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Creating Dictionaries

+ dictionary

Returns an empty dictionary. Usually used to create NSMutableDictionaries. NSDictionaries created with this method are permanently empty.

		// Most common use

		id mutableDictionary = [NSMutableDictionary 
dictionary];



		// May not be what you want

		id dictionary = [NSDictionary dictionary];

+ dictionaryWithObjects:forKeys:

Returns a dictionary containing entries constructed from the contents of a specified array of objects and a specified array of keys. The two arrays must have the same number of elements.

		id preferences = [NSMutableDictionary

			dictionaryWithObjects:@("window", "non-
smoking", "747")

			forKeys:@("seatAssignment", "smoking", 
"aircraft")];

+ dictionaryWithObjectsAndKeys:

Returns a dictionary containing entries constructed from a specified set of objects and keys. This method takes a variable number of arguments: a list of alternating objects and keys ending with nil.

		id customerPreferences = [NSDictionary 
dictionaryWithObjectsAndKeys:

			seatingPreference, @"seatAssignment", 

			smokingPreference, @"smoking",

			aircraftPreference, @"aircraft", nil];

+ dictionaryWithDictionary:

Returns a dictionary containing the contents of a specified dictionary. Usually used to create an NSMutableDictionary from an immutable NSDictionary.

+ dictionaryWithContentsOfFile:

Returns a dictionary initialized from the contents of a specified file. The specified file can be a full or relative pathname; the file that it names must contain a string representation of a dictionary, such as that produced by the writeToFile:atomically: method.

See also description.

- allKeys

Returns an array containing the dictionary's keys or an empty array if the dictionary has no entries. This method is useful for accessing all the entries in a dictionary. For example, the following code excerpt creates the NSArray keys and uses it to access the value of each entry in the dictionary:

		id index;

		id keys = [dictionary allKeys];

		for (index = 0; index < [keys count]; index++) {

			value = [dictionary objectForKey:[keys 

			objectAtIndex:index]];

			// Use the value

		}

- allKeysForObject:

Returns an array containing all the keys corresponding to values equivalent to a specified object. Equivalency is determined using the isEqual: method. If the specified object isn't equivalent to any of the values in the receiver, this method returns nil.

- allValues:

Returns an array containing the dictionary's values, or an empty array if the dictionary has no entries.

Note that the array returned from allValues may have a different count than the array returned from allKeys. An object can be in a dictionary more than once if it corresponds to multiple keys.

- keysSortedByValueUsingSelector:

Returns an NSArray containing the dictionary's keys such that their corresponding values are sorted in ascending order, as determined by a specified method. For example, the following code excerpt creates the NSArray keys containing the string "Pasta" at index 0, "Seafood" at index 1, and "Steak" at index 2:

		id choices = @{"Steak" = 3; "Seafood" = 2; "Pasta" = 
1};

		id keys = [choices 
keysSortedByValueUsingSelector:@"compare:"];

- count

Returns the number of entries currently in the dictionary.

- isEqual:

Returns YES if the specified object is a dictionary and has contents equivalent to the receiver; NO, otherwise. Two dictionaries have equivalent contents if they each hold the same number of entries and, for a given key, the corresponding value objects in each dictionary satisfy the isEqual: test.

- objectForKey:

Returns the object that corresponds to a specified key. For example, the following code excerpt produces the NSString sectionPreference with the contents "non-smoking":

		id preferences = [NSMutableDictionary

			dictionaryWithObjects:@("window", "non-
smoking", "747")

			forKeys:@("seatAssignment", "section", 
"aircraft")];

		id sectionPreference = [dictionary 
objectForKey:@"section"];

- setObject:forKey:

Adds an entry to the receiver, consisting of a specified key and its corresponding value object. If the receiver already has an entry for the specified key, the previous value for that key is replaced with the argument for setObject:. For example, the following code excerpt produces the NSMutableDictionary dictionary with the value "non-smoking" for the key "section" and the value "aisle" for the key "seatAssignment." Notice that the original value for "seatAssignment" is replaced:

		id dictionary = [NSMutableDictionary 
dictionaryWithDictionary:

			@{"seatAssignment" = "window"}];

		[dictionary setObject:@"non-smoking" 
forKey:@"section"];

		[dictionary setObject:@"aisle" 
forKey:@"seatAssignment"];

It is an error to specify nil as an argument for setObject: or forKey:. You can't put nil in a dictionary as a key or as a value.

- addEntriesFromDictionary:

Adds the entries from a specified dictionary to the receiver. If both dictionaries contain the same key, the receiver's previous value for that key is replaced with the new value.

- removeAllObjects

Empties the dictionary of its entries.

- removeObjectForKey:

Removes the entry for a specified key. Releases the object and the key.

- removeObjectsForKeys:

Removes the entries for each key in a specified array. Releases the object and the key.

- setDictionary:

Removes all the entries in the receiver, then adds the entries from a specified dictionary.

- description

Returns a string that represents the contents of the receiver. For example, the following code excerpt produces the string "{"seatAssignment" = "Window"; "section" = "Non-smoking"; "aircraft" = "747"}":

		id preferences = [NSMutableDictionary

			dictionaryWithObjects:@("window", "non-
smoking", "747")

			forKeys:@("seatAssignment", "section", 
"aircraft")];

		id description = [preferences description];

- writeToFile:atomically:

Writes the dictionary's string representation to a specified file using the description method. Returns YES on success and NO on failure. If YES is specified for atomically:, this method attempts to write the file safely so that an existing file with the specified path is not overwritten. It does not create a new file at the specified path unless the write is successful. The resulting file is suitable for use with dictionaryWithContentsOfFile:. For example, the following excerpt creates an NSMutableDictionary from the contents of the file specified by path, updates the object for the key @"Language", and saves the updated dictionary back to the same file:

		id defaults = [NSMutableDictionary

			dictionaryWithContentsOfFile:path];

		[defaults setObject:newLanguagePreference 
forKey:@"Language"];

		[defaults writeToFile:path atomically:YES];

See also description.

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