Are negative array indexes allowed in C?

Question

I was just reading some code and found that the person was using arr[-2] to access the 2nd element before the arr, like so:

|a|b|c|         


        

Answer 1

I know the question is answered, but I couldn't resist sharing this explanation.

I remember Principles of Compiler design: Let's assume a is an int array and size of int is 2, and the base address for a is 1000.

How will a[5] work ->

Base Address of your Array a + (index of array *size of(data type for array a))
Base Address of your Array a + (5*size of(data type for array a))
i.e. 1000 + (5*2) = 1010

This explanation is also the reason why negative indexes in arrays work in C; i.e., if I access a[-5] it will give me:

Base Address of your Array a + (index of array *size of(data type for array a))
Base Address of your Array a + (-5 * size of(data type for array a))
i.e. 1000 + (-5*2) = 990

It will return the object at location 990. So, by this logic, we can access negative indexes in arrays in C.