Accessing elements in Rust - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Rust code accessing a vector element?

Consider the following Rust code that accesses elements in a vector. What will it print?

Rust

fn main() {
    let numbers = vec![10, 20, 30, 40];
    println!("{}", numbers[2]);
}

A Index out of bounds panic

B 20

C 40

D 30

💡 Hint

Remember that Rust vectors are zero-indexed, so the first element is at index 0.

The vector has elements [10, 20, 30, 40]. Accessing numbers[2] gets the third element, which is 30.

❓ Predict Output

intermediate

What happens when accessing a vector element with an invalid index?

What will happen when running this Rust code?

Rust

fn main() {
    let letters = vec!['a', 'b', 'c'];
    println!("{}", letters[5]);
}

A Panics with index out of bounds error

B Prints 'a'

C Prints 'c'

D Prints nothing

💡 Hint

Check what happens if you try to access an index outside the vector's length.

Accessing letters[5] is invalid because the vector has only 3 elements (indices 0 to 2). Rust panics at runtime with an index out of bounds error.

❓ Predict Output

advanced

What is the output when using get() method to access vector elements?

Analyze this Rust code and determine its output.

Rust

fn main() {
    let nums = vec![1, 2, 3];
    match nums.get(1) {
        Some(val) => println!("{}", val),
        None => println!("No value"),
    }
    match nums.get(5) {
        Some(val) => println!("{}", val),
        None => println!("No value"),
    }
}

A 1\n3

B 2\nNo value

C No value\nNo value

D 2\n3

💡 Hint

The get() method returns an Option type. If the index is valid, it returns Some(value), else None.

nums.get(1) returns Some(2), so it prints 2. nums.get(5) returns None, so it prints 'No value'.

❓ Predict Output

advanced

What is the output of this code accessing elements in a tuple?

What will this Rust program print?

Rust

fn main() {
    let tup = (100, 200, 300);
    println!("{}", tup.1);
}

A 200

B 300

C Compilation error

D 100

💡 Hint

Tuple elements are accessed by dot followed by the index starting at 0.

tup.1 accesses the second element of the tuple, which is 200.

❓ Predict Output

expert

What is the output of this Rust code accessing elements in a nested vector?

Examine the code and determine what it prints.

Rust

fn main() {
    let matrix = vec![
        vec![1, 2, 3],
        vec![4, 5, 6],
        vec![7, 8, 9],
    ];
    println!("{}", matrix[1][2]);
}

A 9

B 5

C 6

D Index out of bounds panic

💡 Hint

Remember that matrix[1] accesses the second vector, then [2] accesses its third element.

matrix[1] is vec![4,5,6], and matrix[1][2] is 6.

Practice

1. In Rust, how do you access the first element of an array named arr?

easy

A. arr[0]

B. arr[1]

C. arr.first()

D. arr.get(1)

Solution

1. Step 1: Understand Rust indexing

   Rust arrays and slices use zero-based indexing, so the first element is at index 0.

2. Step 2: Access the element using square brackets

   Using arr[0] accesses the first element directly.

3. Final Answer:

   arr[0] -> Option A

4. Quick Check:

   First element index = 0 [OK]

Hint: Remember: Rust arrays start at index 0 [OK]

Common Mistakes:

• Using index 1 for the first element
• Confusing .first() with indexing
• Using get(1) which accesses second element

2. Which of the following is the correct syntax to access the third element of a vector v in Rust?

easy

A. v.get[3]

B. v(3)

C. v[3]

D. v[2]

Solution

1. Step 1: Recall zero-based indexing

   The third element is at index 2 because counting starts at zero.

2. Step 2: Use square brackets with index 2

   The correct syntax is v[2] to access the third element.

3. Final Answer:

   v[2] -> Option D

4. Quick Check:

   Third element index = 2 [OK]

Hint: Count from zero: third element is index 2 [OK]

Common Mistakes:

• Using parentheses instead of square brackets
• Using index 3 for third element
• Incorrect method call syntax like get[3]

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

let arr = [10, 20, 30, 40];
println!("{}", arr[1]);

medium

A. 20

B. 40

C. 30

D. 10

Solution

1. Step 1: Identify the array elements and their indexes

   The array arr has elements: 10 at index 0, 20 at index 1, 30 at index 2, and 40 at index 3.

2. Step 2: Access element at index 1

   The code prints arr[1], which is 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   arr[1] = 20 [OK]

Hint: Index 1 means second element in Rust arrays [OK]

Common Mistakes:

• Printing arr[0] instead of arr[1]
• Confusing index with element value
• Expecting output 30 or 40

4. Consider this Rust code snippet:

let v = vec![1, 2, 3];
let x = v[3];

What is the problem here?

medium

A. Syntax error: wrong vector declaration

B. Index out of bounds panic at runtime

C. Compile-time error: invalid index

D. No problem, x will be 3

Solution

1. Step 1: Check vector size and index used

   The vector v has 3 elements at indexes 0, 1, and 2. Index 3 is out of range.

2. Step 2: Understand Rust's runtime behavior

   Accessing v[3] causes a panic at runtime due to index out of bounds.

3. Final Answer:

   Index out of bounds panic at runtime -> Option B

4. Quick Check:

   Invalid index causes panic [OK]

Hint: Index must be less than vector length [OK]

Common Mistakes:

• Thinking it causes compile error
• Assuming x will be 3
• Ignoring runtime panic possibility

5. You have a nested vector let data = vec![vec![1, 2], vec![3, 4], vec![5, 6]];. How do you access the number 4?

hard

A. data[0][1]

B. data[2][0]

C. data[1][1]

D. data[1][0]

Solution

1. Step 1: Identify the outer and inner indexes

   The outer vector has 3 inner vectors: index 0 is [1, 2], index 1 is [3, 4], index 2 is [5, 6].

2. Step 2: Access the inner element 4

   Within the inner vector at index 1 ([3, 4]), the element 4 is at index 1.

3. Final Answer:

   data[1][1] -> Option C

4. Quick Check:

   Nested access: outer 1, inner 1 = 4 [OK]

Hint: First index outer vec, second index inner vec [OK]

Common Mistakes:

• Mixing up outer and inner indexes
• Using wrong inner index for element 4
• Accessing out-of-range indexes