String Traversal and Character Access in DSA Rust - Build from Scratch

String Traversal and Character Access

📖 Scenario: You are working on a simple text analyzer that needs to look at each letter in a word to count vowels.

🎯 Goal: Build a Rust program that goes through each character in a string and prints them one by one.

📋 What You'll Learn

Create a string variable with a specific word

Create a counter variable to count vowels

Use a for loop to traverse each character in the string

Print each character on a new line

💡 Why This Matters

🌍 Real World

Text processing programs often need to look at each letter to analyze or transform text.

💼 Career

Understanding string traversal is essential for roles in software development, data processing, and automation.

1

Create the string variable

Create a string variable called word and set it to "hello" using String::from.

DSA Rust

# Create a string variable called word and set it to "hello"
# Your code here

Hint

Use let word = String::from("hello"); to create the string.

2

Create a vowel counter

Create a variable called vowel_count and set it to 0 to count vowels.

DSA Rust

let word = String::from("hello");
# Create a variable vowel_count and set it to 0
# Your code here

Hint

Use let vowel_count = 0; to start counting vowels.

3

Traverse each character in the string

Use a for loop with variable ch to iterate over word.chars().

DSA Rust

let word = String::from("hello");
let vowel_count = 0;
# Use a for loop with ch to iterate over word.chars()
# Your code here

Hint

Use for ch in word.chars() { } to go through each character.

4

Print each character

Inside the for loop, use println!("{}", ch); to print each character on a new line.

DSA Rust

let word = String::from("hello");
let vowel_count = 0;
for ch in word.chars() {
    # Print each character ch
    # Your code here
}

Hint

Use println!("{}", ch); inside the loop to print each letter.

Practice

1. What does the following Rust code print?
let s = "hello";
for c in s.chars() {
print!("{}-", c);
}

easy

A. h-e-l-l-o-

B. hello

C. h e l l o

D. h,e,l,l,o,

Solution

1. Step 1: Understand the chars() method

   The chars() method returns an iterator over each character in the string s.

2. Step 2: Trace the for loop printing

   Each character is printed followed by a dash, so output is h-e-l-l-o-.

3. Final Answer:

   h-e-l-l-o- -> Option A

4. Quick Check:

   chars() + print with dash = h-e-l-l-o- [OK]

Hint: chars() iterates characters; print each with dash [OK]

Common Mistakes:

• Thinking chars() returns words
• Ignoring the dash printed after each char
• Confusing chars() with bytes()

2. Which of the following is the correct way to access the third character of a Rust string s safely?

easy

A. let c = s.bytes().nth(2).unwrap();

B. let c = s[2];

C. let c = s.get(2).unwrap();

D. let c = s.chars().nth(2).unwrap();

Solution

1. Step 1: Understand string indexing in Rust

   Rust strings cannot be indexed directly by integer due to UTF-8 encoding, so s[2] is invalid.

2. Step 2: Use chars().nth() to get character safely

   The chars() iterator allows accessing the nth character safely with nth(2) and unwrap() to get the value.

3. Final Answer:

   let c = s.chars().nth(2).unwrap(); -> Option D

4. Quick Check:

   chars().nth(2).unwrap() = correct char access [OK]

Hint: Use chars().nth(index).unwrap() for safe char access [OK]

Common Mistakes:

• Using direct indexing like s[2]
• Using get(2) which returns Option<&str> slice, not char
• Using bytes() which returns u8, not char

3. What is the output of this Rust code?

let s = "rust";
let mut result = String::new();
for c in s.chars() {
    if c == 'u' {
        result.push('U');
    } else {
        result.push(c);
    }
}
println!("{}", result);

medium

A. rust

B. rUSt

C. rUst

D. Rust

Solution

1. Step 1: Iterate over each character in "rust"

   The loop goes over 'r', 'u', 's', 't' one by one.

2. Step 2: Replace 'u' with 'U' and keep others same

   When character is 'u', push 'U' to result; else push original char.

3. Final Answer:

   rUst -> Option C

4. Quick Check:

   Only 'u' replaced by 'U' = rUst [OK]

Hint: Check character equality and push accordingly [OK]

Common Mistakes:

• Replacing wrong characters
• Not pushing characters correctly
• Confusing uppercase and lowercase

4. Identify the error in this Rust code snippet:

let s = "hello";
let c = s[1];
println!("{}", c);

medium

A. s is not declared mutable

B. Cannot index string with integer directly

C. Missing semicolon after println!

D. println! macro syntax error

Solution

1. Step 1: Check string indexing rules in Rust

   Rust strings are UTF-8 encoded and do not support direct indexing by integer like arrays.

2. Step 2: Identify the invalid line

   The line let c = s[1]; causes a compile-time error because of invalid indexing.

3. Final Answer:

   Cannot index string with integer directly -> Option B

4. Quick Check:

   Direct indexing on String = error [OK]

Hint: Rust strings can't be indexed by integer directly [OK]

Common Mistakes:

• Assuming s[1] works like array indexing
• Ignoring UTF-8 encoding complexity
• Thinking semicolon or macro syntax is the error

5. Given a string s, how can you create a new string containing only the vowels from s in Rust?
Choose the correct code snippet.

hard

A. let vowels: String = s.chars().filter(|c| "aeiou".contains(*c)).collect();

B. let vowels: String = s.bytes().filter(|b| b == b'a' || b == b'e').collect();

C. let vowels: String = s.get(0..5).unwrap().to_string();

D. let vowels: String = s.chars().map(|c| if "aeiou".contains(c) { c } else { ' ' }).collect();

Solution

1. Step 1: Use chars() to iterate characters

   We need to check each character, so chars() is appropriate.

2. Step 2: Filter vowels and collect into String

   Filter characters that are vowels using filter and collect them into a new String.

3. Final Answer:

   let vowels: String = s.chars().filter(|c| "aeiou".contains(*c)).collect(); -> Option A

4. Quick Check:

   chars() + filter vowels + collect() = correct [OK]

Hint: Filter chars with contains("aeiou") and collect to String [OK]

Common Mistakes:

• Using bytes() which returns u8, not char
• Using get() which slices string, not filters vowels
• Replacing non-vowels with spaces instead of filtering