Is chars[4] and 4[chars] the same in C? Why?

Question

I've read this and don't believe it :) I've no compiler here to test.

score 9 · Accepted Answer · answered Nov 21 '09 at 20:22

9

In raw C, the [] notation is just a pointer math helper. Before [], you'd look for the fourth char in the block pointed to by ptr like:

*(ptr+4)

Then, they introduced a shortcut which looked better:

ptr[4]

Which transaltes to the earlier expression. But, if you'd write it like:

4[ptr]

This would translate to:

*(4+ptr)

Which is indeed the same thing.

answered Nov 21 '09 at 20:22

Andomar

so chars is a pointer to the begining of the my char[] array. Right? – Juanjo Conti Nov 21 '09 at 20:28
Yes, that's exactly what a C array is. It's a pointer to the first element of the array – Andomar Nov 21 '09 at 20:33
4

Technically not quite - see: http://www.lysator.liu.se/c/c-faq/c-2.html (but for the purposes of this answer, they're close enough!) – SimonJ Nov 21 '09 at 20:38

score 5 · Answer 2 · answered Nov 21 '09 at 20:13

5

Because a[b] is exactly the same as *(a+b), and + is commutatitve.

chars[4] is *(chars+4), and 4[chars] is *(4+chars)

answered Nov 21 '09 at 20:13

Ned Batchelder

What's char+4? and what's *(chars+4)? – Juanjo Conti Nov 21 '09 at 20:20
char + 4 is a pointer to the fourth character in the array and *(chars+4) dereferences this pointer to give the character at the chars[4] location – rzrgenesys187 Nov 21 '09 at 20:25

score 4 · Answer 3 · answered Nov 21 '09 at 20:13

4

answered Nov 21 '09 at 20:13

micmoo

3 Answers3