NSUInteger in reversed loop confusion?

Question

I wonder if someone can explain something, I setup a loop where I wanted to count backwards from 10 to 0 :

for(NSUInteger index = 10; index >= 0; index--) {
    NSLog(@"INDEX: %ld", (long)index);
}

This loop runs forever, it does not stop at 0, but keeps going into negative numbers. When I noticed this I changed my code to :

for(NSInteger index = 10; index >= 0; index--) {
    NSLog(@"INDEX: %ld", (long)index);
}

The above works fine, but I am curious, why the first example does not work as the numbers generated are all unsigned integers?

Answer 1

An unsigned type can't "keep going into negative numbers". After the iteration when index = 0, index-- becomes 0xFFFFFFFF, which is still more than zero. Easy mistake to make, I've done it myself.

The static analyzer will actually warn you about this ("Condition index >= 0 is always true" or such like.) I highly recommend setting it to run automatically on debug builds.

Answer 2

Note that an NSUInteger is always >= 0, by consequence, your loop condition will always be true.