指针增量减量语法差异

Question

when I work on pointers on a website I stick at a point.

When I write the given example I can assign adresses of array on a pointer for an increment a pointer example like ptr = arr; and it works correctly, but when I do the same thing for the decrement example it doesn't work it works only when I write like this ptr = &arr[2] . Why I have to write ampersand for decrement example? what is the difference between those two?

int main()
{
    int arr[3]={10,20,30};
    int *ptr,i;

    ptr=arr;

    for(i=0;i<3;i++)
    {
        printf("adress of variable arr[%d] %x\n",i+1,ptr);
        printf("value of arr[%d] = %d\n",i+1,*ptr);
        ptr++;
    }
    return 0;
}

Answer 1

You don't have to write it, just use the pointer:

 ptr = var + 2 ;

for(i=0;i<3;i++)
{ 
    printf("%d" , *ptr ) ;
    ptr-- ;

Note that the last element is at +2 not +3.

Answer 2

in case of an int arr[3] , &arr[0] and arr points to the same thing, the base address of the array, or, the address of the first element in the array. That's why in your increment case, you're allowed to write ptr = var which is nothing but storing the starting address in a separate pointer.

In case of decrements, there is nothing which can point directly to the end-of-array element address. So , you have to use the address-of-last-element [ &arr[n-1] , n being the size] to denote the address of the last element.

Answer 3

The difference is that in "decrement" case you use [] . Think of arr as reference to whole array thus it is ok (compiler wise) to do ptr=arr or ptr=&arr with same result. On the other hand arr[3] is a reference to object in array, so you need to get its address explicitly (no compiler optimization for you).