Using unsigned indexes would be fine if the program automatically throws an error on wrapping. Otherwise, it’s an accident waiting to happen, since you have to

Using unsigned indexes would be fine if the program automatically throws an error on wrapping. Otherwise, it’s an accident waiting to happen, since you have to code all of your arithmetic keeping in mind the fact that the numbers are trying to blow up in your face. If x is unsigned, then you can’t rearrange an inequality like x >= 4 into x - 4 >= 0, since the first inequality is only true sometimes, whereas the second is true always. This looks obvious, but bury it a little way into some nontrivial index arithmetic and boom. Another more aesthetic reason is that indices can represent offsets into a structure relative to some other index, and in this case you really need negative numbers.