Substring Search Patterns in DSA C - Build from Scratch

Substring Search Patterns

📖 Scenario: You are building a simple text search tool. It helps find if a small word or pattern exists inside a bigger text.Imagine you have a book and want to find if a certain word appears in it.

🎯 Goal: Build a program that checks if a given pattern is inside a text using substring search.You will create the text and pattern, then write code to find if the pattern exists in the text.

📋 What You'll Learn

Create a string variable called text with the value "hello world"

Create a string variable called pattern with the value "world"

Write a function called contains_pattern that takes text and pattern and returns 1 if pattern is found in text, else 0

Use a simple loop and comparison to check for the pattern inside the text

Print "Pattern found" if the pattern exists in the text, else print "Pattern not found"

💡 Why This Matters

🌍 Real World

Substring search is used in text editors, search engines, and data filtering tools to find words or patterns inside larger texts.

💼 Career

Understanding substring search helps in software development roles involving text processing, search features, and data analysis.

1

Create the text and pattern strings

Create a string variable called text and set it to "hello world". Also create a string variable called pattern and set it to "world".

DSA C

# Your code here to create text and pattern strings

Hint

Use char text[] = "hello world"; and char pattern[] = "world"; to create the strings.

2

Write the contains_pattern function header

Write a function called contains_pattern that takes two parameters: char *text and char *pattern. The function should return an int.

DSA C

#include 

char text[] = "hello world";
char pattern[] = "world";

// Write function header for contains_pattern
int contains_pattern(char *text, char *pattern) {
    // Your code here
}

Hint

Write int contains_pattern(char *text, char *pattern) to start the function.

3

Implement the substring search logic

Inside the contains_pattern function, write code to check if pattern exists inside text. Return 1 if found, else return 0. Use loops and character comparisons.

DSA C

#include 

char text[] = "hello world";
char pattern[] = "world";

int contains_pattern(char *text, char *pattern) {
    // Use loops to check if pattern is in text
    // Your code here
}

Hint

Use nested loops to compare each character of pattern with text starting at each position.

4

Print the search result

Call contains_pattern(text, pattern) in main. If it returns 1, print "Pattern found". Otherwise, print "Pattern not found".

DSA C

#include 

char text[] = "hello world";
char pattern[] = "world";

int contains_pattern(char *text, char *pattern) {
    int i, j;
    for (i = 0; text[i] != '\0'; i++) {
        for (j = 0; pattern[j] != '\0'; j++) {
            if (text[i + j] != pattern[j]) {
                break;
            }
        }
        if (pattern[j] == '\0') {
            return 1;
        }
    }
    return 0;
}

int main() {
    // Your code here
}

Hint

Use if (contains_pattern(text, pattern)) and print accordingly.

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