How can we store into an NSDictionary? What is the difference between NSDictionary and NSMutableDictionary?

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The NSDictionary and NSMutableDictionary docs are probably your best bet. They even have some great examples on how to do various things, like…

…create an NSDictionary

NSArray *keys = [NSArray arrayWithObjects:@"key1", @"key2", nil];
NSArray *objects = [NSArray arrayWithObjects:@"value1", @"value2", nil];
NSDictionary *dictionary = [NSDictionary dictionaryWithObjects:objects 
                                                       forKeys:keys];

…iterate over it

for (id key in dictionary) {
    NSLog(@"key: %@, value: %@", key, [dictionary objectForKey:key]);
}

…make it mutable

NSMutableDictionary *mutableDict = [dictionary mutableCopy];

Note: historic version before 2010: [[dictionary mutableCopy] autorelease]

…and alter it

[mutableDict setObject:@"value3" forKey:@"key3"];

…then store it to a file

[mutableDict writeToFile:@"path/to/file" atomically:YES];

…and read it back again

NSMutableDictionary *anotherDict = [NSMutableDictionary dictionaryWithContentsOfFile:@"path/to/file"];

…read a value

NSString *x = [anotherDict objectForKey:@"key1"];

…check if a key exists

if ( [anotherDict objectForKey:@"key999"] == nil ) NSLog(@"that key is not there");

…use scary futuristic syntax

From 2014 you can actually just type dict[@”key”] rather than [dict objectForKey:@”key”]

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