Pointer
1. A pointer is a place in memory that keeps address of another place inside.
2. Pointer can’t be initialized at definition.
3. Pointer is dynamic in nature. The memory allocation can be resized or freed later.
4. The assembly code of Pointer is different than Array.
Array
1. An array is a single, pre allocated chunk of contiguous elements (all of the same type), fixed in size and location.
2. Array can be initialized at definition. For Example
int num[] = { 2, 4, 5}
3. They are static in nature. Once memory is allocated , it cannot be resized or freed dynamically.
4. The assembly code of Array is different than Pointer.
Pointers are used for storing address of dynamically allocated arrays and for arrays which are passed as arguments to functions. In other contexts, arrays and pointer are two different things, see the following programs to justify this statement.
Behavior of sizeof operator
// 1st program to show that array and pointers are different
#include <stdio.h>
int main()
{
int arr[] = {10, 20, 30, 40, 50, 60};
int *ptr = arr;
// sizof(int) * (number of element in arr[]) is printed
printf("Size of arr[] %d\n", sizeof(arr));
// sizeof a pointer is printed which is same for all type
// of pointers (char *, void *, etc)
printf("Size of ptr %d", sizeof(ptr));
return 0;
}
Output:
Size of arr[] 24
Size of ptr 4
Although array and pointer are different things, following properties of array make them look similar.
1) Array name gives address of first element of array.
Consider the following program for example.
#include <stdio.h>
int main()
{
int arr[] = {10, 20, 30, 40, 50, 60};
int *ptr = arr; // Assigns address of array to ptr
printf("Value of first element is %d", *ptr)
return 0;
}
Output:
Value of first element is 10
2) Array members are accessed using pointer arithmetic.
Compiler uses pointer arithmetic to access array element. For example, an expression like “arr[i]” is treated as *(arr + i) by the compiler. That is why the expressions like *(arr + i) work for array arr, and expressions like ptr[i] also work for pointer ptr.
#include <stdio.h>
int main()
{
int arr[] = {10, 20, 30, 40, 50, 60};
int *ptr = arr;
printf("arr[2] = %d\n", arr[2]);
printf("*(ptr + 2) = %d\n", *(arr + 2));
printf("ptr[2] = %d\n", ptr[2]);
printf("*(ptr + 2) = %d\n", *(ptr + 2));
return 0;
}
Output:
arr[2] = 30
*(ptr + 2) = 30
ptr[2] = 30
*(ptr + 2) = 30
3) Array parameters are always passed as pointers, even when we use square brackets.
#include <stdio.h>
int fun(int ptr[])
{
int x = 10;
// size of a pointer is printed
printf("sizeof(ptr) = %d\n", sizeof(ptr));
// This allowed because ptr is a pointer, not array
ptr = &x;
printf("*ptr = %d ", *ptr);
return 0;
}
int main()
{
int arr[] = {10, 20, 30, 40, 50, 60};
fun(arr);
return 0;
}
Output:
sizeof(ptr) = 4
*ptr = 10
