Different way of accessing array elements in C

Question

I am a teaching assistant for a C programming course, and I came across the following line of C code:

char str[] = "My cat's name is Wiggles.";
printf("%c %c %c %c\n", str[5], *(str + 5), *(5 + str), 5[str]);

I never came across the very last argument (5[str]) before, and neither did my professor. I don't think it's mentioned in K&R and C Primer Plus. I found this piece of code in a set of technical interview questions. Does anyone know why C allows you to access an array element that way also? I never heard of an index being outside the set of brackets and the name of an array inside the brackets.

Your help will be greatly appreciated!

Answer 1

It's basically just the way C works. str[5] is really equivelent to *(str + 5). Since str + 5 and 5 + str are the same, this means that you can also do *(5 + str), or 5[str].

It helps if you don't think of "5" as an index, but rather just that addition in C is commutative.

Answer 2

Perfectly valid C. From Wikipedia:

Similarly, since the expression a[i] is semantically equivalent to *(a+i), which in turn is equivalent to *(i+a), the expression can also be written as i[a] (although this form is rarely used).

Wacky, but valid.

Answer 3

str[5] directly translates to *(str + 5), and 5[str] directly translates to *(5 + str). Same thing =)

Answer 4

Similarly, since the expression a[i] is semantically equivalent to *(a+i), which in turn is equivalent to *(i+a), the expression can also be written as i[a] (although this form is rarely used).

http://en.wikipedia.org/wiki/C_syntax#Accessing_elements