Pointers and Arrays

In C, the names of arrays are deeply intertwined with pointers. In most contexts, an array name is treated as a constant pointer to the address of its first element. This equivalence is fundamental to C programming.

1. The Array Name as a Pointer

When you use an array name without brackets ([]), it automatically decays into a pointer to the first element.

Consider an array:

int data[5] = {10, 20, 30, 40, 50};

The following three expressions all represent the same memory address (the start of the array):

data
&data[0]
(int *)data 

2. Accessing Elements Using Pointers

Because of this equivalence, you can access array elements using standard pointer notation instead of array indexing ([]).

Example: Pointer Notation Access

int numbers[3] = {100, 200, 300};

// Accessing the second element (index 1)
int value1 = numbers[1];     // value1 is 200 (standard method)
int value2 = *(numbers + 1); // value2 is 200 (pointer method)

3. Pointer Arithmetic

When performing arithmetic on a pointer, C automatically scales the calculation by the size of the data type the pointer points to. This is called pointer arithmetic.

• If ptr is a pointer to an int (typically 4 bytes), adding 1 to the pointer (ptr + 1) doesn’t add 1 byte; it moves the pointer forward by 1 unit of the data type (4 bytes) to point exactly to the next integer in memory.
• ptr + i moves the pointer $i \times \text{sizeof(dataType)}$ bytes forward.

Example of Pointer Increment

int arr[] = {10, 20, 30};
int *ptr = arr; // ptr points to arr[0] (value 10)

printf("Value at arr[0]: %d\n", *ptr);

ptr++; // Pointer arithmetic moves ptr to arr[1] (20)

printf("Value at arr[1]: %d\n", *ptr);
// If 'arr' started at memory 1000, 'ptr' moved to 1004.

4. Arrays as Function Arguments

When you pass an array to a function, C always passes it by reference (meaning the function gets access to the original memory block). Even if the function signature uses array notation, the compiler treats the parameter as a pointer.

// The parameter declaration is identical to: void process(int *arr)
void processArray(int arr[], int size) {
    arr[0] = 99; // Changes the original array in main()
}

int main() {
    int scores[5] = {1, 2, 3, 4, 5};
    processArray(scores, 5); // Pass the array name (pointer to first element)
    printf("New score: %d\n", scores[0]); // Output: 99
    return 0;
}