Substring Search Patterns in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Naive Substring Search

What is the output of the following code that searches for the pattern "ana" in the text "banana" using a naive substring search?

DSA C

const char* text = "banana";
const char* pattern = "ana";

for (int i = 0; text[i] != '\0'; i++) {
    int j = 0;
    while (pattern[j] != '\0' && text[i + j] == pattern[j]) {
        j++;
    }
    if (pattern[j] == '\0') {
        printf("%d ", i);
    }
}
printf("\n");

A 0 2

B 1 2 3

C 2 4

D 1 3

💡 Hint

Check each position in the text where the pattern matches fully.

The pattern "ana" appears starting at index 1 ("banana") and index 3 ("banana"). So the output prints "1 3 ".

❓ Predict Output

intermediate

Output of KMP Prefix Table Construction

What is the output array (prefix table) constructed by the KMP algorithm for the pattern "ABABAC"?

DSA C

int prefix[] = {0, 0, 0, 0, 0, 0};
char pattern[] = "ABABAC";
int len = 0;
for (int i = 1; pattern[i] != '\0'; i++) {
    while (len > 0 && pattern[i] != pattern[len]) {
        len = prefix[len - 1];
    }
    if (pattern[i] == pattern[len]) {
        len++;
        prefix[i] = len;
    }
}
for (int i = 0; i < 6; i++) {
    printf("%d ", prefix[i]);
}
printf("\n");

A 0 0 1 2 3 1

B 0 1 0 1 2 0

C 0 0 1 2 3 0

D 0 0 1 2 0 1

💡 Hint

The prefix table stores the length of the longest proper prefix which is also suffix for each prefix of the pattern.

For pattern "ABABAC", the prefix table is [0,0,1,2,3,0]. This means at index 4, the longest prefix-suffix length is 3, but at index 5 it resets to 0.

🔧 Debug

advanced

Identify the error in Rabin-Karp hash update

What error will occur when running this Rabin-Karp rolling hash update code snippet for substring search?

DSA C

int base = 256;
int prime = 101;
int h = 1;
int pattern_len = 3;
for (int i = 0; i < pattern_len - 1; i++) {
    h = (h * base) % prime;
}
int text_hash = 0;
int pattern_hash = 0;
char text[] = "abcd";
char pattern[] = "bcd";
for (int i = 0; i < pattern_len; i++) {
    pattern_hash = (base * pattern_hash + pattern[i]) % prime;
    text_hash = (base * text_hash + text[i]) % prime;
}
// Update hash for next window
text_hash = (base * (text_hash - text[0] * h) + text[pattern_len]) % prime;
if (text_hash < 0) text_hash += prime;
printf("%d\n", text_hash);

A Runtime error: array index out of bounds

B No error, prints 31

C Wrong hash value due to missing parentheses

D Negative hash value without correction

💡 Hint

Check the calculation of the rolling hash and the array indices used.

The code correctly computes the rolling hash and adjusts negative values. The output is 31, no runtime error occurs.

❓ Predict Output

advanced

Output of Boyer-Moore Bad Character Table

What is the value of bad character table entry for character 'c' in the pattern "abcabc" using Boyer-Moore preprocessing?

DSA C

int badChar[256];
char pattern[] = "abcabc";
int m = 6;
for (int i = 0; i < 256; i++) {
    badChar[i] = -1;
}
for (int i = 0; i < m; i++) {
    badChar[(unsigned char)pattern[i]] = i;
}
printf("%d\n", badChar['c']);

A 5

B 2

C -1

D 3

💡 Hint

The bad character table stores the last occurrence index of each character in the pattern.

The character 'c' appears at indices 2 and 5. The table stores the last occurrence, which is 5.

🧠 Conceptual

expert

Minimum number of comparisons in substring search

Which substring search algorithm guarantees the minimum worst-case number of character comparisons for searching a pattern of length m in a text of length n?

A Knuth-Morris-Pratt (KMP) algorithm

B Naive substring search algorithm

C Rabin-Karp algorithm

D Boyer-Moore algorithm

💡 Hint

Consider worst-case linear time complexity and character comparisons.

KMP guarantees O(n) worst-case comparisons by using prefix table to avoid re-examining characters.

Practice

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

easy

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

B. To sort the characters of a string alphabetically

C. To reverse the entire string

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

Solution

1. Step 1: Understand substring search purpose

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

2. Step 2: Compare options with definition

   Only To find if a smaller string exists inside a larger string describes finding a smaller string inside a larger one, which matches substring search.

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 larger [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 of the following is the correct syntax to check if string pattern is a substring of string text in C?

easy

A. if (text.find(pattern)) { /* found */ }

B. if (text.contains(pattern)) { /* found */ }

C. if (pattern in text) { /* found */ }

D. if (strstr(text, pattern) != NULL) { /* found */ }

Solution

1. Step 1: Recall C substring search function

   In C, strstr() returns a pointer to the first occurrence of pattern in text, or NULL if not found.

2. Step 2: Check each option's syntax

   if (strstr(text, pattern) != NULL) { /* found */ } uses strstr correctly with NULL check. Options A, B, C use syntax from other languages or invalid in C.

3. Final Answer:

   if (strstr(text, pattern) != NULL) { /* found */ } -> Option D

4. Quick Check:

   C substring search uses strstr() [OK]

Hint: Use strstr() in C to find substring, check for NULL [OK]

Common Mistakes:

• Using methods from other languages like contains() or find()
• Not checking if strstr() returns NULL
• Confusing order of arguments in strstr()

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

char text[] = "hello world";
char *pos = strstr(text, "wor");
if (pos != NULL) {
    printf("%ld", pos - text);
} else {
    printf("-1");
}

medium

A. 6

B. 0

C. -1

D. 3

Solution

1. Step 1: Understand strstr() return value

   strstr(text, "wor") returns pointer to first occurrence of "wor" in "hello world", which starts at index 6.

2. Step 2: Calculate pointer difference

   Subtracting text pointer from pos gives index 6, which is printed.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Substring "wor" starts at index 6 [OK]

Hint: Pointer difference in strstr() gives substring start index [OK]

Common Mistakes:

• Printing pointer value instead of index difference
• Assuming substring starts at 0 or 3 incorrectly
• Not checking if strstr() returns NULL before subtracting

4. Identify the error in the following C code for substring search:

char *find_substring(char *text, char *pattern) {
    for (int i = 0; i <= strlen(text); i++) {
        if (strncmp(&text[i], pattern, strlen(pattern)) == 0) {
            return &text[i];
        }
    }
    return NULL;
}

medium

A. Function should return int index, not pointer

B. strncmp() is used incorrectly; should use strcmp() instead

C. The loop condition should be 'i < strlen(text) - strlen(pattern) + 1' to avoid overflow

D. No error; code works correctly

Solution

1. Step 1: Analyze loop boundary

   The loop runs from 0 to strlen(text), but comparing substrings beyond text length minus pattern length causes overflow.

2. Step 2: Correct loop condition

   Loop should run until i <= strlen(text) - strlen(pattern) to safely compare substrings of pattern length.

3. Final Answer:

   The loop condition should be 'i < strlen(text) - strlen(pattern) + 1' to avoid overflow -> Option C

4. Quick Check:

   Loop boundary must prevent substring overflow [OK]

Hint: Loop till text length minus pattern length to avoid overflow [OK]

Common Mistakes:

• Looping beyond valid substring start positions
• Using strcmp instead of strncmp for partial comparison
• Ignoring return type correctness

5. You want to find all occurrences of substring pat in string txt using C. Which approach correctly finds all start indices of pat in txt?

hard

A. Use strstr() once and return the first found position only

B. Use a loop with strstr() starting from last found position + 1 until strstr() returns NULL

C. Use strcmp() on entire text and pattern directly

D. Use strlen() to compare lengths and decide if substring exists

Solution

1. Step 1: Understand strstr() usage for multiple occurrences

   strstr() returns pointer to first occurrence. To find all, repeatedly call strstr() starting after last found position.

2. Step 2: Evaluate options

   Use a loop with strstr() starting from last found position + 1 until strstr() returns NULL correctly describes looping with strstr() advancing pointer. Use strstr() once and return the first found position only finds only first occurrence. Options C and D do not find all occurrences.

3. Final Answer:

   Use a loop with strstr() starting from last found position + 1 until strstr() returns NULL -> Option B

4. Quick Check:

   Loop with strstr() to find all occurrences [OK]

Hint: Loop with strstr() advancing pointer to find all matches [OK]

Common Mistakes:

• Using strstr() only once for all matches
• Using strcmp() which compares whole strings only
• Relying on length checks without searching