Difference Between Array Name and Pointer

DIFFERENCE BETWEEN ARRAY NAME AND POINTER

When memory is allocated to an array, its base address is fixed and it cannot be changed during program execution. In other words, an array name is an address constant. Therefore, its value cannot be changed. To ensure that the address of the array does not get changed even inadvertently, C does not allow array names to be used as an Ivalue. Hence, array names cannot appear on the left side of the assignment operator.

However, you may declare a pointer variable of appropriate type that points to the first element of the array and use it as Ivalue. Figure 7.5 shows two sets of codes. The first code gives an error as the array name is being used as an Ivalue for the ++ operator. The second code shows the correct way of doing the same thing.

Second thing to remember is that an array cannot be assigned to another array. This is because an array name cannot be used as the lvalue.

int arr1 []={1,2,3,4,5); *

int arr2 [5];

arr2 = arr1; // ERROR

But one pointer variable can be assigned to another pointer variable of the same type. Therefore, the following statements are valid in C.

int arr1 []={1,2,3,4,5);

int *ptrl, *ptr2;

ptr1 = arr1;

ptr2 = ptr1;

When we write ptr2 = ptr1, we are not copying the data pointed to. Rather, we are just making two pointers point to the same location. This is shown in Figure 7.6.

Third point of difference lies with the return value of the address operator. The address operator returns the address of the operand. But when an address operator is applied to an array name, it gives the same value as the array reference without the operator. Therefore, arr and &arr give the same value. However, this is not true for a pointer variable.

Last but not the least, the sizeof operator when applied to an array name returns the number of bytes allocated for the array. But in case of a pointer variable, the sizeof operator returns the number of bytes used for the pointer variable (machine dependent). Look at the following code which illustrates this concept.

main()

{

int arr[]={1,2,3,4,5);

int *ptr;

ptr = arr;

printf("\n Size of array = %d", sizeof (arr));

printf("\n Size of pointer variable = %d", sizeof (ptr));

}

Output (On Turbo C)

Size of array = 10

70 Size of pointer variable = 2