Dword.len

Attempts:

3 left

💡 Hint

Common Mistakes

Using the whole slice as key or value

Trying to call len as a method

Using wrong variable names

We use word as the key and len(word) as the value for each word longer than 3 characters.

Note: Go does not support map comprehensions like this; this is a conceptual exercise.

5fill in blank

hard

Fill all three blanks to create a deferred function that recovers from panic and prints the panic message.

Go

func main() {
    defer func() {
        if r := [1](); r != nil {
            fmt.[2]("Recovered panic: %v", [3])
        }
    }()
    panic("unexpected error")
}

Drag options to blanks, or click blank then click option'

Apanic

Brecover

Cr

DPrintf

Attempts:

3 left

💡 Hint

Common Mistakes

Using panic instead of recover

Using fmt.Println instead of fmt.Printf

Not passing the recovered value to print

Use recover() to catch the panic, fmt.Printf to print formatted output, and r as the recovered panic message.

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