String Pattern Matching Naive in DSA Rust - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Naive Pattern Matching Function

What is the output of the following Rust code that uses a naive string pattern matching function to find all occurrences of a pattern in a text?

DSA Rust

fn naive_pattern_search(text: &str, pattern: &str) -> Vec<usize> {
    let n = text.len();
    let m = pattern.len();
    let mut result = Vec::new();
    for i in 0..=n - m {
        let mut j = 0;
        while j < m && &text[i + j..i + j + 1] == &pattern[j..j + 1] {
            j += 1;
        }
        if j == m {
            result.push(i);
        }
    }
    result
}

fn main() {
    let text = "abracadabra";
    let pattern = "abra";
    let positions = naive_pattern_search(text, pattern);
    println!("{:?}", positions);
}

A [0, 7]

B [0, 3, 7]

C [3, 7]

D [7]

💡 Hint

Check where the substring "abra" appears exactly in "abracadabra".

The pattern "abra" appears at index 0 and index 7 in the text "abracadabra". The naive search checks each position and finds matches only at these two indices.

❓ Predict Output

intermediate

Result of Pattern Not Found

What will be the output of this Rust code when the pattern does not exist in the text?

DSA Rust

fn naive_pattern_search(text: &str, pattern: &str) -> Vec<usize> {
    let n = text.len();
    let m = pattern.len();
    let mut result = Vec::new();
    for i in 0..=n - m {
        let mut j = 0;
        while j < m && &text[i + j..i + j + 1] == &pattern[j..j + 1] {
            j += 1;
        }
        if j == m {
            result.push(i);
        }
    }
    result
}

fn main() {
    let text = "hello world";
    let pattern = "abc";
    let positions = naive_pattern_search(text, pattern);
    println!("{:?}", positions);
}

A [3]

B [0, 3]

C [0]

D []

💡 Hint

If the pattern is not found, what does the function return?

Since "abc" does not appear in "hello world", the function returns an empty vector indicating no matches.

🔧 Debug

advanced

Identify the Error in Naive Pattern Matching Code

What error will this Rust code produce when run, and why?

DSA Rust

fn naive_pattern_search(text: &str, pattern: &str) -> Vec<usize> {
    let n = text.len();
    let m = pattern.len();
    let mut result = Vec::new();
    for i in 0..=n - m {
        let mut j = 0;
        while j < m && &text[i + j..i + j + 1] == &pattern[j..j + 1] {
            j += 1;
        }
        if j == m {
            result.push(i);
        }
    }
    result
}

fn main() {
    let text = "abc";
    let pattern = "abcd";
    let positions = naive_pattern_search(text, pattern);
    println!("{:?}", positions);
}

A compilation error due to type mismatch

B compiles and outputs []

C panic at runtime due to index out of bounds

D outputs [0]

💡 Hint

Check what happens when pattern length is greater than text length.

The code tries to loop from 0 to n - m inclusive, but if m > n, this causes an underflow in the range, leading to a panic at runtime due to index out of bounds.

🧠 Conceptual

advanced

Time Complexity of Naive Pattern Matching

What is the worst-case time complexity of the naive string pattern matching algorithm for a text of length n and a pattern of length m?

A O(n * m)

B O(n + m)

C O(n * log m)

D O(m * log n)

💡 Hint

Consider how many comparisons are done in the worst case.

The naive algorithm checks for the pattern starting at each position in the text, and for each position, it may compare up to m characters, resulting in O(n * m) time complexity.

🚀 Application

expert

Find All Occurrences of Overlapping Patterns

Given the naive pattern matching function, what will be the output when searching for the pattern "aa" in the text "aaaaa"?

DSA Rust

fn naive_pattern_search(text: &str, pattern: &str) -> Vec<usize> {
    let n = text.len();
    let m = pattern.len();
    let mut result = Vec::new();
    for i in 0..=n - m {
        let mut j = 0;
        while j < m && &text[i + j..i + j + 1] == &pattern[j..j + 1] {
            j += 1;
        }
        if j == m {
            result.push(i);
        }
    }
    result
}

fn main() {
    let text = "aaaaa";
    let pattern = "aa";
    let positions = naive_pattern_search(text, pattern);
    println!("{:?}", positions);
}

A [0, 2, 4]

B [0, 1, 2, 3]

C [1, 3]

D [0, 1, 2]

💡 Hint

Check all starting indices where the pattern "aa" appears, including overlapping ones.

The pattern "aa" appears starting at indices 0, 1, 2, and 3 in the text "aaaaa" because overlapping matches are counted.

Practice

1. What is the main idea behind the naive string pattern matching algorithm?

easy

A. Check every possible position in the text for the pattern

B. Use a hash function to find the pattern quickly

C. Build a prefix table to skip unnecessary checks

D. Sort the text and pattern before matching

Solution

1. Step 1: Understand naive matching approach

   The naive algorithm tries to match the pattern starting at every position in the text.

2. Step 2: Compare with other methods

   Other methods like hashing or prefix tables are not part of naive matching.

3. Final Answer:

   Check every possible position in the text for the pattern -> Option A

4. Quick Check:

   Naive = Check all positions [OK]

Hint: Naive means try all positions one by one [OK]

Common Mistakes:

• Confusing naive with hashing or prefix methods
• Thinking naive skips positions
• Assuming sorting is involved

2. Which Rust code snippet correctly starts the naive pattern matching loop over text and pattern?

easy

A. for i in 0..text.len() + pattern.len() { /* check pattern */ }

B. for i in 0..=text.len() - pattern.len() { /* check pattern */ }

C. for i in 0..text.len() { /* check pattern */ }

D. for i in 0..pattern.len() { /* check pattern */ }

Solution

1. Step 1: Determine valid start positions

   The pattern can only start at positions where it fits fully in the text, so from 0 to text length minus pattern length.

2. Step 2: Check Rust range syntax

   Using 0..=text.len() - pattern.len() includes the last valid index.

3. Final Answer:

   for i in 0..=text.len() - pattern.len() { /* check pattern */ } -> Option B

4. Quick Check:

   Loop range = 0 to text.len()-pattern.len() [OK]

Hint: Loop till text length minus pattern length inclusive [OK]

Common Mistakes:

• Looping beyond valid start positions causing panic
• Using pattern length as loop limit
• Off-by-one errors in range

3. What will be the output of this Rust code using naive pattern matching?

let text = "abracadabra";
let pattern = "abra";
let mut positions = Vec::new();
for i in 0..=text.len() - pattern.len() {
    if &text[i..i+pattern.len()] == pattern {
        positions.push(i);
    }
}
println!("{:?}", positions);

medium

A. [3, 7]

B. [0, 3, 7]

C. [7]

D. [0, 7]

Solution

1. Step 1: Identify pattern matches in text

   The pattern "abra" appears at index 0 ("abra") and index 7 ("abra") in "abracadabra".

2. Step 2: Confirm positions collected

   Positions vector collects indices 0 and 7 where pattern matches exactly.

3. Final Answer:

   [0, 7] -> Option D

4. Quick Check:

   Matches at 0 and 7 only [OK]

Hint: Check substring slices equal to pattern [OK]

Common Mistakes:

• Including partial matches like at index 3
• Off-by-one errors in slicing
• Missing second occurrence

4. Identify the bug in this naive pattern matching Rust code:

let text = "hello";
let pattern = "ll";
for i in 0..text.len() - pattern.len() {
    if &text[i..i+pattern.len()] == pattern {
        println!("Found at {}", i);
    }
}

medium

A. Range should be 0..=text.len() - pattern.len() to include last position

B. Pattern length is used incorrectly in slicing

C. Text and pattern should be converted to chars before slicing

D. println! macro syntax is wrong

Solution

1. Step 1: Check loop range correctness

   The loop uses 0..text.len() - pattern.len(), which excludes the last valid start index.

2. Step 2: Fix range to include last index

   Using 0..=text.len() - pattern.len() includes the last position where pattern can start.

3. Final Answer:

   Range should be 0..=text.len() - pattern.len() to include last position -> Option A

4. Quick Check:

   Use inclusive range for last valid index [OK]

Hint: Use inclusive range to cover all valid start indices [OK]

Common Mistakes:

• Using exclusive range causing missed matches
• Incorrect slicing indices
• Unnecessary conversion to chars

5. Given a text and a pattern, how can you modify the naive pattern matching to count overlapping occurrences of the pattern in Rust?
Example: text = "aaaa", pattern = "aa" should count 3 occurrences.

hard

A. Increment index by pattern length after each match

B. Use a hash map to store counts

C. Increment index by 1 after each check instead of pattern length

D. Reverse the text and pattern before matching

Solution

1. Step 1: Understand overlapping matches

   Overlapping means after finding a match, next check starts just one position ahead, not skipping full pattern length.

2. Step 2: Adjust loop increment

   Increment index by 1 each time to allow overlapping matches to be found.

3. Final Answer:

   Increment index by 1 after each check instead of pattern length -> Option C

4. Quick Check:

   Step by 1 to count overlaps [OK]

Hint: Move index by 1 to catch overlaps [OK]

Common Mistakes:

• Skipping by pattern length missing overlaps
• Using hash map unnecessarily
• Reversing strings does not help