Does 2[arr] = 5 Compile and Pass Assertion in C and C ?

Accessing Arrays by Index[array] in C and C

In a knowledge test sometimes posed by interviewers, the following code is presented:

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

It might initially appear that the expression 2[arr] should fail to compile, as it seems to be attempting to index an array using an integer as the array name. However, this unexpected syntax is indeed valid in both C and C .

To understand why, let's delve into the technicalities of the [] operator in these languages.

C and C Array Access Semantics

According to the C99 standard (6.5.2.1 paragraph 1), the [] operator expects arguments in the form:

• One expression of pointer-to-object type
• One expression of integer type
• Resulting expression of the same type as the object

Further, paragraph 2 of the same section explains that E1[E2] is equivalent to (*((E1) (E2))). This indicates that the expression E1[E2] can be interpreted as a pointer manipulation:

• E1 is converted to a pointer to the initial element of an array.
• E2 is added to the pointer, resulting in a new pointer that points to the E2-th element of the array.
• Dereferencing the resulting pointer returns the value of the selected array element.

Crucially, there is no requirement within the standard that the order of arguments to [] be sane. Therefore, the expression 2[arr] is treated as equivalent to (*((2) (arr))).

• 2, an integer, is added to a pointer pointing to the first element of arr.
• The resulting pointer points to the third element of arr.
• Dereferencing the resulting pointer (i.e., accessing arr[2]) assigns the value 5 to the third element.

Therefore, both the assignment and the subsequent assertion succeed, as expected.

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