Does `2[arr]` Work in C and C ?

Accessing Arrays Using Index[array] in C and C

Consider the following code snippet:

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

Some interviewers might ask why this code snippet compiles and executes without errors.

Understanding the Code

The expression 2[arr] is equivalent to *(2 arr). Since addition is commutative in C and C , the order of the arguments to [] does not matter. Therefore, 2[arr] effectively accesses the third element of the array arr.

Is This Guaranteed by the Language Specification?

Yes. The C and C standards explicitly state this behavior:

• C99 Standard: Paragraph 6.5.2.1 states that E1[E2] is equivalent to *((E1) (E2)).
• C 11 Standard: Paragraph 5.2.1.2 states that "E1[E2] is equivalent to "(*(E1) (E2))"

Therefore, accessing arrays using index[array] is guaranteed by the language specification, even if it seems unusual or counter-intuitive.

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