Different way of accessing array elements in C

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 mentio开发者_Go百科ned 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!

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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.

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

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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.

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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

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Its all same. *ptr or ptr[0] actually means *(ptr+0). So whenever you write *ptr or ptr[0] it goes as *(ptr+0). Let say you want value at ptr[4] so it means you can also write it as *(ptr+4). Now whether you write it as *(ptr+4) or *(4+ptr), it's same. so just for understading if you can write *(ptr+4) as ptr[4] same way *(4+ptr) is same as 4[ptr]. Please go through http://en.wikipedia.org/wiki/C_syntax#Accessing_elements for more details.

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if str is an array of type char, then we can access any index say i as below-

1. str[i]
2. *(str + i)
3. char *p = str, then access the index i as p[i] or *(p+i)

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It's a funky syntax for sure, but...

str[5] would mean *(str+5)

And

5[str] would mean *(5+str)

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Example Code.

#include<stdio.h>
int main(){
    int arr[] = {1, 2, 3};
    for(int i = 0; i <= 2; i++){
        printf("%d\t", i[arr]);
    }
    return 0;
}

Output:1 2 3