Constants in Rust - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of constant usage in Rust

What is the output of this Rust program that uses a constant?

Rust

const MAX_POINTS: u32 = 100_000;

fn main() {
    println!("Max points: {}", MAX_POINTS);
}

A Max points: 100_000

B Compilation error: constants must be mutable

C Max points: 100000

D Runtime error: constant not initialized

💡 Hint

Constants in Rust are immutable and can be used anywhere after declaration.

The constant MAX_POINTS is set to 100_000, which is just a numeric literal with an underscore for readability. The program prints the value as 100000 without underscores.

❓ Predict Output

intermediate

Constant shadowing and output

What will this Rust code print when run?

Rust

const VALUE: i32 = 10;

fn main() {
    let VALUE = 20;
    println!("Value is: {}", VALUE);
}

A Value is: 20

B Runtime error: variable VALUE not found

C Compilation error: cannot shadow constant

D Value is: 10

💡 Hint

Local variables can shadow constants with the same name.

The local variable VALUE shadows the constant VALUE inside main, so the printed value is 20.

❓ Predict Output

advanced

Constant expression evaluation

What is the output of this Rust program using constant expressions?

Rust

const BASE: u32 = 5;
const MULTIPLIER: u32 = 3;
const RESULT: u32 = BASE * MULTIPLIER;

fn main() {
    println!("Result: {}", RESULT);
}

A Compilation error: cannot multiply constants

B Result: 15

C Result: 8

D Runtime error: constant not initialized

💡 Hint

Constants can be computed at compile time using expressions.

The constant RESULT is computed as 5 * 3 = 15 at compile time and printed.

❓ Predict Output

advanced

Constant type mismatch error

What error does this Rust code produce?

Rust

const PI: f32 = 3.14;
const RADIUS: u32 = 5;
const AREA: u32 = PI * RADIUS * RADIUS;

fn main() {
    println!("Area: {}", AREA);
}

A Compilation error: mismatched types between f32 and u32

B Area: 78

C Runtime error: cannot multiply float and integer

D Compilation error: constant AREA not initialized

💡 Hint

Rust requires matching types in constant expressions.

The constant AREA tries to multiply a float (PI) with integers (RADIUS), causing a type mismatch error at compile time.

🧠 Conceptual

expert

Why use constants instead of variables in Rust?

Which of the following is the best reason to use a constant instead of a variable in Rust?

A Constants allow you to use mutable references safely

B Constants can be changed at runtime for flexibility

C Constants are faster because they are stored on the heap

D Constants guarantee immutability and can be used in any scope without allocation

💡 Hint

Think about immutability and compile-time guarantees.

Constants in Rust are immutable, have no fixed memory address, and can be used anywhere without runtime overhead, unlike variables.