In a “for” statement, should I use `!=` or `<`?

Question

I\'ve seen both of these two for statements:

for(i=0;i<10;i++) 

for(i=0;i!=10;i++)

I know they all stop when i reaches 10 , bu

Answer 1

I usually use < in for-loops for the reasons stated by others. In while-loops and more advanced for-loops though, != makes it much easier to reason about what my program does.

Say I want to find the position after the first run of '5's in an array like [5,5,5,3,5,2,3,2,0]. That is we want k such that 0 <= i < k => a[i] = 5:

int pos = 0;
// Inv: a[0..pos) = 5
while (pos != N && a[pos] == 5)
    pos = pos+1;

Once the loop has executed we know that the inverse of the loop guard is true: pos == N || a[pos] != 5. In either case we have the pos we want.

Now say we had used < in the loop guard, then all we would have known afterwards was pos >= N || a[pos] != 5, but that's not the situation we wanted to be in. Doing a lot more work, we can prove that we can't possible be in pos > N, but that seams like a waste of time compared to just using != in the loop guard.