String Constants:

String constants are declared using double quotes. They are initialized with the string literal inside the quotes. Individual characters within a string constant can be accessed using the [] operator. String constants are immutable and stored in read-only memory.

Eg.
"Programming", "Data Structures"

String:

Declaring strings

Strings can be declared using character arrays:

  char name[20];   // character array of size 20

Initializing strings

Strings can be initialized using:

• String literal:

    char name[] = "John";
  

• Character array:

    char name[] = {'J', 'o', 'h', 'n', '\0'};
  

The null character '\0' is required to indicate the end of the string.

Accessing Characters

Individual characters can be accessed using the [] operator:

  char first = name[0];   // 'J'
  char last = name[3];   // 'n'

Example

  #include <stdio.h>

  int main() {

    char string[100];

    // Taking string as input using scanf() 
    printf("Enter a string: "); 
    scanf("%s", string);

    // Printing string using printf()  
    printf("You entered: %s\n", string);  

    // Taking string as input using gets() 
    printf("Enter another string: ");
    gets(string);

    // Printing string using puts()
    puts(string);  

    // Initializing string  
    strcpy(string, "Hello");  

    // Printing string using while loop
    int i = 0; 
    while (string[i] != '\0') {
        printf("%c", string[i]); 
        i++;
    }
    printf("\n");

    return 0;
  }

String Library Functions:

The string library functions make it easy to perform common string operations in C like copying, concatenating, comparing, searching and manipulating strings.

strlen() - Returns the length of a string

  int len = strlen(string);

strcpy() - Copies one string to another

  strcpy(dest, source);

strcat() - Concatenates two strings

  strcat(string1, string2);

strcmp() - Compares two strings

  int result = strcmp(string1, string2);

strchr() - Finds the first occurrence of a character in a string

  char *p = strchr(string, 'c');

strstr() - Finds the first occurrence of a substring

  char *p = strstr(string, "sub");

Example:

  char str1[20] = "Hello";
  char str2[20] = "World";

  int len = strlen(str1);   
  // len = 5

  strcpy(str1, str2);       
  // str1 = "World"

  strcat(str1, str2);       
  // str1 = "WorldWorld"

  int result = strcmp(str1, str2);  
  // result < 0 as str1 is lexicographically less than str2

  char *p = strchr(str1, 'r');
  // p points to the first occurrence of 'r' in str1

  char *p = strstr(str1, "orl");
  // p points to the first occurrence of "orl" in str1

String Pointers:

Declaring string pointers

A string pointer is declared just like any other pointer but it points to a string (array of characters).

  char *str;

Initializing string pointers

A string pointer can be initialized in two ways:

1. Pointing to a string literal:

    char *str = "Hello";
   

2. Pointing to an array name:

    char array[100];
    char *str = array;
   

Example

  #include <stdio.h>

  int main() {

    char *str = "Hello"; 

    printf("Length: %d\n", strlen(str));

    char array[100]; 
    char *ptr = array;

    printf("Enter a string: ");
    gets(array);  

    printf("You entered: ");
    puts(ptr);

    strcpy(ptr, "How are you?");
    printf("%s\n", ptr);

    char *sub = strstr(ptr, "are");
    printf("%s\n", sub); 

    char *ch = strchr(ptr, 'u');
    printf("%c", *ch);

    return 0;
  }

Multidimensional String Arrays:

2D Character arrays can be used to store strings.

For eg.

char names[4][20] = {"Mathematics", "Algorithms", "Programming", "TOC"}

Passing Strings to Functions:

Passing string constants

We can pass string constants to functions like this:

  void printString(char* str){
    printf("%s", str);
  }

  int main(){
    printString("Hello");  
  }

Here we are passing the string literal "Hello" to the printString() function.

Passing character arrays

We can also define a character array and pass it to a function:

  #include <stdio.h>
  void printString(char str[]){
    printf("%s", str);  
  }

  int main(){
    char str[6] = "Hello";
    printString(str); 
    return 0;   
  }

Here we are passing the character array str to the function.

Passing string pointers

  #include <stdio.h>

  void printString(char *str){
    printf("%s", str);
  }

  int main(){
    char *str = "Hello";
    printString(str);    
  }

Array of String Pointers:

Declaring an array of string pointers

We declare an array of string pointers like this:

  char *strings[n];

Here strings is an array of n string pointers.

Initializing array of string pointers

We can initialize an array of string pointers like this:

  char *strings[3] = {"Hello", "World", "C"};

Here we are initializing the string pointers to point to string literals.

Accessing elements

We access elements of an array of string pointers using the [] operator:

  printf("%s", strings[0]); // Prints Hello
  printf("%s", strings[1]); // Prints World

Example

  #include <stdio.h>

  int main() {
    char *strings[3];

    strings[0] = "Hello";
    strings[1] = "World"; 
    strings[2] = "C";

    for (int i = 0; i < 3; i++)
      printf("%s\n", strings[i]); 

    return 0;
  }

Output:

  Hello  
  World
  C