Why doing minus operation for NSUInteger while comparing crashed? - objective-c

Question

I'm comparing two NSUInteger, I kept getting crash thats says -[__NSCFNumber length]: unrecognized selector sent to instance
NSUInteger index = [masterArray indexOfObject:object];
if (index != NSNotFound){
if (index < [anArray count] - 1 ){
//Do something
}
else{
//Do something
}
}
Reading old post of similar question, but I still can't figure this out. I've tried to cast and it still crash:
NSUInteger index = [masterArray indexOfObject:object];
if (index != NSNotFound){
if (index < (NSUInteger)((int)[anArray count] - 1) ){
//Do something
}
else{
//Do something
}
}
However, without any minus operation, it works.
NSUInteger index = [masterArray indexOfObject:object];
if (index != NSNotFound){
if (index < [anArray count]){
//Do something
}
else{
//Do something
}
}
Any idea why? Thanks in advance.

Answer 1

the array is not the problem.
See what happens in the following example.
NSUInteger i = 0;
NSLog(#"%#", i-1==NSUIntegerMax ? #"YES" : #"NO" );
//output: YES
NSLog(#"NSUIntegerMax=%lu", NSUIntegerMax );
//output: NSUIntegerMax=18446744073709551615
NSLog(#"%lu", i-1 );
//output: 18446744073709551615
so when array count is 0 and you subtract 1 you end up with the NSUIntegerMax.
You compare array.count - 1 against a range of numbers that can't express signed (negative) numbers.
array.count is a property of type NSUInteger.
so in the following scenario will come up
when array.count == 0,
array.count - 1 == 18446744073709551615 // not!! -1
your if statements will not work as expected unless you really wanted to compare to NSUIntegerMax.
As you discovered casting to integer may help you out..
you will want to
if (index < ((int)array.count) - 1) {
} else {
}
because casting (NSUInteger)((int)array.count - 1) is still 18446744073709551615 == NSUIntegerMax when array.count < 1
you could also go without casting for this solution
if (index + 1 >= array.count) {
} else {
}