Vector creation in Rust - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Rust code creating a vector?

Consider this Rust code that creates a vector and prints it. What will be printed?

Rust

fn main() {
    let v = vec![1, 2, 3, 4];
    println!("{:?}", v);
}

A [1, 2, 3, 4]

B 1 2 3 4

C [1 2 3 4]

D vec![1, 2, 3, 4]

💡 Hint

Look at how the vector is printed with {:?} in println!

Using {:?} in println! prints the vector in debug format with square brackets and commas.

❓ Predict Output

intermediate

What is the length of the vector after this code runs?

Look at this Rust code that creates a vector and pushes elements. What is the length of the vector at the end?

Rust

fn main() {
    let mut v = Vec::new();
    v.push(10);
    v.push(20);
    v.push(30);
    println!("{}", v.len());
}

A 3

B 0

C 2

D None

💡 Hint

Count how many times push is called.

Each push adds one element, so after three pushes, length is 3.

🔧 Debug

advanced

What error does this Rust code produce?

This Rust code tries to create a vector with a fixed size but fails. What error will it produce?

Rust

fn main() {
    let v = vec![0;];
    println!("{:?}", v);
}

A RuntimeError: index out of bounds

B TypeError: mismatched types

C SyntaxError: expected expression after semicolon

D No error, prints an empty vector

💡 Hint

Look carefully at the vec! macro syntax with semicolons.

The vec![0;] macro expects a number after the semicolon to specify size, missing here causes syntax error.

❓ Predict Output

advanced

What is the output of this Rust code using vector initialization?

What will this Rust program print?

Rust

fn main() {
    let v = vec![5; 3];
    println!("{:?}", v);
}

A [5, 3]

B [5, 5, 5]

C [5; 3]

D [3, 3, 3, 3, 3]

💡 Hint

vec![value; count] creates a vector with count copies of value.

vec![5; 3] creates a vector with three 5s: [5, 5, 5].

🧠 Conceptual

expert

How many elements does this vector contain after execution?

Analyze this Rust code snippet. How many elements does the vector contain after the loop finishes?

Rust

fn main() {
    let mut v = Vec::new();
    for i in 0..5 {
        if i % 2 == 0 {
            v.push(i);
        }
    }
    println!("{}", v.len());
}

A 5

B 0

C 2

D 3

💡 Hint

Count how many numbers from 0 to 4 are even.

The loop pushes only even numbers: 0, 2, 4, so length is 3.

Practice

1. Which of the following correctly creates an empty vector of integers in Rust?

easy

A. let v: Vec<i32> = Vec::new();

B. let v = vec!();

C. let v = Vector::new();

D. let v = vec![0];

Solution

1. Step 1: Understand how to create an empty vector

   In Rust, an empty vector is created using Vec::new() with the type specified if needed.

2. Step 2: Check each option

   let v: Vec<i32> = Vec::new(); uses Vec::new() with type i32, which is correct. let v = vec!(); uses vec!() which is invalid syntax. let v = Vector::new(); uses Vector::new() which is not a Rust type. let v = vec![0]; creates a vector with one element 0, not empty.

3. Final Answer:

   let v: Vec<i32> = Vec::new(); -> Option A

4. Quick Check:

   Empty vector = Vec::new() [OK]

Hint: Empty vector uses Vec::new() with type [OK]

Common Mistakes:

• Using vec!() with empty parentheses
• Using Vector instead of Vec
• Creating vector with one element instead of empty

2. Which syntax correctly creates a vector with the values 1, 2, and 3 in Rust?

easy

A. let v = vec{1, 2, 3};

B. let v = Vec[1, 2, 3];

C. let v = vec(1, 2, 3);

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

Solution

1. Step 1: Recall vector creation with values

   Rust uses the macro vec![...] with square brackets and exclamation mark to create vectors with initial values.

2. Step 2: Evaluate each option

   let v = vec![1, 2, 3]; uses correct syntax vec![1, 2, 3]. let v = Vec[1, 2, 3]; uses square brackets without exclamation mark, invalid. let v = vec(1, 2, 3); uses parentheses, invalid. let v = vec{1, 2, 3}; uses curly braces, invalid.

3. Final Answer:

   let v = vec![1, 2, 3]; -> Option D

4. Quick Check:

   Vector with values = vec![...] [OK]

Hint: Use vec! macro with square brackets for values [OK]

Common Mistakes:

• Missing exclamation mark after vec
• Using parentheses or curly braces instead of brackets
• Using Vec[...] which is invalid syntax

3. What is the output of this Rust code?

let mut v = Vec::new();
v.push(10);
v.push(20);
println!("{:?}", v);

medium

A. [10, 20]

B. [20, 10]

C. 10 20

D. Error: cannot push to vector

Solution

1. Step 1: Understand vector push operations

   The vector starts empty. push(10) adds 10 at the end, then push(20) adds 20 after 10.

2. Step 2: Check the printed output

   The println!("{:?}", v) prints the vector in debug format showing all elements in order: [10, 20].

3. Final Answer:

   [10, 20] -> Option A

4. Quick Check:

   Push adds elements in order [OK]

Hint: push adds elements at the end, print shows all [OK]

Common Mistakes:

• Assuming push adds at the front
• Printing vector without debug format
• Confusing order of elements

4. Identify the error in this Rust code snippet:

let v = Vec::new();
v.push(5);

medium

A. push method does not exist for vectors

B. Vec::new() does not create a vector

C. Cannot push to an immutable vector

D. Missing semicolon after Vec::new()

Solution

1. Step 1: Check vector mutability

   The vector v is declared without mut, so it is immutable by default.

2. Step 2: Understand push method requirements

   The push method requires the vector to be mutable to add elements. Trying to push to an immutable vector causes a compile error.

3. Final Answer:

   Cannot push to an immutable vector -> Option C

4. Quick Check:

   push needs mutable vector [OK]

Hint: Add mut to vector to allow push [OK]

Common Mistakes:

• Forgetting mut keyword
• Thinking Vec::new() returns non-vector
• Assuming push works on immutable vectors

5. You want to create a vector of strings from an array of &str slices and then add a new string. Which code correctly does this?

hard

A. let v = vec!["a", "b", "c"]; v.push("d");

B. let mut v: Vec<String> = ["a", "b", "c"].iter().map(|&s| s.to_string()).collect(); v.push("d".to_string());

C. let mut v = Vec::new(); v.push(["a", "b", "c"]); v.push("d");

D. let mut v = vec![String::from("a"), String::from("b"), String::from("c")]; v.push('d');

Solution

1. Step 1: Convert &str slices to String vector

   The array of &str slices needs to be converted to a vector of String using iter(), map() with to_string(), then collect().

2. Step 2: Add a new String element

   After creating a mutable vector, use push with "d".to_string() to add a new string.

3. Step 3: Check other options

   let v = vec!["a", "b", "c"]; v.push("d"); tries to push to an immutable vector of &str slices, invalid. let mut v = Vec::new(); v.push(["a", "b", "c"]); v.push("d"); tries to push an array inside vector, invalid. let mut v = vec![String::from("a"), String::from("b"), String::from("c")]; v.push('d'); pushes a char instead of String, invalid.

4. Final Answer:

   let mut v: Vec<String> = ["a", "b", "c"].iter().map(|&s| s.to_string()).collect(); v.push("d".to_string()); -> Option B

5. Quick Check:

   Convert slices to String vector, then push String [OK]

Hint: Map &str to String, collect to Vec<String>, then push [OK]

Common Mistakes:

• Trying to push to immutable vector
• Pushing wrong types (char instead of String)
• Not converting &str slices to String