Table of Contents
Modifying Strings
Accessing string elements refers to retrieving or modifying individual characters within a string by using their index position. In C, since strings are arrays of characters, each character is stored in a contiguous memory location starting from index 0.
Syntax:
string_name[index]
#include <stdio.h>
int main() {
char str[] = "Hello";
str[0] = 'Y'; // Valid: Modifying the array
printf("Result 1: %s\n", str); // Output: Yello
return 0;
}
// Result 1: Yello
Pointer and String
When you declare a string using a pointer, the string literal is stored in a read-only section of memory, and the pointer holds the memory address of the first character.
Syntax:
Char * ptr = “string_literal”;
#include <stdio.h>
int main() {
char *p = "C String"; // p points to C
printf(" %c", *(p + 2));
return 0;
}
// S
The code defines a pointer that stores the address of the first letter in the string “C String”. By using pointer arithmetic, *(p + 2) moves two positions forward from the start—skipping the letter ‘C’ and the space—to access and print the character at the third position, which is ‘S’.
| Index | Address | Character |
|---|---|---|
| 0 | p + 0 | 'C' |
| 1 | p + 1 | ' ' (Space) |
| 2 | p + 2 | 'S' |
| 3 | p + 3 | 't' |
| 4 | p + 4 | 'r' |
Passing Strings to Functions
Explanation: When a string is declared as a parameter using square brackets, the function signals that it expects a sequence of characters, though internally the compiler treats this as a pointer to the first element.
Syntax:
void function_name(char str[]){…}
Method 2: Using Pointer Notation
Explanation: This method explicitly uses a pointer variable to receive the starting memory address of the string, providing a direct way to navigate the string using pointer arithmetic.
Syntax:
void function_name(char * str){…}
#include <stdio.h>
// Method 1: Using Character Array Notation
void displayUsingArray(char str[]) {
printf("Array Notation: %s\n", str);
}
// Method 2: Using Pointer Notation
void displayUsingPointer(char *ptr) {
printf("Pointer Notation: %s\n", ptr);
}
int main() {
char myString[] = "Hello Qubrica";
// Calling both functions with the same string
displayUsingArray(myString);
displayUsingPointer(myString);
return 0;
}
// Array Notation: Hello Qubrica
// Pointer Notation: Hello Qubrica
Write a C program to convert a string to uppercase and then reverse it.
#include <stdio.h>
#include <string.h>
#include <ctype.h>
// Application 1: Convert to Uppercase using Array Notation
void toUpperCase(char str[]) {
for (int i = 0; str[i] != '\0'; i++) {
str[i] = toupper(str[i]);
}
}
// Application 2: Reverse String using Pointer Notation
void reverseString(char *ptr) {
int len = strlen(ptr);
char *start = ptr;
char *end = ptr + len - 1;
char temp;
while (start < end) {
temp = *start;
*start = *end;
*end = temp;
start++;
end--;
}
}
int main() {
char message[] = "program";
// Call Method 1
toUpperCase(message);
printf("Uppercase: %s\n", message);
// Call Method 2
reverseString(message);
printf("Reversed: %s\n", message);
return 0;
}
// Uppercase: PROGRAM
// Reversed: MARGORP
| Index (i) | str[i] (Before) | toupper() Action | str[i] (After) | Resulting String |
|---|---|---|---|---|
| 0 | 'p' | 'p' -> 'P' | 'P' | Program |
| 1 | 'r' | 'r' -> R | 'R' | PRogram |
| 2 | 'o' | 'o' -> O | 'O' | PROgram |
| 3 | 'g' | 'g' -> G | 'G' | PROGram |
| 4 | 'r' | 'r' -> R | 'R' | PROGRam |
| 5 | 'a' | 'a' -> A | 'A' | PROGRAm |
| 6 | 'm' | 'm' -> M | 'M' | PROGRAM |
Write a c program to check if the entered string is a palindrome.
#include <stdio.h>
#include <string.h>
int main() {
char s[] = "radar";
int len = strlen(s);
int flag = 1;
for(int i = 0; i < len/2; i++) {
if(s[i] != s[len-i-1]) {
flag = 0;
break;
}
}
if(flag) printf("It is a Palindrome.");
else printf("Not a Palindrome.");
return 0;
}
// It is a Palindrome.
| Iteration (i) | s[i] | s[len-i-1] | Comparison (!=) | Action | flag |
|---|---|---|---|---|---|
| i =0 | 'r' | 'r' | 'r' != 'r' (False) | Continue | 1 |
| i = 1 | 'a' | 'a' | 'a' != 'a' (False) | Continue | 1 |
| End | - | - | - | Loop finished | 1 (True) |