How to check if an NSDictionary or NSMutableDictionary contains a key?

Question

I need to check if an dict has a key or not. How?

Answer 1

One very nasty gotcha which just wasted a bit of my time debugging - you may find yourself prompted by auto-complete to try using doesContain which seems to work.

Except, doesContain uses an id comparison instead of the hash comparison used by objectForKey so if you have a dictionary with string keys it will return NO to a doesContain.

NSMutableDictionary* keysByName = [[NSMutableDictionary alloc] init];
keysByName[@"fred"] = @1;
NSString* test = @"fred";

if ([keysByName objectForKey:test] != nil)
    NSLog(@"\nit works for key lookups");  // OK
else
    NSLog(@"\nsod it");

if (keysByName[test] != nil)
    NSLog(@"\nit works for key lookups using indexed syntax");  // OK
else
    NSLog(@"\nsod it");

if ([keysByName doesContain:@"fred"])
    NSLog(@"\n doesContain works literally");
else
    NSLog(@"\nsod it");  // this one fails because of id comparison used by doesContain