An answer to the problem in 18.3 but also a problem with changing a key?

[MarkReid]

Code: (Objective-C)

// Create an NSDictionary dictionary object and fill it with some key/object pairs. Then make both
// mutable and immutable copies. Are these deep copies or shallow copies that are made? Verify your answer.

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
	
	// Create object and key pairs to populate dictionary
	
	NSMutableString *key1 = [NSMutableString stringWithFormat: @"First key"];
	NSMutableString *key2 = [NSMutableString stringWithFormat: @"Second key"];
	NSMutableString *key3 = [NSMutableString stringWithFormat: @"Third key"];
	NSMutableString *obj1 = [NSMutableString stringWithFormat: @"First object"];
	NSMutableString *obj2 = [NSMutableString stringWithFormat: @"Second object"]; 
	NSMutableString *obj3 = [NSMutableString stringWithFormat: @"Third object"]; 
	
	// Create dictionary and copies
	
	NSDictionary *dict = [NSDictionary dictionaryWithObjectsAndKeys: obj1, key1, obj2, key2, obj3, key3, nil];
	NSMutableDictionary *mutableDict;
	NSDictionary *immutableDict;
	
	mutableDict = [dict mutableCopy];
	immutableDict = [dict copy];
	
	// Change the first object and print dictionaries to see results
	
	[obj1 setString: @"This isn't the same as it was before"];
	
	NSLog(@"Original dictionary %@\n\n", dict);
	NSLog(@"Mutable dictionary %@\n\n", mutableDict);
	NSLog(@"Immutable dictionary %@\n\n", immutableDict);
	
	NSLog(@"The first object in the dictionaries change when the original is changed therefore they are shallow copies.");
	
	[mutableDict release];
	[immutableDict release];
	
	[pool drain];
    return 0;
}

That's my code for the solution to the problem. As expected the copies are shallow.

One issue however, if I use the code

Code: (Objective-C)

[key1 setString: @"A different key"];

in my program right where I change the object it's associated with, the key in the dictionary objects does not change the way I'd expect it to.

Is there some reason that a key cannot be changed while the objects can?

[skochan]

I don't think I mentioned in the text that the keys are copied while the objects are retained (or was that the point of the exercise)?

Cheers,

Steve

[MarkReid]

Thanks Steve, it didn't occur to me that would be possible so thought it was a problem with the code but it makes sense to me now.