How to add to an NSDictionary

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];

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

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"];