Defining modules in Rust - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested module function call

What is the output of this Rust program?

Rust

mod outer {
    pub mod inner {
        pub fn greet() {
            println!("Hello from inner module!");
        }
    }
}

fn main() {
    outer::inner::greet();
}

A Hello from inner module!

B Compilation error: function not found

C Runtime error: module not found

D No output

💡 Hint

Check how the function is called using the full path with ::.

The function greet is public inside the nested module inner, which is inside outer. Calling outer::inner::greet() prints the message.

❓ Predict Output

intermediate

Accessing private module item

What error does this Rust code produce?

Rust

mod my_mod {
    fn secret() {
        println!("Secret function");
    }
}

fn main() {
    my_mod::secret();
}

A No error, prints 'Secret function'

B error[E0425]: cannot find function `secret` in `my_mod`

C error[E0433]: failed to resolve: use of undeclared crate or module `my_mod`

D error[E0603]: function `secret` is private

💡 Hint

Functions are private by default inside modules unless marked pub.

The function secret is private because it lacks pub. Trying to call it from outside the module causes an access error.

🔧 Debug

advanced

Fix the module import error

Given this Rust code, which option fixes the compilation error?

mod utils {
    pub fn helper() {
        println!("Helping");
    }
}

fn main() {
    helper();
}

A Add <code>use crate::utils::helper;</code> before <code>fn main()</code>

B Change <code>helper();</code> to <code>utils::helper();</code>

C Make <code>helper</code> function private by removing <code>pub</code>

D Add <code>mod helper;</code> inside <code>utils</code> module

💡 Hint

Functions inside modules need to be called with their full path or imported.

Calling helper() directly fails because it's inside utils. Using utils::helper() calls it correctly.

📝 Syntax

advanced

Identify the syntax error in module declaration

Which option contains the syntax error in defining a module?

Rust

mod math {
    pub fn add(a: i32, b: i32) -> i32 {
        a + b
    }
}

A mod math pub fn add(a: i32, b: i32) -> i32 { a + b }

B mod math { pub fn add(a: i32, b: i32) -> i32 { return a + b; } }

C mod math { pub fn add(a: i32, b: i32) -> i32 { a + b } }

D } } b + a { 23i >- )23i :b ,23i :a(dda nf bup { htam dom

💡 Hint

Check the syntax for module declaration braces.

Option A misses the braces { } after mod math, causing a syntax error.

🚀 Application

expert

Number of items in a nested module

Consider this Rust code:

mod outer {
    pub mod inner {
        pub fn f1() {}
        pub fn f2() {}
        fn f3() {}
    }
    fn f4() {}
}

fn main() {
    let count = count_pub_functions();
    println!("{}", count);
}

fn count_pub_functions() -> usize {
    // Imagine this function counts all public functions inside outer and its submodules
    0 // placeholder
}

What should count_pub_functions() return for the number of public functions inside outer and its nested modules?

A 4

B 3

C 2

D 1

💡 Hint

Count all pub fn inside outer and its nested modules.

Inside outer, f4 is private. Inside inner, f1 and f2 are public, f3 is private. So total public functions are 2 in inner + 0 in outer = 2.

But the question asks for all public functions inside outer and nested modules, so total is 2.