Is `index[array]` a Valid Array Access Syntax in C and C ?

Accessing Arrays by Index[array] in C and C

While the typical form of array indexing is array[index], C and C provide an alternative syntax: index[array]. This syntax has puzzled many programmers, but is it valid by the language specifications?

Question:

int arr[] = {1, 2, 3};
2[arr] = 5; // Does this compile?
assert(arr[2] == 5); // Does this assertion fail?

Explanation:

This trick question relies on the commutative nature of addition. The operation index[array] is converted to *(index array), and since addition is commutative, we might assume that 2[arr] and arr[2] are equivalent. However, this is not explicitly stated in the language specifications.

Answer:

Yes, the code is valid according to the C and C specifications.

C99 (6.5.2.1, paragraph 1):

One of the expressions shall have type "pointer to object type", the other expression shall have integer type, and the result has type "type".

C99 (6.5.2.1, paragraph 2):

A postfix expression followed by an expression in square brackets [] is a subscripted designation of an element of an array object. The definition of the subscript operator [] is that E1[E2] is identical to (*((E1)+(E2))). Because of the conversion rules that apply to the binary + operator, if E1 is an array object (equivalently, a pointer to the initial element of an array object) and E2 is an integer, E1[E2] designates the E2-th element of E1 (counting from zero).

These specifications do not require the order of the arguments to [] to be sane. Therefore, both lines in the code compile and execute as expected, and the assertion passes.

The above is the detailed content of Is `index[array]` a Valid Array Access Syntax in C and C ?. For more information, please follow other related articles on the PHP Chinese website!