NSDictionary

Syntax#

@{ key: value, … }
[NSDictionary dictionaryWithObjectsAndKeys: value, key, …, nil];
dict[key] = value;
id value = dict[key];

Remarks#

The NSDictionary class declares the programmatic interface to objects that manage immutable associations of keys and values. Use this class or its subclass NSMutableDictionary when you need a convenient and efficient way to retrieve data associated with an arbitrary key. NSDictionary creates static dictionaries, and NSMutableDictionary creates dynamic dictionaries. (For convenience, the term dictionary refers to any instance of one of these classes without specifying its exact class membership.)

A key-value pair within a dictionary is called an entry. Each entry consists of one object that represents the key and a second object that is that key’s value. Within a dictionary, the keys are unique. That is, no two keys in a single dictionary are equal (as determined by isEqual:). In general, a key can be any object (provided that it conforms to the NSCopying protocol—see below), but note that when using key-value coding the key must be a string (see Key-Value Coding Fundamentals). Neither a key nor a value can be nil; if you need to represent a null value in a dictionary, you should use NSNull.

NSDictionary is “toll-free bridged” with its Core Foundation counterpart, CFDictionaryRef. See Toll-Free Bridging for more information on toll-free bridging.

Creating using literals

NSDictionary *inventory = @{
    @"Mercedes-Benz SLK250" : @(13),
    @"BMW M3 Coupe" : @(self.BMWM3CoupeInventory.count),
    @"Last Updated" : @"Jul 21, 2016",
    @"Next Update"  : self.nextInventoryUpdateString
};

Creating using dictionaryWithObjectsAndKeys:

NSDictionary *inventory = [NSDictionary dictionaryWithObjectsAndKeys:
    [NSNumber numberWithInt:13], @"Mercedes-Benz SLK250",
    [NSNumber numberWithInt:22], @"Mercedes-Benz E350",
    [NSNumber numberWithInt:19], @"BMW M3 Coupe",
    [NSNumber numberWithInt:16], @"BMW X6",
    nil];

nil must be passed as the last parameter as a sentinel signifying the end.

It’s important to remember that when instantiating dictionaries this way the values go first and the keys second. In the example above the strings are the keys and the numbers are the values. The method’s name reflects this too: dictionaryWithObjectsAndKeys . While this is not incorrect, the more modern way of instantiating dictionaries (with literals) is prefered.

Creating using plists

NSString *pathToPlist = [[NSBundle mainBundle] pathForResource:@"plistName" 
    ofType:@"plist"];
NSDictionary *plistDict = [[NSDictionary alloc] initWithContentsOfFile:pathToPlist];

Setting a Value in NSDictionary

There are multiple ways to set a key’s object in an NSDictionary, corresponding to the ways you get a value. For instance, to add a lamborghini to a list of cars

Standard

[cars setObject:lamborghini forKey:@"Lamborghini"];

Just like any other object, call the method of NSDictionary that sets an object of a key, objectForKey:. Be careful not to confuse this with setValue:forKey:; that’s for a completely different thing, Key Value Coding

Shorthand

cars[@"Lamborghini"] = lamborghini;

This is the syntax that you use for dictionaries in most other languages, such as C#, Java, and Javascript. It’s much more convenient than the standard syntax, and arguably more readable (especially if you code in these other languages), but of course, it isn’t standard. It’s also only available in newer versions of Objective C

Getting a Value from NSDictionary

There are multiple ways to get an object from an NSDictionary with a key. For instance, to get a lamborghini from a list of cars

Standard

Car * lamborghini = [cars objectForKey:@"Lamborghini"];

Just like any other object, call the method of NSDictionary that gives you an object for a key, objectForKey:. Be careful not to confuse this with valueForKey:; that’s for a completely different thing, Key Value Coding

Shorthand

Car * lamborghini = cars[@"Lamborghini"];

Check if NSDictionary already has a key or not

Objective c:

   //this is the dictionary you start with. 
 NSDictionary *dict = [NSDictionary dictionaryWithObjectsAndKeys:@"name1", @"Sam",@"name2", @"Sanju",nil];

//check if the dictionary contains the key you are going to modify.  In this example, @"Sam"
if (dict[@"name1"] != nil) {
    //there is an entry for Key name1       
}
else {
    //There is no entry for name1       
}