Threads overview in Rust - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Rust thread code?

Consider the following Rust program that creates a thread and prints a message. What will it print?

Rust

use std::thread;

fn main() {
    let handle = thread::spawn(|| {
        println!("Hello from the thread!");
    });
    handle.join().unwrap();
    println!("Hello from the main thread!");
}

A Hello from the main thread!\nHello from the thread!

B Hello from the thread!\nHello from the main thread!

C Hello from the thread!

D Hello from the main thread!

💡 Hint

Remember that join() waits for the thread to finish before continuing.

The spawned thread prints first, then join() waits for it to finish. After that, the main thread prints its message.

🧠 Conceptual

intermediate

Which statement about Rust threads is true?

Choose the correct statement about Rust threads.

A Rust threads require explicit synchronization to share data safely.

B Rust threads automatically clone all variables from the main thread.

C Rust threads cannot be joined once spawned.

D Rust threads share memory by default without any safety checks.

💡 Hint

Think about Rust's ownership and safety guarantees.

Rust enforces safe concurrency by requiring explicit synchronization (like Mutex or channels) to share data between threads.

🔧 Debug

advanced

What error does this Rust thread code produce?

Examine the code below. What error will the Rust compiler report?

Rust

use std::thread;

fn main() {
    let v = vec![1, 2, 3];
    let handle = thread::spawn(|| {
        println!("{:?}", v);
    });
    handle.join().unwrap();
}

A No error, prints [1, 2, 3]

B error: use of moved value `v`

C error: closure may outlive the current function, but it borrows `v`, which is owned by the current function

D error: expected `FnOnce` closure, found `FnMut` closure

💡 Hint

Think about how closures capture variables and thread lifetimes.

The closure borrows `v` but the thread may outlive `v`'s scope, so Rust prevents this with a lifetime error.

❓ Predict Output

advanced

What is the output of this Rust thread code with move closure?

What will this Rust program print?

Rust

use std::thread;

fn main() {
    let v = vec![10, 20, 30];
    let handle = thread::spawn(move || {
        println!("{:?}", v);
    });
    handle.join().unwrap();
}

A error: use of moved value `v`

B error: closure does not implement `FnOnce`

C []

D [10, 20, 30]

💡 Hint

Consider what the move keyword does to variable capture.

The move keyword transfers ownership of v into the closure, so it can safely be used inside the thread.

🧠 Conceptual

expert

Which Rust synchronization primitive allows multiple threads to read but only one to write at a time?

Choose the synchronization primitive that permits multiple readers or one writer concurrently.

A RwLock

B AtomicBool

C Mutex

D Condvar

💡 Hint

Think about read-write locks.

RwLock allows multiple readers or one writer at a time, enabling more concurrency than a Mutex.

Practice

1. What is the main purpose of using threads in Rust?

easy

A. To make the program run slower

B. To avoid using variables

C. To write code without functions

D. To run multiple parts of a program at the same time

Solution

1. Step 1: Understand what threads do

   Threads allow a program to perform multiple tasks simultaneously, improving efficiency.

2. Step 2: Match purpose with options

   Only To run multiple parts of a program at the same time correctly states that threads run parts of a program at the same time.

3. Final Answer:

   To run multiple parts of a program at the same time -> Option D

4. Quick Check:

   Threads = run tasks simultaneously [OK]

Hint: Threads let programs do many things at once [OK]

Common Mistakes:

• Thinking threads slow down the program
• Confusing threads with functions
• Believing threads remove variables

2. Which of the following is the correct way to start a new thread in Rust?

easy

A. thread::start(|| println!("Hello"));

B. thread::spawn(|| println!("Hello"));

C. thread::run(|| println!("Hello"));

D. thread::create(|| println!("Hello"));

Solution

1. Step 1: Recall Rust thread syntax

   Rust uses thread::spawn to create a new thread.

2. Step 2: Compare options with correct syntax

   Only thread::spawn(|| println!("Hello")); uses thread::spawn correctly.

3. Final Answer:

   thread::spawn(|| println!("Hello")); -> Option B

4. Quick Check:

   Start thread = thread::spawn [OK]

Hint: Use thread::spawn to start threads in Rust [OK]

Common Mistakes:

• Using thread::start instead of spawn
• Using thread::run or thread::create which don't exist
• Missing the closure syntax ||

3. What will the following Rust code print?

use std::thread;

fn main() {
    let handle = thread::spawn(|| {
        println!("Inside thread");
    });
    handle.join().unwrap();
    println!("Thread finished");
}

medium

A. Inside thread
Thread finished

B. Thread finished

C. Inside thread

D. Thread finished
Inside thread

Solution

1. Step 1: Understand thread::spawn and join

   The thread prints "Inside thread". The main thread waits for it to finish using join().

2. Step 2: Determine print order

   Because join() waits, "Inside thread" prints first, then "Thread finished" prints.

3. Final Answer:

   Inside thread Thread finished -> Option A

4. Quick Check:

   join() waits, so inside thread prints before main [OK]

Hint: join() waits for thread, so thread prints before main continues [OK]

Common Mistakes:

• Assuming main prints first without waiting
• Ignoring join() effect
• Expecting only one print line

4. Identify the error in this Rust code that uses threads:

use std::thread;

fn main() {
    let handle = thread::spawn(|| {
        println!("Hello from thread");
    });
    println!("Main thread");
}

medium

A. Missing call to join(), so main may end before thread finishes

B. thread::spawn is used incorrectly

C. println! macro is used wrong

D. No error, code runs perfectly

Solution

1. Step 1: Check thread usage

   The code starts a thread but does not wait for it to finish with join().

2. Step 2: Understand consequences

   Without join(), the main thread may finish and exit before the spawned thread prints.

3. Final Answer:

   Missing call to join(), so main may end before thread finishes -> Option A

4. Quick Check:

   Always join threads to ensure they complete [OK]

Hint: Always join threads to wait for their completion [OK]

Common Mistakes:

• Forgetting to call join()
• Thinking thread::spawn syntax is wrong
• Assuming println! is incorrect

5. You want to run two threads that each print a message, then print "Done" after both finish. Which code correctly does this?

hard

A. thread::spawn(|| println!("Thread 1")).join().unwrap();
thread::spawn(|| println!("Thread 2"));
println!("Done");

B. let t1 = thread::spawn(|| println!("Thread 1"));
let t2 = thread::spawn(|| println!("Thread 2"));
println!("Done");

C. let t1 = thread::spawn(|| println!("Thread 1"));
let t2 = thread::spawn(|| println!("Thread 2"));
t1.join().unwrap();
t2.join().unwrap();
println!("Done");

D. thread::spawn(|| println!("Thread 1"));
thread::spawn(|| println!("Thread 2"));
thread::spawn(|| println!("Done"));

Solution

1. Step 1: Understand thread creation and waiting

   To ensure "Done" prints after both threads finish, we must join both threads before printing.

2. Step 2: Analyze options for correct join usage

   let t1 = thread::spawn(|| println!("Thread 1")); let t2 = thread::spawn(|| println!("Thread 2")); t1.join().unwrap(); t2.join().unwrap(); println!("Done"); creates two threads, joins both, then prints "Done". This guarantees order.

3. Final Answer:

   let t1 = thread::spawn(|| println!("Thread 1")); let t2 = thread::spawn(|| println!("Thread 2")); t1.join().unwrap(); t2.join().unwrap(); println!("Done"); -> Option C

4. Quick Check:

   Join all threads before final print to ensure order [OK]

Hint: Join all threads before final action to ensure completion [OK]

Common Mistakes:

• Not joining threads before printing "Done"
• Joining threads immediately after spawn (serializing)
• Spawning extra thread for "Done" message