String Basics and Memory Representation in DSA Rust - Build from Scratch

String Basics and Memory Representation

📖 Scenario: Imagine you are creating a simple program to store and display a person's full name. Understanding how strings work and how they are stored in memory is important for this task.

🎯 Goal: You will create a string variable to hold a full name, add a middle name to it, and then print the full name. This will help you understand how strings are created, modified, and displayed in Rust.

📋 What You'll Learn

Create a string variable with a first and last name

Add a middle name to the existing string

Print the final full name

💡 Why This Matters

🌍 Real World

Manipulating strings is common in programs that handle user names, messages, or any text data.

💼 Career

Understanding string basics and memory helps in writing efficient and bug-free code in Rust, which is valuable for systems programming and backend development.

1

Create a String with First and Last Name

Create a String variable called full_name and set it to the value "John Doe".

DSA Rust

# Create a String variable called full_name with value "John Doe"
# Your code here

Hint

Use String::from to create a new string in Rust.

2

Add a Middle Name to the String

Add the middle name "Michael " to the beginning of the full_name string using the insert_str method.

DSA Rust

fn main() {
    let mut full_name = String::from("John Doe");
    // Add "Michael " at the start of full_name
    # Your code here
}

Hint

The insert_str method inserts a string slice at a specified position.

3

Print the Full Name

Use println! to print the full_name string.

DSA Rust

fn main() {
    let mut full_name = String::from("John Doe");
    full_name.insert_str(0, "Michael ");
    // Print the full_name string
    # Your code here
}

Hint

Use println!("{}", variable) to print a variable's value.

4

Run and Verify the Output

Run the program and verify that the output is exactly Michael John Doe.

DSA Rust

fn main() {
    let mut full_name = String::from("John Doe");
    full_name.insert_str(0, "Michael ");
    println!("{}", full_name);
    // Verify output
    # Your code here
}

Hint

Check the console output matches the expected full name.

Practice

1. What does the String type in Rust represent?

easy

A. A growable, heap-allocated string

B. A fixed-size array of characters

C. A pointer to a string literal

D. An immutable string slice

Solution

1. Step 1: Understand Rust string types

   Rust has two main string types: String and string slices (&str). String is growable and stored on the heap.

2. Step 2: Compare options with definitions

   A growable, heap-allocated string correctly describes String as growable and heap-allocated. Others describe slices or fixed arrays.

3. Final Answer:

   A growable, heap-allocated string -> Option A

4. Quick Check:

   String = growable heap string [OK]

Hint: Remember: String is growable and heap-allocated [OK]

Common Mistakes:

• Confusing String with &str (string slice)
• Thinking String is fixed size
• Assuming String is immutable

2. Which of the following is the correct way to create a new empty String in Rust?

easy

A. let s = String::new();

B. let s = String();

C. let s = new String();

D. let s = String[];

Solution

1. Step 1: Recall Rust String creation syntax

   In Rust, String::new() creates a new empty String.

2. Step 2: Check each option syntax

   let s = String::new(); uses correct Rust syntax. The other options use invalid Rust syntax for creating a String.

3. Final Answer:

   let s = String::new(); -> Option A

4. Quick Check:

   String::new() creates empty String [OK]

Hint: Use String::new() to create empty String [OK]

Common Mistakes:

• Using parentheses like a function call without ::new()
• Trying to use new keyword like other languages
• Using array brackets for String

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

let s = String::from("hello");
let slice = &s[0..2];
println!("{}", slice);

medium

A. Runtime error

B. hel

C. hello

D. he

Solution

1. Step 1: Understand string slicing in Rust

   The slice &s[0..2] takes characters from index 0 up to but not including 2.

2. Step 2: Extract substring from "hello"

   Characters at index 0 and 1 are 'h' and 'e', so the slice is "he".

3. Final Answer:

   he -> Option D

4. Quick Check:

   Slice 0..2 of "hello" = "he" [OK]

Hint: Slice 0..n means characters from 0 up to n-1 [OK]

Common Mistakes:

• Assuming slice includes index 2
• Expecting full string output
• Thinking slicing causes runtime error

4. Identify the error in this Rust code snippet:

let s = String::from("hello");
let slice = &s[0..4];
println!("{}", slice);
let ch = &s[4];

medium

A. Slice range 0..4 is invalid

B. Cannot index String with single usize

C. Missing semicolon after println!

D. String::from is not valid

Solution

1. Step 1: Check indexing rules for String

   Rust does not allow indexing a String with a single usize like s[4] because strings are UTF-8 encoded and indexing by byte is unsafe.

2. Step 2: Verify other parts

   Slice &s[0..4] is valid. Semicolons and String::from usage are correct.

3. Final Answer:

   Cannot index String with single usize -> Option B

4. Quick Check:

   Single index on String causes error [OK]

Hint: Use slices, not single index, to access String parts [OK]

Common Mistakes:

• Trying to access String by single index
• Confusing slice range validity
• Ignoring Rust UTF-8 string rules

5. Given a UTF-8 encoded Rust String containing emoji and letters, which method safely returns the number of characters (not bytes)?

hard

A. s.len()

B. s.as_bytes().len()

C. s.chars().count()

D. s.bytes().count()

Solution

1. Step 1: Understand difference between bytes and characters

   UTF-8 strings store characters as 1 or more bytes. len() returns byte count, not character count.

2. Step 2: Identify method counting characters

   s.chars().count() iterates over Unicode scalar values (characters) and counts them correctly.

3. Final Answer:

   s.chars().count() -> Option C

4. Quick Check:

   chars().count() counts characters, not bytes [OK]

Hint: Use chars().count() for character count, not len() [OK]

Common Mistakes:

• Using len() which counts bytes, not characters
• Confusing bytes() with chars()
• Assuming as_bytes().len() counts characters