C++ modulus operator circular array

Question

I am trying to achieve a "wrap around" effect in a circular array. When I push an item to front, I want it to be stored in decreasing locations. When I push to front, I'm trying something like this:

items[front] = ch;
front = (front - 1) % capacity;

But this does not wrap around like I would expect. Front starts out at 0, and when the second line executes, front becomes -1. Shouldn't front become 6 after the second line executes? My data structures textbook seems to think so.

Could you give us more code for context? What data type is `front`? What is the value of `capacity`? — user3288829, May 04 '14 at 01:12
If `front` isn't an `unsigned` type, then this behavior is perfectly normal. — Chnossos, May 04 '14 at 01:13
Especially pre-C++11, take note of http://stackoverflow.com/questions/7594508/modulo-operator-with-negative-values — chris, May 04 '14 at 01:17
Front is an int. I guess I just didn't realize that is how modulus works... — chillpenguin, May 04 '14 at 01:19

score 3 · Answer 1 · edited May 04 '14 at 15:56

3

That is how % works in c++. To get the effect you want try this:

front = (front + capacity - 1) % capacity;

edited May 04 '14 at 15:56

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answered May 04 '14 at 01:14

The Dark

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`front = (front > 0 ? front : capacity) - 1;` sounds better to me. Also, if `front` is an `unsigned` type then OP's operation should be correct too. – Chnossos May 04 '14 at 01:15

C++ modulus operator circular array

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