Question

In my iPhone app, I need to get some values from an array like so:

string1 = [array objectAtIndex:0];
string2 = [array objectAtIndex:1];

However, sometimes, the object won’t exist in the array depending on the user data. When a line like this is called and the object doesn’t exist my app crashes naturally with an out of bounds error.

So how can I get around this? How can I detect when that object doesn’t exist and set the string accordingly?

E.g.
if object at index does exist, string = …

else if object doesn’t exist at index, string = …

Answer 1

NSArray has a count property you can use to determine whether an index will be valid.

if(array.count < index)
{ string1 = [array objectAtIndex:index]; } 
else
{ string1 = @"Not found"; }

If you want this to be universal functionality, you can use categories to add it as a method to NSArray:

@interface NSArray()
- (NSString*) objectAtIndexWithCheck:(int) index;
@end

@implementation NSArray
-(NSString*) objectAtIndexWithCheck:(int) index
{
   if(self.count < index)
   { return [self objectAtIndex:index]; }
   else
   { return @"Not found"; }
}
@end