Mutable variables in Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean when a variable is mutable in Rust?

A mutable variable in Rust means you can change its value after it is first set. You declare it with mut before the variable name.

beginner

How do you declare a mutable variable named count with the value 5 in Rust?

You write: <code>let mut count = 5;</code> This means <code>count</code> can be changed later.

beginner

What happens if you try to change a variable that is not mutable in Rust?

Rust will give a compile-time error because variables are immutable by default. You must declare them as mutable to change their value.

intermediate

Why does Rust make variables immutable by default?

Immutability helps prevent bugs by avoiding unexpected changes. It makes programs safer and easier to understand.

beginner

Show an example of changing a mutable variable's value in Rust.

Example:<br><code>let mut score = 10;<br>score = 20;<br>println!("Score is {}", score);</code><br>This prints: <code>Score is 20</code>

How do you declare a mutable variable in Rust?

A var variable_name = value;

B let mut variable_name = value;

C mutable variable_name = value;

D let variable_name = mut value;

In Rust, you use let mut to declare a mutable variable.

What is the default mutability of variables in Rust?

A Mutable

B Depends on the type

C Immutable

D Mutable only for integers

Variables are immutable by default in Rust unless declared with mut.

What error occurs if you try to change an immutable variable?

A Compile-time error

B Warning only

C No error, it works fine

D Runtime error

Rust prevents changing immutable variables at compile time, so you get a compile-time error.

Why is immutability preferred by default in Rust?

A To prevent unexpected changes and bugs

B To make code run faster

C Because mutable variables are not supported

D To save memory

Immutability helps keep code safe and easier to understand by avoiding unexpected changes.

Which of the following is a correct way to change a mutable variable's value?

A let x = mut 5; x = 10;

B let x = 5; x = 10;

C let mut x = 5; mut x = 10;

D let mut x = 5; x = 10;

You must declare the variable as mutable with mut and then you can assign a new value.

Explain what mutable variables are in Rust and how to declare them.

Think about how you can change a variable's value after setting it.

Describe why Rust makes variables immutable by default and the benefits of this choice.

Consider how fixing values helps avoid mistakes.