Adding elements to vectors in Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a vector in Rust?

A vector in Rust is a growable list type that can store multiple values of the same type in a contiguous memory block. It allows adding, removing, and accessing elements dynamically.

beginner

How do you add a single element to a vector in Rust?

You use the push() method. For example: vec.push(5); adds the value 5 to the end of the vector.

beginner

What happens if you try to add elements to a vector that is not mutable?

You will get a compile-time error because vectors must be declared as mut to allow adding or changing elements.

intermediate

How can you add multiple elements to a vector at once?

You can use the extend() method with an iterable, like another vector or slice. Example: vec.extend(&[1, 2, 3]); adds three elements.

intermediate

What is the difference between push() and extend() when adding elements to a vector?

push() adds a single element to the vector, while extend() adds multiple elements from an iterable collection.

Which method adds a single element to the end of a Rust vector?

A push()

B extend()

C insert()

D append()

The push() method adds one element to the end of the vector.

What must you declare a vector as to add elements to it?

A const

B static

C mut

D let

Vectors must be declared as mut to allow adding or modifying elements.

Which method adds multiple elements from a slice or another vector?

A extend()

B push()

C insert()

D remove()

extend() adds multiple elements from an iterable like a slice or vector.

What will happen if you try to push an element to a non-mutable vector?

A It will add the element successfully

B The vector will be cloned

C Runtime error

D Compile-time error

Rust requires vectors to be mutable to change them; otherwise, it causes a compile-time error.

Which of these is NOT a valid way to add elements to a vector?

A vec.push(10)

B vec.add(5)

C vec.extend(&[1, 2, 3])

D vec.insert(0, 7)

add() is not a method for vectors; use push(), extend(), or insert() instead.

Explain how to add elements to a vector in Rust and why mutability matters.

Think about how you change a list in real life and what Rust requires to allow changes.

Describe the difference between push() and extend() when working with vectors.

Compare adding one item versus adding a group of items to a shopping cart.

Practice

1. What does the push method do when used on a Rust vector?

easy

A. Removes the last element from the vector

B. Adds a new element to the end of the vector

C. Creates a new vector with one element

D. Sorts the elements in the vector

Solution

1. Step 1: Understand the purpose of push

   The push method is used to add elements to a vector in Rust.

2. Step 2: Identify what push does specifically

   It adds a new element to the end of the vector, increasing its size by one.

3. Final Answer:

   Adds a new element to the end of the vector -> Option B

4. Quick Check:

   push adds element [OK]

Hint: Remember: push always adds at the end [OK]

Common Mistakes:

• Confusing push with pop (which removes elements)
• Thinking push creates a new vector
• Assuming push sorts the vector

2. Which of the following is the correct way to declare a mutable vector and add the number 10 to it in Rust?

easy

A. let v = Vec::new(); v.add(10);

B. let v = Vec::new(); v.push(10);

C. let mut v = Vec::new(); v.add(10);

D. let mut v = Vec::new(); v.push(10);

Solution

1. Step 1: Check vector mutability

   To add elements, the vector must be mutable, so let mut v is required.

2. Step 2: Use correct method to add element

   The method to add an element is push, not add.

3. Final Answer:

   let mut v = Vec::new(); v.push(10); -> Option D

4. Quick Check:

   Mutable vector + push method [OK]

Hint: Mutable vector + push method adds elements [OK]

Common Mistakes:

• Forgetting to make the vector mutable
• Using a non-existent method like add
• Trying to push on an immutable vector

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

let mut v = vec![1, 2];
v.push(3);
println!("{:?}", v);

medium

A. [1, 2]

B. Compilation error

C. [1, 2, 3]

D. [3, 1, 2]

Solution

1. Step 1: Understand initial vector content

   The vector starts with elements [1, 2].

2. Step 2: Apply the push method

   The push(3) adds 3 at the end, so vector becomes [1, 2, 3].

3. Final Answer:

   [1, 2, 3] -> Option C

4. Quick Check:

   push adds element at end [OK]

Hint: push adds element at the end of vector [OK]

Common Mistakes:

• Expecting push to add at the start
• Thinking push replaces elements
• Assuming code causes error

4. Identify the error in this Rust code snippet:

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

medium

A. Vector is not mutable, so push cannot be called

B. push method does not exist for vectors

C. Vec::new() cannot create an empty vector

D. Missing semicolon after Vec::new()

Solution

1. Step 1: Check vector mutability

   The vector is declared as immutable with let v, so it cannot be changed.

2. Step 2: Understand push method requirements

   The push method requires the vector to be mutable to add elements.

3. Final Answer:

   Vector is not mutable, so push cannot be called -> Option A

4. Quick Check:

   push needs mutable vector [OK]

Hint: Make vector mutable to use push [OK]

Common Mistakes:

• Assuming push works on immutable vectors
• Thinking Vec::new() is invalid
• Ignoring mut keyword necessity

5. You want to add multiple elements [4, 5, 6] to an existing mutable vector v. Which code correctly adds all these elements efficiently?

hard

A. v.extend([4, 5, 6]);

B. v.push(4, 5, 6);

C. v.add([4, 5, 6]);

D. v.push([4, 5, 6]);

Solution

1. Step 1: Understand adding multiple elements

   Using push adds one element at a time, so multiple calls are needed for multiple elements.

2. Step 2: Use the efficient method for multiple elements

   The extend method adds all elements from an iterable at once, which is more efficient.

3. Final Answer:

   v.extend([4, 5, 6]); -> Option A

4. Quick Check:

   Use extend for multiple elements [OK]

Hint: Use extend to add many elements at once [OK]

Common Mistakes:

• Using push with a whole array (causes error)
• Using non-existent add method
• Calling push multiple times instead of extend