How to add to an NSDictionary

Question

I was using a NSMutableArray and realized that using a dictionary is a lot simpler for what I am trying to achieve.

I want to save a key as a NSString and a value as an int in the dictionary. How is this done? Secondly, what is the difference between mutable and a normal dictionary?

Answer 1

A mutable dictionary can be changed, i.e. you can add and remove objects. An immutable is fixed once it is created.

create and add:

NSMutableDictionary *dict = [[NSMutableDictionary alloc]initWithCapacity:10];
[dict setObject:[NSNumber numberWithInt:42] forKey:@"A cool number"];

and retrieve:

int myNumber = [[dict objectForKey:@"A cool number"] intValue];

Answer 2

By setting you'd use setValue:(id)value forKey:(id)key method of NSMutableDictionary object:

NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
[dict setValue:[NSNumber numberWithInt:5] forKey:@"age"];

Or in modern Objective-C:

NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
dict[@"age"] = @5;

The difference between mutable and "normal" is, well, mutability. I.e. you can alter the contents of NSMutableDictionary (and NSMutableArray) while you can't do that with "normal" NSDictionary and NSArray

Answer 3

When ever the array is declared, then only we have to add the key-value's in NSDictionary like

NSDictionary *normalDict = [[NSDictionary alloc]initWithObjectsAndKeys:@"Value1",@"Key1",@"Value2",@"Key2",@"Value3",@"Key3",nil];

we cannot add or remove the key values in this NSDictionary

Where as in NSMutableDictionary we can add the objects after intialization of array also by using this method

NSMutableDictionary *mutableDict = [[NSMutableDictionary alloc]init];'
[mutableDict setObject:@"Value1" forKey:@"Key1"];
[mutableDict setObject:@"Value2" forKey:@"Key2"];
[mutableDict setObject:@"Value3" forKey:@"Key3"];

for removing the key value we have to use the following code

[mutableDict removeObject:@"Value1" forKey:@"Key1"];

Answer 4

Update version

Objective-C

Create:

NSDictionary *dictionary = @{@"myKey1": @7, @"myKey2": @5}; 

Change:

NSMutableDictionary *mutableDictionary = [dictionary mutableCopy];     //Make the dictionary mutable to change/add
mutableDictionary[@"myKey3"] = @3;

The short-hand syntax is called Objective-C Literals.

Swift

Create:

var dictionary = ["myKey1": 7, "myKey2": 5]

Change:

dictionary["myKey3"] = 3

Answer 5

You want to ask is "what is the difference between a mutable and a non-mutable array or dictionary." Many times there different terms are used to describe things that you already know about. In this case, you can replace the term "mutable" with "dynamic." So, a mutuable dictionary or array is one that is "dynamic" and can change at runtime, whereas a non-mutable dictionary or array is one that is "static" and defined in your code and does not change at runtime (in other words, you will not be adding, deleting or possibly sorting the elements.)

As to how it is done, you are asking us to repeat the documentation here. All you need to do is to search in sample code and the Xcode documentation to see exactly how it is done. But the mutable thing threw me too when I was first learning, so I'll give you that one!

Answer 6

For reference, you can also utilize initWithDictionary to init the NSMutableDictionary with a literal one:

NSMutableDictionary buttons = [[NSMutableDictionary alloc] initWithDictionary: @{
    @"touch": @0,
    @"app": @0,
    @"back": @0,
    @"volup": @0,
    @"voldown": @0
}];