Why defer is used in Go - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to ensure the file is closed after opening it.

Go

file, err := os.Open("example.txt")
if err != nil {
    log.Fatal(err)
}
[1] file.Close()

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

Afunc

Bgo

Cdefer

Dreturn

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'go' instead of 'defer' causes the file to close immediately or not at the right time.

The defer keyword schedules the file.Close() call to run after the surrounding function finishes, ensuring the file is closed properly.

2fill in blank

medium

Complete the code to print 'Done' after the function finishes.

Go

func process() {
    [1] fmt.Println("Done")
    fmt.Println("Processing...")
}

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

Afunc

Bgo

Creturn

Ddefer

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'go' runs the print concurrently, not after the function ends.

Using defer before fmt.Println("Done") delays printing 'Done' until process finishes.

3fill in blank

hard

Fix the error by completing the code to close the database connection after use.

Go

func queryDB() {
    db, err := sql.Open("driver", "source")
    if err != nil {
        log.Fatal(err)
    }
    [1] db.Close()
    // perform queries
}

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

Adefer

Bgo

Creturn

Dfunc

Attempts:

3 left

💡 Hint

Common Mistakes

Not using defer causes the connection to remain open longer than needed.

defer ensures db.Close() runs after queryDB finishes, properly closing the connection.

4fill in blank

hard

Fill both blanks to defer closing the file and unlocking the mutex.

Go

func safeWrite() {
    mu.Lock()
    [1] mu.Unlock()
    file, err := os.Create("data.txt")
    if err != nil {
        log.Fatal(err)
    }
    [2] file.Close()
    // write data
}

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

Adefer

Bgo

Creturn

Dfunc

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'go' instead of 'defer' causes unlocking or closing to happen too early or concurrently.

Using defer before mu.Unlock() and file.Close() ensures both run after safeWrite finishes, keeping code safe and clean.

5fill in blank

hard

Fill all three blanks to defer closing the file, unlocking the mutex, and printing 'Finished'.

Go

func processFile() {
    mu.Lock()
    [1] mu.Unlock()
    file, err := os.Open("file.txt")
    if err != nil {
        log.Fatal(err)
    }
    [2] file.Close()
    [3] fmt.Println("Finished")
    // process file
}

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

Ago

Bdefer

Creturn

Dfunc

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'go' runs the functions concurrently and immediately, not after the function ends.

All three calls are deferred to run after processFile ends, ensuring proper cleanup and final message.