Pointers with Arrays and Strings

Pointers, arrays, and strings are closely connected in C. Understanding their relationship helps you master memory management, performance, and clean syntax.

📦Pointers and Arrays: The Relationship

🧠 In C, the name of an array is itself a pointer to the first element.

int arr[3] = {10, 20, 30};

printf("%p\\n", arr);     // Address of arr[0]
printf("%p\\n", &arr[0]); // Same address

✅ Example:

int arr[5] = {1, 2, 3, 4, 5};
int *ptr = arr;  // or &arr[0]

for(int i = 0; i < 5; i++) {
    printf("%d ", *(ptr + i));  // OR ptr[i]
}

1 2 3 4 5

📍 Memory Mapping of Arrays and Pointers

Let’s say:

int arr[3] = {5, 10, 15};

• arr → 1000

• arr + 1 → 1004

• (arr + 2) → 15

🔁 Pointer Arithmetic with Arrays

int arr[3] = {10, 20, 30};
int *p = arr;

printf("%d\\n", *p);     // 10
printf("%d\\n", *(p+1)); // 20
printf("%d\\n", *(p+2)); // 30

📌 Pointer arithmetic moves based on data type size

If int is 4 bytes → p+1 adds 4 bytes, not 1.

📬 Passing Arrays to Functions using Pointers

✅ Function Declaration:

void printArray(int *arr, int size);

✅ Call:

int numbers[] = {1, 2, 3};
printArray(numbers, 3);

✅ Function Definition:

void printArray(int *arr, int size) {
    for(int i = 0; i < size; i++) {
        printf("%d ", arr[i]);  // or *(arr + i)
    }
}

👉 When you pass numbers, you're passing the address of the first element.

🧵 Strings and Character Arrays

A string in C is simply a char array ending with a null terminator '\\0'.

char str[] = "Hello";

🧠 This is equivalent to:

char str[] = {'H', 'e', 'l', 'l', 'o', '\\0'};

📌 Pointer to String

char *s = "Hello";

This creates a pointer to a string literal. It is read-only — do not modify.

🔎 Accessing Characters in a String

char *s = "Hello";

printf("%c\\n", s[1]);     // 'e'
printf("%c\\n", *(s + 4)); // 'o'

⚠️ Beware of Pointer Modifications

char str[] = "C Lang";
char *p = str;

*p = 'D';  // ✅ Works (modifiable)

char *q = "C Lang";
*q = 'D';  // ❌ Undefined behavior (read-only)

• str[] → modifiable

• "C Lang" → stored in read-only memory

🧮 Traversing String Using Pointer

char *s = "World";

while (*s != '\\0') {
    printf("%c ", *s);
    s++;
}

W o r l d

🧪 Example: Reversing a String Using Pointers

void reverse(char *str) {
    char *end = str;
    while (*end) end++;    // Move to end
    end--;                 // Back from null terminator

    while (str < end) {
        char temp = *str;
        *str = *end;
        *end = temp;
        str++;
        end--;
    }
}

✅ Real-Life Usage

• 💬 Printing and manipulating strings

• 🔁 Passing arrays to functions efficiently

• 🛠️ Building custom versions of strlen, strcpy, etc.

• 🧪 Interview questions on pointer arithmetic