Question

I have NSDictionaries in NSArray just like below.

array(dictionary(“user”:1, “p1”:1), dictionary(“user”:2, “p1”:3),
dictionary(“user”:1, “p1”:5), dictionary(“user”:2, “p1”:7))

And I want to turn this array into dictionary like below.

NSArray *u1 = [NSArray arrayWithObjects:@"1", @"5", nil];
NSArray *u2 = [NSArray arrayWithObjects:@"3", @"7", nil];
keys = [NSArray arrayWithObjects:@"u1", @"u2", nil];
points = [NSDictionary dictionaryWithObjectsAndKeys:u1, @"u1", u2, @"u2", nil];

How can I do that? I am lost, can you guys please help me?

Answer 1

Another possible solution (works with an arbitrary number of users):

NSArray *orig = @[
    @{@"user" : @"1", @"p1" : @"1"},
    @{@"user" : @"2", @"p1" : @"3"},
    @{@"user" : @"1", @"p1" : @"5"},
    @{@"user" : @"2", @"p1" : @"7"},
];

// Create set of all users (without duplicates)
NSSet *users = [NSSet setWithArray:[orig valueForKey:@"user"]];

NSMutableDictionary *points = [NSMutableDictionary dictionary];
for (NSString *user in users) {
    // newKey = "u" + username, e.g. "u1" or "u2":
    NSString *newKey = [@"u" stringByAppendingString:user];

    // newValue = array of "p1" values of the current user:
    NSPredicate *pred = [NSPredicate predicateWithFormat:@"user == %@", user];
    NSArray *newValue = [[orig filteredArrayUsingPredicate:pred] valueForKey:@"p1"];

    // Add to dictionary:
    [points setObject:newValue forKey:newKey];
}
NSLog(@"%@", points);

Output:

{
    u1 =     (
        1,
        5
    );
    u2 =     (
        3,
        7
    );
}

And the keys can be obtained by

NSArray *keys = [points allKeys];