Substring Search Patterns in DSA Rust - Build from Scratch

Substring Search Patterns

📖 Scenario: Imagine you are building a simple text search tool that helps users find if a small word or phrase exists inside a larger text. This is useful in many apps like search engines, text editors, or chat filters.

🎯 Goal: You will create a Rust program that checks if a given pattern (substring) exists inside a larger text string. You will learn how to set up the text and pattern, write the search logic, and print the result.

📋 What You'll Learn

Create a variable for the main text string

Create a variable for the pattern string to search

Write a loop to check if the pattern exists inside the text

Print whether the pattern was found or not

💡 Why This Matters

🌍 Real World

Searching for words or phrases inside larger texts is common in search engines, text editors, and chat filters.

💼 Career

Understanding substring search helps in building efficient search features and text processing tools in software development.

1

Create the main text string

Create a variable called text and set it to the string "hello world from rust".

DSA Rust

# Create a variable called text with the value "hello world from rust"
# Your code here

Hint

Use let to create a variable and assign the exact string.

2

Create the pattern string to search

Add a variable called pattern and set it to the string "world".

DSA Rust

fn main() {
    let text = "hello world from rust";
    # Create a variable called pattern with the value "world"
    # Your code here
}

Hint

Use let to create the pattern variable with the exact string.

3

Check if the pattern exists in the text

Write an if statement that uses text.contains(pattern) to check if pattern is inside text. Inside the if, create a variable found and set it to true. Otherwise, set found to false.

DSA Rust

fn main() {
    let text = "hello world from rust";
    let pattern = "world";
    # Use if statement with text.contains(pattern) to set found true or false
    # Your code here
}

Hint

Use text.contains(pattern) inside an if to decide the value of found.

4

Print the search result

Write a println! statement to print "Pattern found: " followed by the value of found.

DSA Rust

fn main() {
    let text = "hello world from rust";
    let pattern = "world";
    let found;
    if text.contains(pattern) {
        found = true;
    } else {
        found = false;
    }
    # Print "Pattern found: " and the value of found
    # Your code here
}

Hint

Use println!("Pattern found: {}", found); to print the result.

Practice

1. What is the main purpose of substring search algorithms in strings?

easy

A. To find if a smaller string exists inside a larger string

B. To sort characters in a string alphabetically

C. To reverse the order of characters in a string

D. To count the total number of characters in a string

Solution

1. Step 1: Understand substring search goal

   Substring search algorithms check if a smaller string (pattern) appears inside a bigger string (text).

2. Step 2: Compare with other string operations

   Sorting, reversing, and counting characters are different operations unrelated to searching for a substring.

3. Final Answer:

   To find if a smaller string exists inside a larger string -> Option A

4. Quick Check:

   Substring search = find smaller string inside bigger string [OK]

Hint: Substring search means finding a smaller string inside a bigger one [OK]

Common Mistakes:

• Confusing substring search with sorting or reversing strings
• Thinking substring search counts characters
• Mixing substring search with string length calculation

2. Which Rust code snippet correctly checks if the string needle is a substring of haystack?

easy

A. if haystack.index_of(needle) { println!("Found"); }

B. if haystack.find(needle) { println!("Found"); }

C. if needle.in(haystack) { println!("Found"); }

D. if haystack.contains(needle) { println!("Found"); }

Solution

1. Step 1: Identify correct Rust substring method

   Rust's standard library provides contains method on strings to check substring presence, returning a boolean.

2. Step 2: Check other options for syntax errors

   find returns an Option, not a boolean; in is not a Rust keyword; index_of does not exist in Rust standard strings.

3. Final Answer:

   if haystack.contains(needle) { println!("Found"); } -> Option D

4. Quick Check:

   Rust substring check uses contains() [OK]

Hint: Use .contains() to check substring presence in Rust strings [OK]

Common Mistakes:

• Using .find() as a boolean without checking Option
• Trying Python or other language syntax like 'in'
• Using non-existent methods like index_of

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

let text = "hello world";
let pattern = "wor";
if text.contains(pattern) {
    println!("Pattern found");
} else {
    println!("Pattern not found");
}

medium

A. Compilation error

B. Pattern found

C. Pattern not found

D. Runtime panic

Solution

1. Step 1: Check if pattern is substring of text

   The string "hello world" contains "wor" starting at index 6.

2. Step 2: Evaluate the if condition

   Since text.contains(pattern) returns true, the code prints "Pattern found".

3. Final Answer:

   Pattern found -> Option B

4. Quick Check:

   "hello world" contains "wor" = true [OK]

Hint: Check if substring exists literally inside the main string [OK]

Common Mistakes:

• Assuming contains() checks for whole words only
• Confusing contains() with starts_with() or ends_with()
• Expecting compilation or runtime errors without syntax issues

4. Identify the error in this Rust code snippet for substring search:

let text = "rustacean";
let pattern = "ace";
if text.find(pattern) {
    println!("Found");
} else {
    println!("Not found");
}

medium

A. Missing semicolon after let statements

B. Using contains() instead of find()

C. Using find() without handling Option result

D. Pattern string is empty

Solution

1. Step 1: Understand return type of find()

   In Rust, find() returns an Option<usize>, not a boolean.

2. Step 2: Check if condition usage

   The if condition expects a boolean, but text.find(pattern) returns Option, causing a type mismatch error.

3. Final Answer:

   Using find() without handling Option result -> Option C

4. Quick Check:

   find() returns Option, not bool [OK]

Hint: Remember find() returns Option, not boolean; unwrap or check Some [OK]

Common Mistakes:

• Treating find() as boolean directly in if
• Ignoring Option type returned by find()
• Confusing contains() and find() return types

5. Given a large text and multiple patterns, which substring search approach is best to efficiently find all patterns in the text?

hard

A. Using a Trie or Aho-Corasick algorithm to search all patterns simultaneously

B. Naive search checking each pattern separately in the text

C. Sorting all patterns and text then binary searching each pattern

D. Reversing the text and patterns before searching

Solution

1. Step 1: Analyze naive approach inefficiency

   Checking each pattern separately in a large text is slow because it repeats scanning multiple times.

2. Step 2: Understand advanced multi-pattern search

   Trie-based algorithms like Aho-Corasick build a structure to search all patterns at once efficiently.

3. Step 3: Evaluate other options

   Sorting and binary searching patterns is not suitable for substring search; reversing strings does not help search efficiency.

4. Final Answer:

   Using a Trie or Aho-Corasick algorithm to search all patterns simultaneously -> Option A

5. Quick Check:

   Multi-pattern search uses Trie/Aho-Corasick [OK]

Hint: Use Trie or Aho-Corasick for multiple substring searches efficiently [OK]

Common Mistakes:

• Using naive repeated searches for many patterns
• Trying sorting-based methods for substring search
• Thinking reversing strings improves search speed