String Pattern Matching Naive in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the length of the pattern string.

DSA C#

int patternLength = pattern.[1];

Drag options to blanks, or click blank then click option'

ACount

BLength

CSize

DLength()

Attempts:

3 left

💡 Hint

Common Mistakes

Using Length() as if it were a method.

Using Count which is not a string property.

The Length property returns the number of characters in the string.

2fill in blank

medium

Complete the code to compare characters of text and pattern at given indices.

DSA C#

if (text[i + j] == pattern.[1]) {

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A[j]

BLength

CElementAt(j)

DSubstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using Substring which returns a string, not a character.

Using ElementAt without importing LINQ.

Use the indexer [j] to get the character at position j in the pattern string.

3fill in blank

hard

Fix the error in the loop condition to avoid going out of bounds in the text string.

DSA C#

for (int i = 0; i <= text.Length - [1]; i++) {

Drag options to blanks, or click blank then click option'

Apattern.Length + 1

Bpattern.Length * 2

Cpattern.Length - 1

Dpattern.Length

Attempts:

3 left

💡 Hint

Common Mistakes

Using pattern.Length + 1 which causes index out of range.

Using pattern.Length - 1 which misses some valid positions.

We subtract the pattern length to ensure the pattern fits inside the text when matching.

4fill in blank

hard

Fill both blanks to complete the inner loop that checks each character of the pattern against the text.

DSA C#

for (int j = 0; j < [1]; j++) {
    if (text[i + j] != pattern.[2]) {
        break;
    }
}

Drag options to blanks, or click blank then click option'

Apattern.Length

Btext.Length

C[j]

D[i]

Attempts:

3 left

💡 Hint

Common Mistakes

Using text.Length for the loop limit which is incorrect.

Accessing pattern with [i] instead of [j].

The inner loop runs for the length of the pattern, and compares characters at index j in the pattern.

5fill in blank

hard

Fill all three blanks to complete the naive pattern matching function that returns the first index of the pattern in the text or -1 if not found.

DSA C#

public static int NaiveSearch(string text, string pattern) {
    int n = text.Length;
    int m = pattern.[1];
    for (int i = 0; i <= n - m; i++) {
        int j;
        for (j = 0; j < m; j++) {
            if (text[i + j] != pattern.[2]) {
                break;
            }
        }
        if (j == [3]) {
            return i;
        }
    }
    return -1;
}

Drag options to blanks, or click blank then click option'

ALength

B[j]

Cm

D[i]

Attempts:

3 left

💡 Hint

Common Mistakes

Using [i] instead of [j] to access pattern characters.

Comparing j to n instead of m.

Using Length() as a method instead of property.

The pattern length is stored in m. Characters are compared at index j in the pattern. If all characters match (j == m), return the current index i.

Practice

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

easy

A. Check every possible position in the text to find the pattern

B. Use a hash function to find the pattern quickly

C. Build a prefix table to skip unnecessary comparisons

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 used in naive matching.

3. Final Answer:

   Check every possible position in the text to find 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 of the following is the correct syntax to start a loop for naive pattern matching in C# if text length is n and pattern length is m?

easy

A. for (int i = 0; i <= n; i++)

B. for (int i = 0; i < n; i++)

C. for (int i = 0; i < m; i++)

D. for (int i = 0; i <= n - m; i++)

Solution

1. Step 1: Determine loop range for pattern matching

   The loop must run from 0 to n - m inclusive to avoid overflow when matching pattern of length m.

2. Step 2: Check each option

   for (int i = 0; i <= n - m; i++) correctly uses i <= n - m. Others either go beyond or are too short.

3. Final Answer:

   for (int i = 0; i <= n - m; i++) -> Option D

4. Quick Check:

   Loop till n - m inclusive [OK]

Hint: Loop till n - m to fit pattern fully [OK]

Common Mistakes:

• Looping till n causes index out of range
• Looping till m is too short
• Using < instead of <= misses last position

3. What will be the output of the following C# code snippet?

string text = "abcabcabc";
string pattern = "abc";
int count = 0;
for (int i = 0; i <= text.Length - pattern.Length; i++) {
    int j;
    for (j = 0; j < pattern.Length; j++) {
        if (text[i + j] != pattern[j])
            break;
    }
    if (j == pattern.Length)
        count++;
}
Console.WriteLine(count);

medium

A. 1

B. 2

C. 3

D. 0

Solution

1. Step 1: Identify pattern occurrences in text

   The pattern "abc" appears at positions 0, 3, and 6 in the text "abcabcabc".

2. Step 2: Count matches in the loop

   The inner loop confirms full matches at these positions, so count increments 3 times.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   Pattern occurs 3 times [OK]

Hint: Count full matches by checking all pattern chars [OK]

Common Mistakes:

• Counting partial matches
• Off-by-one errors in loop limits
• Breaking early without counting

4. The following code tries to implement naive pattern matching but does not work correctly. What is the bug?

string text = "hellohello";
string pattern = "lo";
int count = 0;
for (int i = 0; i < text.Length - pattern.Length; i++) {
    int j;
    for (j = 0; j < pattern.Length; j++) {
        if (text[i + j] != pattern[j])
            break;
    }
    if (j == pattern.Length)
        count++;
}
Console.WriteLine(count);

medium

A. The outer loop should use <= instead of < to include last position

B. The inner loop should start from 1 instead of 0

C. The pattern length is not used correctly

D. The count variable is not initialized

Solution

1. Step 1: Analyze loop boundary

   The outer loop runs from 0 to text.Length - pattern.Length - 1, missing the last valid start position.

2. Step 2: Correct loop condition

   Changing < to <= ensures the last possible position is checked for pattern match.

3. Final Answer:

   The outer loop should use <= instead of < to include last position -> Option A

4. Quick Check:

   Loop boundary off by one [OK]

Hint: Use <= to cover last possible start index [OK]

Common Mistakes:

• Using < causes missing last match
• Changing inner loop start breaks matching
• Ignoring initialization of count

5. Given a text "ababababab" and a pattern "abab", how many times will the naive pattern matching algorithm find the pattern if overlapping matches are counted?

hard

A. 2

B. 4

C. 3

D. 5

Solution

1. Step 1: Identify all overlapping matches

   The pattern "abab" appears starting at indices 0, 2, 4, and 6 in the text "ababababab".

2. Step 2: Count total matches including overlaps

   Counting all these positions gives 4 matches.

3. Final Answer:

   4 -> Option B

4. Quick Check:

   Overlapping matches counted = 4 [OK]

Hint: Check every position, count overlaps too [OK]

Common Mistakes:

• Ignoring overlapping matches
• Counting only non-overlapping occurrences
• Miscounting start indices