What are Strings in C?

In C, a string is a sequence of characters stored in a character array, terminated by a null character (\0) . Unlike other languages, C has no built-in string data type—strings are just arrays of char.

String Representation in Memory

char str[] = "Hello"; // Memory layout: // [H][e][l][l][o][\0] // 0 1 2 3 4 5 // The null character marks the end

Declaring and Initializing Strings

#include <stdio.h> int main() { // Method 1: String literal (automatically adds \0) char str1[] = "Hello"; // Method 2: Character array with size char str2[10] = "Hello"; // Method 3: Character array without size char str3[] = {'H', 'e', 'l', 'l', 'o', '\0'}; // Method 4: Fixed size (remaining are \0) char str4[10] = {'H', 'e', 'l', 'l', 'o', '\0'}; // Method 5: Empty string char empty[10] = ""; char empty2[10] = {'\0'}; printf("str1: %s\n", str1); printf("str2: %s\n", str2); printf("str3: %s\n", str3); return 0; }

String Input and Output

Basic Input/Output

#include <stdio.h> int main() { char name[50]; // Simple output printf("Enter your name: "); // Input (stops at first whitespace) scanf("%s", name); // Note: no & for array printf("Hello, %s!\n", name); return 0; }

Problem with scanf (stops at space)

#include <stdio.h> int main() { char fullname[50]; printf("Enter full name: "); scanf("%s", fullname); // Only reads first word printf("You entered: %s\n", fullname); // "John" only, not "John Doe" return 0; }

Solution: fgets() for Full Line Input

#include <stdio.h> int main() { char fullname[50]; printf("Enter full name: "); fgets(fullname, sizeof(fullname), stdin); // Reads spaces printf("Hello, %s", fullname); // Note: fgets includes newline return 0; }

Removing Newline from fgets

#include <stdio.h> #include <string.h> int main() { char fullname[50]; printf("Enter full name: "); fgets(fullname, sizeof(fullname), stdin); // Remove trailing newline size_t len = strlen(fullname); if(len > 0 && fullname[len-1] == '\n') { fullname[len-1] = '\0'; } printf("Hello, %s!\n", fullname); return 0; }

String Functions (string.h Library)

1. strlen() - Get String Length

#include <stdio.h> #include <string.h> int main() { char str[] = "Hello World"; int len = strlen(str); // Returns 11 (doesn't count \0) printf("Length: %d\n", len); // Manual way (without string.h) int manual_len = 0; while(str[manual_len] != '\0') { manual_len++; } printf("Manual length: %d\n", manual_len); return 0; }

2. strcpy() - Copy String

#include <stdio.h> #include <string.h> int main() { char source[] = "Hello C Programming"; char destination[50]; // Copy source to destination strcpy(destination, source); printf("Source: %s\n", source); printf("Destination: %s\n", destination); // Manual copy char manual[50]; int i = 0; while(source[i] != '\0') { manual[i] = source[i]; i++; } manual[i] = '\0'; printf("Manual copy: %s\n", manual); return 0; }

3. strcat() - Concatenate Strings

#include <stdio.h> #include <string.h> int main() { char str1[50] = "Hello "; char str2[] = "World!"; // Append str2 to str1 strcat(str1, str2); printf("Concatenated: %s\n", str1); // "Hello World!" // Manual concatenation char manual[50] = "Hello "; char str3[] = "C Programming"; int j = strlen(manual); int k = 0; while(str3[k] != '\0') { manual[j] = str3[k]; j++; k++; } manual[j] = '\0'; printf("Manual: %s\n", manual); return 0; }

4. strcmp() - Compare Strings

#include <stdio.h> #include <string.h> int main() { char str1[] = "Apple"; char str2[] = "Apple"; char str3[] = "Banana"; // Returns 0 if strings are equal if(strcmp(str1, str2) == 0) { printf("str1 and str2 are equal\n"); } // Returns negative if str1 < str3 int result = strcmp(str1, str3); printf("Compare Apple vs Banana: %d\n", result); // Negative // Returns positive if str3 > str1 result = strcmp(str3, str1); printf("Compare Banana vs Apple: %d\n", result); // Positive // Manual comparison int i = 0; while(str1[i] == str2[i] && str1[i] != '\0') { i++; } if(str1[i] == str2[i]) { printf("Manually equal\n"); } return 0; }

5. strchr() - Find Character

#include <stdio.h> #include <string.h> int main() { char str[] = "Hello World"; // Find first occurrence of 'o' char *ptr = strchr(str, 'o'); if(ptr != NULL) { printf("Found 'o' at position: %ld\n", ptr - str); // 4 printf("Rest of string: %s\n", ptr); } return 0; }

6. strstr() - Find Substring

#include <stdio.h> #include <string.h> int main() { char str[] = "The quick brown fox jumps"; // Find substring "brown" char *ptr = strstr(str, "brown"); if(ptr != NULL) { printf("Found at position: %ld\n", ptr - str); printf("From found point: %s\n", ptr); } else { printf("Substring not found\n"); } return 0; }

Common String Operations

Reading and Manipulating Strings

#include <stdio.h> #include <string.h> #include <ctype.h> int main() { char str[100]; printf("Enter a sentence: "); fgets(str, sizeof(str), stdin); // Remove newline str[strcspn(str, "\n")] = '\0'; // Convert to uppercase for(int i = 0; str[i] != '\0'; i++) { str[i] = toupper(str[i]); } printf("Uppercase: %s\n", str); // Count vowels int vowels = 0; for(int i = 0; str[i] != '\0'; i++) { char c = tolower(str[i]); if(c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u') { vowels++; } } printf("Vowels: %d\n", vowels); // Reverse string int len = strlen(str); for(int i = 0; i < len/2; i++) { char temp = str[i]; str[i] = str[len-1-i]; str[len-1-i] = temp; } printf("Reversed: %s\n", str); return 0; }

Practical Example 1: String Palindrome Check

#include <stdio.h> #include <string.h> #include <ctype.h> int isPalindrome(char str[]) { int left = 0; int right = strlen(str) - 1; while(left < right) { // Skip non-alphabetic characters while(left < right && !isalpha(str[left])) left++; while(left < right && !isalpha(str[right])) right--; // Compare ignoring case if(tolower(str[left]) != tolower(str[right])) { return 0; // Not palindrome } left++; right--; } return 1; // Is palindrome } int main() { char str1[] = "A man a plan a canal panama"; char str2[] = "Hello World"; // Remove spaces for testing printf("Testing: \"%s\"\n", str1); if(isPalindrome(str1)) { printf("Is a palindrome!\n"); } else { printf("Not a palindrome\n"); } printf("\nTesting: \"%s\"\n", str2); if(isPalindrome(str2)) { printf("Is a palindrome!\n"); } else { printf("Not a palindrome\n"); } return 0; }

Practical Example 2: String Tokenization

#include <stdio.h> #include <string.h> int main() { char str[] = "Apple,Banana,Cherry,Date"; char *token; printf("Original: %s\n", str); printf("Tokens:\n"); // strtok modifies the original string token = strtok(str, ","); while(token != NULL) { printf("- %s\n", token); token = strtok(NULL, ","); } return 0; }

Output:

Original: Apple,Banana,Cherry,Date Tokens: - Apple - Banana - Cherry - Date

Array of Strings (2D Character Array)

#include <stdio.h> #include <string.h> int main() { // Array of strings (3 strings, max 20 chars each) char fruits[3][20] = { "Apple", "Banana", "Cherry" }; // Print all fruits for(int i = 0; i < 3; i++) { printf("Fruit %d: %s\n", i+1, fruits[i]); } // Modify a string strcpy(fruits[1], "Blueberry"); printf("\nAfter modification:\n"); for(int i = 0; i < 3; i++) { printf("Fruit %d: %s\n", i+1, fruits[i]); } return 0; }

String Literals vs Character Arrays

#include <stdio.h> #include <string.h> int main() { // Character array (modifiable) char str1[] = "Hello"; str1[0] = 'J'; // OK - becomes "Jello" printf("str1: %s\n", str1); // String literal (read-only - can cause crash) char *str2 = "World"; // str2[0] = 'J'; // BAD! Undefined behavior // Correct way: str2 points to first character printf("str2: %s\n", str2); // Copy literal to array for modification char buffer[20]; strcpy(buffer, "Hello World"); buffer[0] = 'M'; // OK printf("buffer: %s\n", buffer); return 0; }

Common String Pitfalls

#include <stdio.h> #include <string.h> int main() { // PITFALL 1: Forgetting null terminator char bad[3] = {'A', 'B', 'C'}; // No \0 - NOT a string! // printf("%s", bad); // Dangerous! Prints until random \0 char good[4] = {'A', 'B', 'C', '\0'}; // Proper string printf("Good: %s\n", good); // PITFALL 2: Buffer overflow char small[5]; // strcpy(small, "This is too long"); // BAD! Overflow // Use strncpy for safety strncpy(small, "This is too long", 4); small[4] = '\0'; // Manually null terminate printf("Safe copy: %s\n", small); // PITFALL 3: Comparing strings with == char str1[] = "Hello"; char str2[] = "Hello"; if(str1 == str2) { // WRONG! Compares addresses, not content printf("This won't print\n"); } if(strcmp(str1, str2) == 0) { // CORRECT printf("Correct comparison\n"); } return 0; }

Quick Reference Table

Function	Purpose	Example
`strlen(s)`	Length of string	`int len = strlen("Hi");` // 2
`strcpy(dest, src)`	Copy src to dest	`strcpy(dest, "Hello");`
`strcat(dest, src)`	Append src to dest	`strcat(str, " World");`
`strcmp(s1, s2)`	Compare strings	`if(strcmp(a, b) == 0)`
`strchr(s, c)`	Find char in string	`char *p = strchr(str, 'a');`
`strstr(s, sub)`	Find substring	`char *p = strstr(str, "sub");`
`strtok(s, del)`	Split string	`token = strtok(str, ",");`
`strncpy(dest, src, n)`	Copy n chars	`strncpy(dest, src, 5);`
`strncat(dest, src, n)`	Append n chars	`strncat(dest, src, 3);`
`strncmp(s1, s2, n)`	Compare n chars	`if(strncmp(a, b, 3) == 0)`

Key Points

Strings are char arrays terminated by \0 (null character)
Use double quotes for string literals: "text"
Include <string.h> for string functions
Always ensure enough space for null terminator
fgets() is safer than scanf() for input with spaces
Strings are 0-indexed like arrays
Never compare strings with ==—use strcmp()

Complete C Programming Guide + Compilers Collection

1. C srand() Function – Understanding Seed Initialization

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7. C log() Function (Alternate Guide)

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8. C log10() Function

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9. C tan() Function

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10. Random Numbers in C (Secure vs Predictable)

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C Functions, Arguments, Parameters & Flow

Mastering Functions in C – Complete Guide

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Covers function structure, modular programming, and real-world usage.

Function Arguments in C

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Explains how arguments are passed and used in function calls.

Function Parameters in C

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Explains defining inputs for functions and matching them with arguments.

Function Declarations in C

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Covers prototypes, syntax rules, and best practices.

Function Calls in C

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Explains execution flow and parameter handling during function calls.

Void Functions in C

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Explains functions that do not return values.

Return Values in C

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Pass-by-Value in C

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Explains how copies of variables are passed into functions.

Pass-by-Reference in C

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C strstr() Function

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C Preprocessor & Macros

C Structures, Memory, Scope & Linkage

C Scope, Storage Classes & Typedef

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