Writing first Rust program - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Rust program?

Look at this Rust code. What will it print when run?

Rust

fn main() {
    let x = 5;
    let y = 10;
    println!("Sum is: {}", x + y);
}

A Sum is: 5 + 10

B Compilation error

C Sum is: 510

D Sum is: 15

💡 Hint

Remember, the + operator adds numbers, not strings.

The variables x and y hold numbers 5 and 10. Adding them gives 15. The println! macro prints "Sum is: 15".

❓ Predict Output

intermediate

What will this Rust program print?

Check this Rust code. What is the output?

Rust

fn main() {
    let name = "Alice";
    println!("Hello, {}!", name);
}

A Hello, Alice!

B Runtime error

C Hello, {}!

D Hello, name!

💡 Hint

The curly braces {} are replaced by the variable value.

The println! macro replaces {} with the value of name, which is "Alice".

❓ Predict Output

advanced

What is the output of this Rust program with a conditional?

What will this Rust program print?

Rust

fn main() {
    let number = 7;
    if number % 2 == 0 {
        println!("Even");
    } else {
        println!("Odd");
    }
}

A Compilation error

B Even

C Odd

D 7

💡 Hint

Check if 7 is divisible by 2 without remainder.

7 % 2 equals 1, so the else branch runs, printing "Odd".

❓ Predict Output

advanced

What does this Rust program print with a loop?

Look at this Rust code. What will it print?

Rust

fn main() {
    let mut count = 0;
    while count < 3 {
        println!("Count: {}", count);
        count += 1;
    }
}

A

Count: 0
Count: 1
Count: 2

B

Count: 0
Count: 1
Count: 2
Count: 3

C

Count: 1
Count: 2
Count: 3

D Infinite loop

💡 Hint

The loop runs while count is less than 3, starting from 0.

The loop prints count values 0, 1, and 2, then stops when count reaches 3.

❓ Predict Output

expert

What is the output of this Rust program using a function?

What will this Rust program print?

Rust

fn greet(name: &str) {
    println!("Hello, {}!", name);
}

fn main() {
    let user = String::from("Bob");
    greet(&user);
}

A Hello, user!

B Hello, Bob!

C Compilation error due to borrowing

D Runtime error

💡 Hint

Check how the string is passed to the function.

The function greet takes a string slice (&str). Passing &user works fine and prints "Hello, Bob!".

Practice

1. What is the correct way to start a basic Rust program?

easy

A. By defining a fn main() function

B. By writing start() function

C. By using begin() function

D. By declaring a program() function

Solution

1. Step 1: Understand Rust program entry point

   Rust programs always start execution from the main function.

2. Step 2: Identify the correct function name

   The correct function to start a Rust program is fn main().

3. Final Answer:

   By defining a fn main() function -> Option A

4. Quick Check:

   Rust program entry point = fn main() [OK]

Hint: Rust programs always start with fn main() [OK]

Common Mistakes:

• Using incorrect function names like start() or begin()
• Omitting the main function entirely
• Confusing Rust with other languages' entry points

2. Which of the following is the correct syntax to print "Hello, world!" in Rust?

easy

A. print!("Hello, world!");

B. println!("Hello, world!");

C. echo("Hello, world!");

D. console.log("Hello, world!");

Solution

1. Step 1: Identify Rust's print macro

   Rust uses the macro println! to print text with a newline.

2. Step 2: Compare options

   Only println!("Hello, world!"); is valid Rust syntax for printing with a newline.

3. Final Answer:

   println!("Hello, world!"); -> Option B

4. Quick Check:

   Print with newline = println! macro [OK]

Hint: Use println! to print with a newline in Rust [OK]

Common Mistakes:

• Using print! without newline when newline is expected
• Using functions from other languages like echo or console.log
• Missing the exclamation mark after println

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

fn main() {
    println!("{}", 5 + 3);
}

medium

A. 8

B. 5 + 3

C. {}

D. Compilation error

Solution

1. Step 1: Understand the println! macro with placeholders

   The {} inside the string is a placeholder for the value following the string.

2. Step 2: Calculate the expression inside println!

   The expression 5 + 3 evaluates to 8, which replaces the {}.

3. Final Answer:

   8 -> Option A

4. Quick Check:

   5 + 3 = 8 printed [OK]

Hint: {} prints evaluated expression inside println! [OK]

Common Mistakes:

• Expecting the literal string '5 + 3' to print
• Confusing {} as a string literal
• Thinking it causes a compile error

4. Identify the error in this Rust program:

fn main() {
    println!("Hello, world!")
}

medium

A. Missing quotation marks around string

B. Incorrect function name main

C. Missing semicolon after println! macro

D. println! macro does not exist

Solution

1. Step 1: Check syntax of println! macro call

   In Rust, statements must end with a semicolon unless they are the last expression in a block.

2. Step 2: Identify missing semicolon

   The println!("Hello, world!") line is missing a semicolon at the end.

3. Final Answer:

   Missing semicolon after println! macro -> Option C

4. Quick Check:

   Statements need semicolon unless last expression [OK]

Hint: Add semicolon after println! unless last expression [OK]

Common Mistakes:

• Forgetting semicolon after macro calls
• Thinking main function name is wrong
• Assuming println! is not a macro

5. You want to write a Rust program that prints "Hello, <name>!" where <name> is a variable. Which code correctly does this?

hard

A. fn main() {
let name = "Alice";
println!("Hello, {name}!");
}

B. fn main() {
let name = "Alice";
println!("Hello, name!");
}

C. fn main() {
let name = "Alice";
print!("Hello, {}!", name);
}

D. fn main() {
let name = "Alice";
println!("Hello, {}!", name);
}

Solution

1. Step 1: Understand variable interpolation in println!

   Rust uses {} as a placeholder and variables are passed after the string.

2. Step 2: Check each option for correct syntax and output

   fn main() { let name = "Alice"; println!("Hello, {}!", name); } correctly uses println!("Hello, {}!", name); to print the variable's value.

3. Final Answer:

   fn main() { let name = "Alice"; println!("Hello, {}!", name); } -> Option D

4. Quick Check:

   Use {} and pass variable to println! [OK]

Hint: Use {} and pass variable to println! for interpolation [OK]

Common Mistakes:

• Putting variable name inside quotes without {}
• Using print! instead of println! (missing newline)
• Trying to use {name} without Rust 1.58+ raw identifiers