Pointers And Strings in C

Pointers And Strings

• A string is simply an array of characters terminated by a null character \0.

• Pointers are often used to manipulate strings in C, since strings are essentially just arrays of characters and pointers are used to manipulate arrays.

Example 1

#include <stdio.h>

int main()
{
    char str1[] = "hello";
    char *str2 = "world";

    // Print the strings using array notation
    printf("str1: %s\n", str1);
    printf("str2: %s\n", str2);

    // Print the strings using pointer notation
    printf("str1: %s\n", &str1[0]);
    printf("str2: %s\n", str2);

    // Modify the strings using pointers
    str1[0] = 'H';
    *(str2 + 1) = 'u';

    // Print the modified strings
    printf("str1: %s\n", str1);
    printf("str2: %s\n", str2);

    return 0;
}

Output:

Explanation

• We declare two strings: str1 as an array of characters, and str2 as a pointer to a string literal.
• We then use both array notation and pointer notation to print out the contents of the strings.
• Note that when using pointer notation, we can simply pass the pointer to the printf() function instead of passing the address of the first character in the array.
• Next, we use pointers to modify the first character of each string.
• Note that we can use either array notation or pointer notation to modify the strings.

Finally, we print out the modified strings using printf().

Example 2

#include <stdio.h>

int main()
{
    char str[] = "Hello, world!";
    char *ptr = str; // ptr points to the first character in str

    // Print the string using pointer notation
    while (*ptr != '\0')
    {
        printf("%c", *ptr);
        ptr++; // move to the next character
    }
    printf("\n");

    // Modify the string using pointers
    ptr = str;
    *ptr = 'h'; // change the first character to 'h'
    while (*ptr != '\0')
    {
        if (*ptr == 'o')
        {
            *ptr = '0'; // change 'o' to '0'
        }
        ptr++; // move to the next character
    }

    // Print the modified string using array notation
    printf("%s\n", str);

    return 0;
}

Output:

Explanation

• We declare a string str and a pointer ptr that initially points to the first character in str.
• We then use the while loop and pointer notation to print out each character in the string, and move the pointer to the next character.
• Next, we use pointers to modify the string: we change the first character to lowercase, and replace any occurrences of the letter 'o' with the digit '0'.
• We use the while loop and pointer notation again to traverse the string and modify each character.
• Finally, we print out the modified string using array notation.