Puzzling: If statement with <= not working as expected

This is probably obvious, but I cannot see it...

NSLog(@"nthObject = %i, [mutableFetchResults count] - 1 = %i", nthObject, [mutableFetchResults count] - 1);
if (nthObject <= [mutableFetchResults count] - 1) {
    MyObject *myObject = [mutableFetchResults objectAtIndex:nthObject];

The count of mutableFetchResults is zero, so [mutableFetchResults count] - 1 = -1 and nthObject is 0. This is proven by the Log.

nthObject is an int passed into the method.

So the of statement shou开发者_JAVA百科ld be saying if (0 <= -1) and therefore not firing the MyObject *myObject = [mutableFetchResults objectAtIndex:nthObject] line, but it does which then causes a crash as its trying to access an empty array.

Any ideas?

Many thanks,

Chris.

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I'm assuming that mutableFetchResults is an NSMutableArray (a subclass of NSArray). The count member is an NSUInteger which is unsigned, so [mutableFetchResults count] - 1 doesn't go negative - it wraps around to become a very large number.

Change your test to be:

if (nthObject < [mutableFetchResults count])

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Is [mutableFetchResults count] an unsigned int? The "%i" in the format string says to print it as a signed int, but that doesn't mean it is...

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[mutableFetchResults count] returns an NSUInteger.

If count is 0 then 0 - 1 on an NSUInteger will not be -1, it will 'wrap' around and be a massive number.

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[mutableFetchResults count] likely is this, so it yields an unsigned integer. It will wrap around to the max value of an unsigned integer , and your i will also be promoted to an unsigned type.

You comparison ends up as something like (0 < 0xffffffff).