In C++, how to manipulate the elements in an array with pointers

Question

Update the array values from { 5, 10, 15 } to { 30, 20, 10 } using pointers. In the end of the method, print out the elements in the array using pointers as well.

Solution

int main()
{
int arr[3] = { 5, 10, 15 };
int* ptr = arr;
*ptr = 10;          // set arr[0] to 10
*(ptr + 1) = 20;      // set arr[1] to 20
ptr += 2;
ptr[0] = 30;        // set arr[2] to 30
ptr++;
while (ptr > arr)
{
ptr--;
cout << ' ' << *ptr;    // print values
}
cout << endl;
}

Explanation (line by line)

int* ptr = arr;

The above line declares the variable ‘ptr’ to the array. The current memory address that this pointer will have the first element of this array. The ‘*’ is important as it declares the variable as a pointer.

*ptr = 10;

The above line shows that the value 10 is overriding the current value in the pointer.

ptr += 2;
ptr[0] = 30;        // set arr[2] to 30

ptr += 2 is the asme as ptr = ptr+2

This line shifts the pointer by 2 indices to the right. So now the pointer is at index 3. Therefore, ptr[0]=30 will override the 3rd element in the array to the value of 30.