You want to store key-value data in an object, or you would like to retrieve objects from an array using a key into the array, but arrays won’t quite suffice for this purpose, as they do not facilitate finding objects inside the array using a key or a marker for that object.
A dictionary is a special container for objects in which each object is given a key, which itself is an object. That is one of the key differences between dictionaries and arrays. An array has a numeric index into each item/object that it holds whereas a dictionary holds a key to each item. I’ll show you what I mean.
Let’s say we want to store a person’s first name, last name, and age into an array and then into a dictionary. This is how we would store those values in an array:
NSArray *person = [[NSArray alloc] initWithObjects: @"Anthony", @"Robbins", [NSNumber numberWithUnsignedInteger:51], nil]; NSLog(@"First Name = %@", [person objectAtIndex:0]); NSLog(@"Last Name = %@", [person objectAtIndex:1]); NSLog(@"Age = %@", [person objectAtIndex:2]);
You can see that we are using an index into the array to access
each one of these values. With dictionaries, we give each value a
key, which is an object, and then use that key to
access those values. Let’s see the same example, using dictionaries.
We have a "First Name"
key with the
value "Anthony"
and so on:
NSNumber *age = [NSNumber numberWithUnsignedInteger:51]; NSDictionary *person = [[NSDictionary alloc] initWithObjectsAndKeys: @"Anthony", @"First Name", @"Robbins", @"Last Name", age, @"Age", nil]; NSLog(@"First Name = %@", [person objectForKey:@"First Name"]); NSLog(@"Last Name = %@", [person objectForKey:@"Last Name"]); NSLog(@"Age = %@", [person objectForKey:@"Age"]);
The results will then be printed out as shown here:
First Name = Anthony Last Name = Robbins Age = 51
As you can see, we initialized the dictionary with values and
keys. We give a value followed by the key for that value. When we used
NSLog
, we printed out each value by
handing the key to the dictionary’s objectForKey:
method.
The mutable version of NSDictionary
, NSMutableDictionary
, can be modified after
it has been allocated and initialized. For instance, if we want to
remove the object associated with the key
Age
from our dictionary after its
initialization, we would use a mutable dictionary like so:
NSNumber *age = [NSNumber numberWithUnsignedInteger:51]; NSMutableDictionary *person = [[NSMutableDictionary alloc] initWithObjectsAndKeys: @"Anthony", @"First Name", @"Robbins", @"Last Name", age, @"Age", nil]; [person removeObjectForKey:@"Age"]; NSLog(@"First Name = %@", [person objectForKey:@"First Name"]); NSLog(@"Last Name = %@", [person objectForKey:@"Last Name"]); NSLog(@"Age = %@", [person objectForKey:@"Age"]);
We have simply removed the object associated with the key
Age
. The results printed to the console
window will be similar to this:
First Name = Anthony Last Name = Robbins Age = (null)
Note
"Age"
is not just empty,
but totally missing.
If you want to enumerate all keys with their objects inside a
dictionary, you can simply use the enumerateKeysAndObjectsUsingBlock:
method of
the dictionary. In the previous array, the method would print the
"First Name"
and "Last Name"
elements, but not "Age"
, because we removed it. The parameter
to this method is a block object with no return value and three
parameters:
- Key
An
id
that tells you which key is being enumerated at the moment.- Object
An
id
as well, that gives you the object associated with the key being currently enumerated.- A pointer to a value of type
BOOL
At any point during the enumeration, if you want to stop the process, you can simply put the value
YES
into this pointer’s memory address. Keep it untouched if you want to enumerate through all the keys in the dictionary.
Let’s see an example:
NSNumber *age = [NSNumber numberWithUnsignedInteger:51]; NSDictionary *person = [[NSDictionary alloc] initWithObjectsAndKeys: @"Anthony", @"First Name", @"Robbins", @"Last Name", age, @"Age", nil]; [person enumerateKeysAndObjectsUsingBlock: ^(__strong id key, __strong id obj, BOOL *stop) { NSLog(@"Key = %@, Object For Key = %@", key, obj); }];
And the results, which get printed to the console window, are shown here:
Key = Last Name, Object For Key = Robbins Key = First Name, Object For Key = Anthony Key = Age, Object For Key = 51
If you want to do a manual fast enumeration without block
objects, you can use the allKeys
method of the dictionary to go through all methods and, once you
enumerate the keys, use the keys to find the objects associated with
the keys using the objectForKey:
method, like so:
for (id keyInDictionary in [person allKeys]){ id objectForKey = [person objectForKey:keyInDictionary]; NSLog(@"Key = %@, Object For Key = %@", keyInDictionary, objectForKey); }
Bear in mind that you can traverse the keys in a dictionary in
various ways. We’ve just seen two ways of doing this. There is another
method that we can use: calling the keyEnumerator
method of the dictionary to
get an object of type NSEnumerator
.
Here is an example:
NSEnumerator *keys = [person keyEnumerator]; id keyInDictionary = nil; while ((keyInDictionary = [keys nextObject]) != nil){ id objectForKey = [person objectForKey:keyInDictionary]; NSLog(@"Key = %@, Object For Key = %@", keyInDictionary, objectForKey); }
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