Writing first Rust program - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the entry point of a Rust program?

The entry point of a Rust program is the fn main() function. This is where the program starts running.

beginner

How do you print text to the console in Rust?

You use the println! macro to print text to the console. For example: println!("Hello, world!");

intermediate

What does the exclamation mark (!) mean in println!?

The exclamation mark means println! is a macro, not a regular function. Macros can do things functions can't, like generating code.

beginner

Why do Rust programs need a main function?

Rust programs need a main function because it tells the computer where to start running the code, like the front door of a house.

beginner

What file extension do Rust source files use?

Rust source files use the .rs file extension. For example, main.rs is a common file name for the first program.

What keyword starts the main function in Rust?

A fn

B function

C main

D start

The fn keyword is used to define functions in Rust, including main.

Which macro prints text to the console in Rust?

A println!

B console.log

C echo!

D printline!

println! is the correct macro to print text with a newline in Rust.

What file extension should you use for Rust source files?

A .r

B .rust

C .rt

D .rs

Rust source files use the .rs extension.

What does the exclamation mark (!) after println indicate?

A It's a function

B It's a macro

C It's a variable

D It's a comment

The exclamation mark means println! is a macro, not a normal function.

What is the purpose of the main function in Rust?

A To define variables

B To print text

C To start the program

D To import libraries

The main function is where the program starts running.

Explain how to write and run your first Rust program that prints "Hello, world!".

Think about the main function, printing, and file naming.

Describe the role of macros in Rust and how they differ from functions, using println! as an example.

Focus on syntax and purpose differences.

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