Testing for contents of an NSArray without risking range error

Question

I'm foolishly saying:

if ([imageCache objectAtIndex:index]) {

Problem is, on my first time through this, I haven't put ANYTHING in my NSMutableArray *imageCache, and this croaks with a range error.

How can I ask an NSMutableArray whether it has anything for a particular index?

Answer 1

if (index < [imageCache count])
   ...

Answer 2

[imageCache count] will return the number of items in your array. Take it from there :-)

Answer 3

Check the number of items in the array first with [imageCache count]. Don't try to ask for anything with an index greater than that result.

Answer 4

The NSArray cluster class cannot store nil. So I think it is sufficient to simply check the bounds:

NSUInteger index = xyz; 
if (index < [imageCache count]) { 
    id myObject = [imageCache objectAtIndex:index]; 
}

Answer 5

What find really useful is having a safeObjectAtIndex: method. This will do the check for you and will return nil if the index is out of range.

Just create a new category on NSArray and include the following methods:

- (id)safeObjectAtIndex:(NSUInteger)index;
{
    return ([self arrayContainsIndex:index] ? [self objectAtIndex:index] : nil);
}

- (BOOL)arrayContainsIndex:(NSUInteger)index;
{
    return NSLocationInRange(index, NSMakeRange(0, [self count]));
}