NSMutable Array beyond bounds - iphone

Question

I have a block of code which when executed gives me this error. And I am relatively new, I can't seem to fix the problem.
Error:
2011-09-06 12:31:06.094 ForceGauge[266:707] CoreAnimation: ignoring exception: * -[NSMutableArray objectAtIndex:]: index 1 beyond bounds [0 .. 0]
-(void)peakCollector:(NSMutableArray *)testarray {
NSUInteger totalRows = [testarray count];
NSMutableArray *percentArray = [[NSMutableArray alloc]initWithObjects:0, nil];
if(forcecontroller.selectedSegmentIndex==0)
testarray = lbData;
else if(forcecontroller.selectedSegmentIndex==1)
testarray = kgData;
else if(forcecontroller.selectedSegmentIndex ==2)
testarray = ozData;
else if(forcecontroller.selectedSegmentIndex ==3)
testarray = newtonData;
for(int i = 0; i< totalRows-1; i++) {
if ([[testarray objectAtIndex:i+1] doubleValue] >= 1.2 * [[testarray objectAtIndex:i] doubleValue]) {
percentArray = [testarray objectAtIndex:i];
DatabaseTable *tableVC = [[DatabaseTable alloc] initWithStyle:UITableViewStylePlain];
[self.navigationController pushViewController:tableVC animated:YES];
if(forcecontroller.selectedSegmentIndex==0)
[tableVC copydatabase:percentArray];
else if(forcecontroller.selectedSegmentIndex==1)
[tableVC copydatabase:kgData];
else if(forcecontroller.selectedSegmentIndex==2)
[tableVC copydatabase:ozData];
else if(forcecontroller.selectedSegmentIndex==3)
[tableVC copydatabase:newtonData];
[tableVC release];
} else {
[analogData removeAllObjects];
}
}
}

Answer 1

There are multiple issues here:
1) NSArrays can only contains NSObjects.
In your code, you are initializing your NSArray using [[NSMutableArray alloc]initWithObjects:0, nil];, but 0 is an atomic type, not an NSObject
(and basically 0 is the same value as nil (nil and NULL are typically equal to 0, interpreted as the id and void* types, respectively)
You have to encapsulate your 0 value in an NSNumber instead :
[[NSMutableArray alloc]initWithObjects:[NSNumber numberWithInt:0], nil];
Then retrieve the NSNumber using [percentArray objectAtIndex:0] and finally convert back the retrieve NSNumber to int using NSNumber's intValue method:
NSNumber* number = [percentArray objectAtIndex:0]; // returns an NSNumber which is an NSObject encapsulating numbers, see Apple's documentation
int val = [number intValue]; // retrieve the integer value encapsulated in the NSNumber
// or directly:
// int val = [[percentArray objectAtIndex:0] intValue];
2) The exception you got is in fact elsewhere and is much more subtle: you are retrieving the [testarray count] value in an NSUInteger variable, which is an unsigned type. Then totalRows-1 will do some tricky things if totalRows is equal to 0 (which is obviously the case considering the exception you have).
As totalRows is an NSUInteger, when it is equal to 0, totalRows-1 will not be equal to -1, but to... (NSUInteger)-1 (-1 interpreted as an unsigned integer), which is 0xFFFFFFFF, or namely the maximum value of the NSUInteger type!
This is why i is always less than this totalRows-1 value (as this value is not -1 but 0xFFFFFFFF = NSUInteger_MAX).
To solve this issue, cast your totalRows variable to an NSInteger value, or add a condition in your code to treat this special case separately.

Answer 2

Please check the how many objects test array contains at beginning of for loop.
Other thing that can help is that avoid NSUInteger and use simple int for storing array count.
Please post if this don't work.

Answer 3

The error simply means that you're trying to retrieve an object at an index that does not exist. In your specific case, the array you're trying to get the object from does not have an object at index 1
An easy example
[0] => MyCoolObject
[1] => MySecondObject
[2] => ObjectTheThird
You can access indexes 0,1,2 of your array as they contain objects. If you now would try to access index 3 it would throw the Out of bounds exception as index 3 does not exist.