NSArray Equivalent of Map

Question

Given an NSArray of NSDictionary objects (containing similar objects and keys) is it possible to write perform a map to an array of specified key?

Answer 1

For Objective-C, I would add the Higher-Order-Functions to this list of answers: https://github.com/fanpyi/Higher-Order-Functions;

There is a JSON array studentJSONList like this:

[
    {"number":"100366","name":"Alice","age":14,"score":80,"gender":"female"},
    {"number":"100368","name":"Scarlett","age":15,"score":90,"gender":"female"},
    {"number":"100370","name":"Morgan","age":16,"score":69.5,"gender":"male"},
    {"number":"100359","name":"Taylor","age":14,"score":86,"gender":"female"},
    {"number":"100381","name":"John","age":17,"score":72,"gender":"male"}
]
//studentJSONList map to NSArray<Student *>
NSArray *students = [studentJSONList map:^id(id obj) {
return [[Student alloc]initWithDictionary:obj];
}];

// use reduce to get average score
NSNumber *sum = [students reduce:@0 combine:^id(id accumulator, id item) {
Student *std = (Student *)item;
return @([accumulator floatValue] + std.score);
}];
float averageScore = sum.floatValue/students.count;

// use filter to find all student of score greater than 70
NSArray *greaterthan = [students filter:^BOOL(id obj) {
Student *std = (Student *)obj;
return std.score > 70;
}];

//use contains check students whether contain the student named 'Alice'
BOOL contains = [students contains:^BOOL(id obj) {
Student *std = (Student *)obj;
return [std.name isEqual:@"Alice"];
}];

Answer 2

For Objective-C, I would add the ObjectiveSugar library to this list of answers: https://github.com/supermarin/ObjectiveSugar

Plus, its tagline is "ObjectiveC additions for humans. Ruby style." which should suit OP well ;-)

My most common use-case is mapping an dictionary returned by a server call to an array of simpler objects e.g. getting an NSArray of NSString IDs from your NSDictionary posts:

NSArray *postIds = [results map:^NSString*(NSDictionary* post) {
                       return [post objectForKey:@"post_id"];
                   }];

Answer 3

Update: If you're using Swift, see map.

---

BlocksKit is an option:

NSArray *new = [stringArray bk_map:^id(NSString *obj) { 
    return [obj stringByAppendingString:@".png"]; 
}];

---

Underscore is another option. There is a map function, here is an example from the website:

NSArray *tweets = Underscore.array(results)
    // Let's make sure that we only operate on NSDictionaries, you never
    // know with these APIs ;-)
    .filter(Underscore.isDictionary)
    // Remove all tweets that are in English
    .reject(^BOOL (NSDictionary *tweet) {
        return [tweet[@"iso_language_code"] isEqualToString:@"en"];
    })
    // Create a simple string representation for every tweet
    .map(^NSString *(NSDictionary *tweet) {
        NSString *name = tweet[@"from_user_name"];
        NSString *text = tweet[@"text"];

        return [NSString stringWithFormat:@"%@: %@", name, text];
    })
    .unwrap;

Answer 4

I've no idea what that bit of Ruby does but I think you are looking for NSArray's implementation of -valueForKey:. This sends -valueForKey: to every element of the array and returns an array of the results. If the elements in the receiving array are NSDictionaries, -valueForKey: is nearly the same as -objectForKey:. It will work as long as the key doesn't start with an @

Answer 5

Swift introduces a new map function.

Here is an example from the documentation:

let digitNames = [
    0: "Zero", 1: "One", 2: "Two",   3: "Three", 4: "Four",
    5: "Five", 6: "Six", 7: "Seven", 8: "Eight", 9: "Nine"
]
let numbers = [16, 58, 510]

let strings = numbers.map {
    (var number) -> String in
    var output = ""
    while number > 0 {
        output = digitNames[number % 10]! + output
        number /= 10
    }
    return output
}
// strings is inferred to be of type String[]
// its value is ["OneSix", "FiveEight", "FiveOneZero"]

The map function takes a closure which returns a value of any type and maps the existing values in the array to instances of this new type.

Answer 6

I think valueForKeyPath is a good choice.

Sit below has very cool examples. Hopes it is helpful.

http://kickingbear.com/blog/archives/9

Some example:

NSArray *names = [allEmployees valueForKeyPath: @"[collect].{daysOff<10}.name"];
NSArray *albumCovers = [records valueForKeyPath:@"[collect].{artist like 'Bon Iver'}.<NSUnarchiveFromDataTransformerName>.albumCoverImageData"];