Loop construct in Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the basic syntax of an infinite loop in Rust?

In Rust, an infinite loop is created using the loop keyword followed by a block of code:

loop {
    // code to repeat forever
}

beginner

How do you exit a loop early in Rust?

You use the break keyword to exit a loop before it naturally ends.
Example:

loop {
    if condition {
        break;
    }
}

beginner

What is the difference between while and for loops in Rust?

while loops run as long as a condition is true.
for loops iterate over items in a collection or range.
Example:

while x < 5 {
    x += 1;
}
for i in 0..5 {
    println!("{}", i);
}

intermediate

How can a loop return a value in Rust?

A <code>loop</code> can return a value by using <code>break</code> with a value.<br>Example:<br><pre>let result = loop {
    if condition {
        break 42;
    }
};
println!("{}", result); // prints 42</pre>

beginner

What is the purpose of the continue keyword in Rust loops?

continue skips the rest of the current loop iteration and moves to the next one.
Example:

for i in 0..5 {
    if i == 2 {
        continue; // skip printing 2
    }
    println!("{}", i);
}

Which keyword creates an infinite loop in Rust?

A while

B for

C loop

D repeat

The loop keyword creates an infinite loop in Rust.

How do you stop a loop immediately in Rust?

A break

B exit

C stop

D return

The break keyword stops a loop immediately.

What does the continue keyword do inside a loop?

A Ends the loop

B Skips to the next iteration

C Restarts the program

D Pauses the loop

continue skips the rest of the current iteration and moves to the next.

Which loop type is best to iterate over a range of numbers in Rust?

A do-while

B while

C loop

D for

The for loop is used to iterate over ranges or collections.

How can a loop return a value in Rust?

A Using <code>break</code> with a value

B Using <code>yield</code>

C Using <code>continue</code> with a value

D Using <code>return</code> inside the loop

A loop returns a value by using break with that value.

Explain how to create and control loops in Rust, including how to exit or skip iterations.

Think about how you repeat tasks and stop or skip steps.

Describe how a loop can return a value in Rust and give an example.

Remember that break can send a value out of the loop.

Practice

1. What does the loop keyword do in Rust?

easy

A. It runs the loop a fixed number of times.

B. It creates an infinite loop that runs until break is called.

C. It runs the loop only once.

D. It creates a loop that runs only if a condition is true.

Solution

1. Step 1: Understand the loop keyword

   The loop keyword in Rust creates a loop that repeats forever unless stopped.

2. Step 2: Identify how to stop the loop

   To stop a loop, you must use break inside it; otherwise, it runs infinitely.

3. Final Answer:

   It creates an infinite loop that runs until break is called. -> Option B

4. Quick Check:

   loop = infinite loop until break [OK]

Hint: Remember: loop runs forever unless broken [OK]

Common Mistakes:

• Thinking loop runs a fixed number of times
• Confusing loop with for or while
• Assuming loop stops automatically

2. Which of the following is the correct syntax to create an infinite loop in Rust?

easy

A. loop { println!("Hello"); }

B. while True { println!("Hello"); }

C. for i in 0.. { println!("Hello"); }

D. repeat { println!("Hello"); }

Solution

1. Step 1: Check Rust syntax for infinite loops

   Rust uses the keyword loop to create an infinite loop.

2. Step 2: Verify each option

   while True { println!("Hello"); } uses while True which is invalid syntax in Rust. for i in 0.. { println!("Hello"); } uses for with an infinite range, but 0.. is not valid without an end. repeat { println!("Hello"); } uses repeat which does not exist in Rust. loop { println!("Hello"); } correctly uses loop.

3. Final Answer:

   loop { println!("Hello"); } -> Option A

4. Quick Check:

   Correct infinite loop syntax = loop [OK]

Hint: Use loop keyword for infinite loops in Rust [OK]

Common Mistakes:

• Using while True which is invalid in Rust
• Trying to use repeat which doesn't exist
• Using for without a proper range

3. What will be the output of this Rust code?

let mut count = 0;
loop {
    count += 1;
    if count == 3 {
        break;
    }
    println!("{}", count);
}

medium

A. 1 2

B. 0 1 2

C. 1 2 3

D. No output (infinite loop)

Solution

1. Step 1: Trace the loop iterations

   Initially, count = 0. In the first loop, count becomes 1, which is printed. Then count becomes 2 and printed. When count becomes 3, the loop breaks before printing.

2. Step 2: List printed values

   Only 1 and 2 are printed before the break stops the loop.

3. Final Answer:

   1 2 -> Option A

4. Quick Check:

   Loop prints until count == 3, so prints 1 and 2 [OK]

Hint: Check when break happens before printing [OK]

Common Mistakes:

• Printing count after break condition
• Starting count from 0 but expecting 0 to print
• Assuming infinite loop without break

4. Identify the error in this Rust loop code:

let mut i = 0;
loop {
    if i == 5 {
        break;
    }
    println!("{}", i);
}

medium

A. 'println!' macro syntax is wrong.

B. Missing semicolon after 'break'.

C. 'loop' keyword is used incorrectly.

D. Variable 'i' is not incremented, causing an infinite loop.

Solution

1. Step 1: Check loop control variable

   The variable i is initialized but never changed inside the loop.

2. Step 2: Understand loop behavior

   Since i never increments, i == 5 is never true, so break is never reached, causing an infinite loop.

3. Final Answer:

   Variable 'i' is not incremented, causing an infinite loop. -> Option D

4. Quick Check:

   Loop needs variable increment to reach break [OK]

Hint: Always update loop variables to avoid infinite loops [OK]

Common Mistakes:

• Forgetting to increment loop variable
• Assuming break works without condition change
• Misunderstanding loop syntax

5. How can you use a loop to sum numbers from 1 to 5 in Rust?

hard

A. Use loop with a counter but never update sum.

B. Use loop without break and add numbers forever.

C. Use a loop with a counter and break when counter > 5, adding each to sum.

D. Use loop with a counter but break immediately without adding.

Solution

1. Step 1: Plan the loop with counter and sum

   Initialize a counter at 1 and sum at 0. Use loop to add counter to sum.

2. Step 2: Use break to stop after 5

   Increment counter each loop. When counter > 5, use break to exit loop.

3. Step 3: Result is sum of 1 to 5

   This method correctly sums numbers 1+2+3+4+5 = 15.

4. Final Answer:

   Use a loop with a counter and break when counter > 5, adding each to sum. -> Option C

5. Quick Check:

   Loop + counter + break = sum 1 to 5 [OK]

Hint: Increment counter and break after limit to sum [OK]

Common Mistakes:

• Not breaking loop causing infinite sum
• Not incrementing counter
• Not adding counter to sum