what does pointer index -1 means ? Help me understand output of code

Question

Hello

Please refer below code:
https://code.sololearn.com/cIoGSThWA599

I understand that initially ptr is referring to index 0 of array, Then ptr +=2; move ptr reference to two more elements. but what does ptr[-1] and why output is 11?

Answer 1

`arr[i]` is the same as `*(arr + i)`

So after you add 2 to ptr, it points to the third element. Therefor ptr[-1] is the second element, 6.

So ptr == a + 2 + 6!

Answer 2

int* ptr = a;
You point to the first element.

ptr +=2;
You increment the pointer by 2 and now it is at 7.

ptr[-1];
Your pointer takes a step back and it is now at 6.

ptr += ptr[-1] is same as ptr += 6
Your pointer will now move to 11.

Also *(ptr), *(ptr+0), ptr[0] are all the same.

Answer 3

Yo...thanks Avinesh ... Thanks Schindlabua

Answer 4

I am still confused.... By doing arithmetic , we can move left or right but not adding values i.e. 5+6

So just doing something on index means we are moving pointer p accross the array index...isn't it?

Would request to elaborate plz

Answer 5

Ptr[-1] means second element's value that is 6 and that we added to p not values