With arrays why is it the case that a 5 5 a

Question

This property of arrays in C is mentioned in the K & R programming language, as Joel notes in Stack Overflow podcast number 34: a[5] == 5[a]

Joel claims that pointer arithmetic is the cause, but I'm still confused. Why is a[5] equal to 5[a]?

Answer 1

The C standard defines the [] operator as follows:

a[b] == *(a + b)

Therefore a[5] will evaluate to:

*(a + 5)

and 5[a] will evaluate to:

*(5 + a)

The array's first element is indicated by the pointer a. Since we know from elementary school math that these are equal, a[5] is the value that is 5 more elements away from a. It is the same as *(a + 5).