C Strings

C Strings: Working with Text

In C programming, a string is essentially a sequence of characters, such as "Hello World!". Unlike some other programming languages that have a dedicated string data type, C represents strings as arrays of characters.

The most crucial aspect of C strings is that they are null-terminated. This means a special character, the null character (\0), marks the end of the string. This null terminator is vital because it tells C functions where the string ends.

1. Declaring and Initializing Strings

You can declare and initialize strings in a few ways:

a. Character Array Initialization

Declare a character array and initialize it with a string literal. The compiler automatically adds the null terminator.

#include <stdio.h>

int main() {
  char greeting[6] = "Hello"; // An array of 6 characters (5 for "Hello" + 1 for '\0')
  printf("%s\n", greeting);
  return 0;
}

You can also omit the size, and the compiler will calculate it:

#include <stdio.h>

int main() {
  char city[] = "New York"; // Compiler determines size needed (8 for "New York" + 1 for '\0')
  printf("%s\n", city);
  return 0;
}

b. Pointer to a String Literal

You can also declare a pointer to a string literal. String literals are stored in read-only memory.

#include <stdio.h>

int main() {
  char *name = "Alice"; // 'name' points to the string literal "Alice"
  printf("%s\n", name);
  return 0;
}

Important Note: When using char *name = "Alice";, you are creating a pointer to a string literal, which is usually stored in read-only memory. Modifying such a string (e.g., name[0] = 'B';) leads to undefined behavior and often a crash. For modifiable strings, always use character arrays.

2. String Length (strlen)

The strlen() function, found in the <string.h> header, calculates the length of a string, excluding the null terminator.

Syntax:

size_t strlen(const char *str);

#include <stdio.h>
#include <string.h> // Required for strlen()

int main() {
  char text[] = "Programming";
  printf("Length of "%s" is %lu\n", text, strlen(text)); // %lu for size_t
  return 0;
}

3. Copying Strings (strcpy, strncpy)

You cannot assign one character array to another directly using the = operator. Instead, you use strcpy() or strncpy().

strcpy(destination, source): Copies the source string to the destination. Be cautious, as it doesn't check buffer size and can lead to buffer overflows if destination is too small.

strncpy(destination, source, num): Copies at most num characters from source to destination. This is safer but requires careful handling of null termination if source is longer than num.

#include <stdio.h>
#include <string.h> // Required for strcpy() and strncpy()

int main() {
  char original[] = "Hello World";
  char copy[20]; // Make sure destination has enough space
  char partialCopy[5];
  strcpy(copy, original);
  printf("Copied string: %s\n", copy);
  strncpy(partialCopy, original, 4); // Copy first 4 characters
  partialCopy[4] = '\0'; // Manually null-terminate if source is longer than count
  printf("Partially copied string: %s\n", partialCopy);
  return 0;
}

4. Concatenating Strings (strcat, strncat)

The strcat() function appends one string to the end of another.

strcat(destination, source): Appends source to destination. Again, beware of buffer overflows.

strncat(destination, source, num): Appends at most num characters from source to destination. Safer, but still ensure destination has sufficient capacity.

#include <stdio.h>
#include <string.h> // Required for strcat() and strncat()

int main() {
  char firstName[20] = "John";
  char lastName[] = " Doe";
  char buffer[50] = "First Part";
  char secondPart[] = " and Second Part";
  strcat(firstName, lastName); // firstName now holds "John Doe"
  printf("Full Name: %s\n", firstName);
  strncat(buffer, secondPart, 5); // Appends " and "
  printf("Concatenated buffer: %s\n", buffer);
  return 0;
}

5. Comparing Strings (strcmp, strncmp)

To compare two strings, you cannot use == because that would compare their memory addresses, not their content. Use strcmp() or strncmp().

strcmp(str1, str2): Compares str1 and str2 lexicographically.

• Returns 0 if str1 is equal to str2.
• Returns a negative value if str1 is less than str2.
• Returns a positive value if str1 is greater than str2.

strncmp(str1, str2, num): Compares at most the first num characters of str1 and str2. The return values follow the same rules as strcmp().

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

int main() {
  char str1[] = "Apple";
  char str2[] = "App";
  if (strcmp(str1, str2) == 0) {
    printf("Strings are completely equal.\n");
  } else {
    printf("Strings are different.\n");
  }
  if (strncmp(str1, str2, 3) == 0) {
    printf("First 3 characters are equal.\n");
  }
  return 0;
}