Strings in C Programming

In C, strings are not a separate primary data type like they are in Python or Java. Instead, they are treated as a special type of array.

9.1. Defining a String

A string is defined as a one-dimensional array of characters. It is a collection of characters stored in contiguous memory locations.

Syntax

char string_name[size];

Memory Representation

When you define char name[6] = "Sunil";, the memory looks like this:

Index	0	1	2	3	4	5
Value	S	u	n	i	l	\0
Addr	1000	1001	1002	1003	1004	1005

Note: Always allocate N+1 space for a string of length N to accommodate the null terminator.

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9.2. Understanding the NULL Character (\0)

The null character \0 is the most important concept in C strings. It is an escape sequence with an ASCII value of 0.

Why do we need it?

1. End of String Marker: Since character arrays don't store their own size, functions like printf("%s") continue reading memory until they find a \0.
2. Loop Termination: Most string processing is done using loops that run while(str[i] != '\0').
3. Memory Boundary: It prevents functions from accessing memory outside the intended string range.

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9.3. Initialization of Strings

There are four primary ways to initialize strings in C:

1. String Literal (Implicit Null)

The most common way. The compiler automatically adds \0.

char str[] = "C Programming"; 

2. Character Array (Explicit Null)

When using curly braces, you must provide the \0 yourself.

char str[] = {'C', ' ', 'P', 'r', 'o', 'g', '\0'};

3. Pointer Initialization

This creates a string literal in read-only memory and a pointer to it.

char *ptr = "Hello";

Note: Modifying ptr[0] might cause a segmentation fault depending on the compiler.

4. Partial Initialization

If the size is larger than the content, the rest is automatically filled with null characters.

char str[10] = "Hi"; // Index 2-9 are all '\0'

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9.4. Reading and Writing Strings

Reading Strings

A. scanf() (The Simple Way)

• Syntax: scanf("%s", str);
• Issue: It stops reading at the first whitespace (space, tab, or newline). If you enter "John Doe", it only stores "John".
• Risk: It doesn't check for buffer overflow.

B. fgets() (The Safe Way)

• Syntax: fgets(str, size, stdin);
• Advantage: You specify the maximum number of characters to read, preventing memory crashes.
• Note: It includes the newline character \n if there is room.

Writing Strings

A. printf()

• Flexible; allows formatting with other variables.
• printf("Name: %s\n", name);

B. puts()

• Simpler for single strings.
• Automatically appends a newline character \n to the output.

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9.5. Library Functions for Strings (string.h)

To use these functions, you must include #include <string.h>.

1. strlen(str)

• Calculates: Number of characters before the null character.
• Return Type: size_t (unsigned integer).
• Example: strlen("Hi") returns 2.

2. strcpy(dest, src)

• Action: Copies everything from src to dest, including the \0.
• Caution: dest must be large enough to hold src.

3. strcat(dest, src)

• Action: Removes the \0 of dest, appends src, and adds a new \0.
• Example: "Hello " + "Sunil" = "Hello Sunil".

4. strcmp(str1, str2)

• Compares ASCII values of characters one by one.
• 0: Strings are identical.
• Positive: str1 comes after str2 alphabetically.
• Negative: str1 comes before str2 alphabetically.

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Practical Implementation: User Authentication Simulation

This program demonstrates reading strings with spaces and comparing them.

#include <stdio.h>
#include <string.h>

int main() {
    char saved_password[] = "Admin123";
    char input_password[20];
    char username[50];

    printf("--- Welcome to the System ---\n");
    
    // Using fgets to allow names with spaces
    printf("Enter Full Name: ");
    fgets(username, sizeof(username), stdin);
    // Remove trailing newline from fgets
    username[strcspn(username, "\n")] = 0;

    printf("Enter Password: ");
    scanf("%s", input_password);

    if (strcmp(saved_password, input_password) == 0) {
        printf("\nAccess Granted!\n");
        printf("Hello, %s. Your name has %lu characters.\n", username, strlen(username));
    } else {
        printf("\nAccess Denied. Incorrect Password.\n");
    }

    return 0;
}

Key Differences Summary

Feature	scanf	fgets
White Spaces	Stops at space	Includes spaces
Security	Risky (Overflow)	Safe (Size limit)
Newline	Left in buffer	Included in string