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.