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With arrays, why is it the case that a[5] == 5[a]?

As Joel points out in Stack Overflow podcast #34, in C Programming Language (aka: K & R), there is mention of this property of arrays in C: a[5] == 5[a]

Joel says that it's because of pointer arithmetic but I still don't understand. Why does a[5] == 5[a]?

Solution

  • 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)

    a is a pointer to the first element of the array. a[5] is the value that's 5 elements further from a, which is the same as *(a + 5), and from elementary school math we know those are equal (addition is commutative).