Condition checking in a for loop

Question

Consider this code:

int main()
    {
        int i;
        int ints[3];
        ints[0] = 0;
        ints[1] = 1;
        ints[2] = 2;

        for(i = 0; (ints[i])<4 && i<3; i++)
        {
            printf("%d\n", ints[i]);
        }
    }

Is there a reason I shouldn't do this sort of conditioning in the loop? I mean, when i becomes 3, will it look for a non-existing value at ints[3]? And if yes, is it ok?, since the loop is going to terminate anyway? It compiles and runs fine, but I was just wondering if there is some sort of a hidden problem with this. Thanks.

Answer 1

In the last iteration of this loop, the expression ints[i]<4 will access ints[3]. Since this access reads past the end of the array ints, the behavior is undefined in C. So yes, this is not good code. It may work most of the time, but the behavior is ultimately undefined.

Answer 2

It will be better to do i < 3 && ints[i] < 4 because the second part of the statement is evaluated only if the first one is true. The way you have it, it will look for ints[3] that does not exist.