NSArray Equivalent of Map - iphone

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? For example, in Ruby it can be done with:
array.map(&:name)

It only saves a couple lines, but I use a category on NSArray. You need to ensure your block never returns nil, but other than that it's a time saver for cases where -[NSArray valueForKey:] won't work.
#interface NSArray (Map)
- (NSArray *)mapObjectsUsingBlock:(id (^)(id obj, NSUInteger idx))block;
#end
#implementation NSArray (Map)
- (NSArray *)mapObjectsUsingBlock:(id (^)(id obj, NSUInteger idx))block {
NSMutableArray *result = [NSMutableArray arrayWithCapacity:[self count]];
[self enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
[result addObject:block(obj, idx)];
}];
return result;
}
#end
Usage is much like -[NSArray enumerateObjectsWithBlock:]:
NSArray *people = #[
#{ #"name": #"Bob", #"city": #"Boston" },
#{ #"name": #"Rob", #"city": #"Cambridge" },
#{ #"name": #"Robert", #"city": #"Somerville" }
];
// per the original question
NSArray *names = [people mapObjectsUsingBlock:^(id obj, NSUInteger idx) {
return obj[#"name"];
}];
// (Bob, Rob, Robert)
// you can do just about anything in a block
NSArray *fancyNames = [people mapObjectsUsingBlock:^(id obj, NSUInteger idx) {
return [NSString stringWithFormat:#"%# of %#", obj[#"name"], obj[#"city"]];
}];
// (Bob of Boston, Rob of Cambridge, Robert of Somerville)

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 #

To summarize all other answers:
Ruby (as in the question):
array.map{|o| o.name}
Obj-C (with valueForKey):
[array valueForKey:#"name"];
Obj-C (with valueForKeyPath, see KVC Collection Operators):
[array valueForKeyPath:#"[collect].name"];
Obj-C (with enumerateObjectsUsingBlock):
NSMutableArray *newArray = [NSMutableArray array];
[array enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
[newArray addObject:[obj name]];
}];
Swift (with map, see closures)
array.map { $0.name }
And, there are a couple of libraries that allow you to handle arrays in a more functional way. CocoaPods is recommended to install other libraries.

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;

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"];

I'm no Ruby expert so I'm not 100% confident I'm answering correctly, but based on the interpretation that 'map' does something to everything in the array and produces a new array with the results, I think what you probably want is something like:
NSMutableArray *replacementArray = [NSMutableArray array];
[existingArray enumerateObjectsUsingBlock:
^(NSDictionary *dictionary, NSUInteger idx, BOOL *stop)
{
NewObjectType *newObject = [something created from 'dictionary' somehow];
[replacementArray addObject:newObject];
}
];
So you're using the new support for 'blocks' (which are closures in more general parlance) in OS X 10.6/iOS 4.0 to perform the stuff in the block on everything in the array. You're choosing to do some operation and then add the result to a separate array.
If you're looking to support 10.5 or iOS 3.x, you probably want to look into putting the relevant code into the object and using makeObjectsPerformSelector: or, at worst, doing a manual iteration of the array using for(NSDictionary *dictionary in existingArray).

#implementation NSArray (BlockRockinBeats)
- (NSArray*)mappedWithBlock:(id (^)(id obj, NSUInteger idx))block {
NSMutableArray* result = [NSMutableArray arrayWithCapacity:self.count];
[self enumerateObjectsUsingBlock:^(id currentObject, NSUInteger index, BOOL *stop) {
id mappedCurrentObject = block(currentObject, index);
if (mappedCurrentObject)
{
[result addObject:mappedCurrentObject];
}
}];
return result;
}
#end
A slight improvement upon a couple of the answers posted.
Checks for nil—you can use nil to remove objects as you're mapping
Method name better reflects that the method doesn't modify the array it's called on
This is more a style thing but I've IMO improved the argument names of the block
Dot syntax for count

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"];
}];

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"];
}];

There is a special key-path operator for this: #unionOfObjects. Probably it replaced [collect] from previous versions.
Imagine a Transaction class with payee property:
NSArray *payees = [self.transactions valueForKeyPath:#"#unionOfObjects.payee"];
Apple docs on Array Operators in Key-Value coding.

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.

Related

error in array cleaning method

I am attempting to use this array cleaning method, and there seems to be an error. I can't spot it, I know the array goes in with 3116 items, comes out with 3116 (and I know for a fact there are three duplicates.
Please advice, thanks!
-(NSArray*) removeDuplicates:(NSArray*)inputArray{
NSMutableArray *arrayToClean = [NSMutableArray arrayWithArray:inputArray];
for (int i =0; i<[arrayToClean count]; i++) {
for (int j=(i+1); j < [arrayToClean count]; j++) {
if ([[arrayToClean objectAtIndex:i] isEqual:[arrayToClean
objectAtIndex:j]]) {
[arrayToClean removeObjectAtIndex:j];
j--;
}
}
}
NSArray *arrayToReturn = [NSArray arrayWithArray:arrayToClean];
return arrayToReturn;
}
NSSet will make this a lot easier:
-(NSArray *)removeDuplicates:(NSArray *)inputArray {
NSSet *unique = [NSSet setWithArray:inputArray];
return [unique allObjects];
}
Please note that a set has no guaranteed order. If you need the objects in the array to be in a specific order then you should sort the resulting array as needed.
It may also be appropriate to use an NSSet instead of the original array, then you don't need to worry about duplicates at all. But this depends on the other needs of your array.
Hey You can use another alternative for this.You can use the NSSet here for this task.
NSSet declares the programmatic interface for static sets of distinct objects
You can use sets as an alternative to arrays when the order of elements isn’t important and performance in testing whether an object is contained in the set is a consideration—while arrays are ordered, testing for membership is slower than with sets.
You Just need To call below method.
-(NSArray *)removeDuplicates:(NSArray *)inputArray {
NSSet *finalData = [NSSet setWithArray:inputArray];
return [finalData allObjects];
}
If really face any problem in above way of cleaning ducplicates then you can try another Alterantive.
-(NSArray *)removeDuplicates:(NSArray *)inputArray {
NSMutableArray *inputArray1=[NSMutableArray arrayWithArray:inputArray];
NSMutableArray *finalARray=[[NSMutableArray alloc]init];
for (id obj in inputArray1)
{
if (![finalARray containsObject:obj])
{
[finalARray addObject: obj];
}
NSLog(#"new array is %#",finalARray);
}
return finalARray;
}
I hope it may help you ...
Here is a helper function I had in a previous project to do the exact same thing
- (NSMutableArray *)removeDuplicates:(NSMutableArray *)sortedArray{
NSMutableSet* valuesAdded = [NSMutableSet set];
NSMutableArray* filteredArray = [[NSMutableArray alloc] init];
NSString* object;
/* Iterate over the array checking if the value is a member of the set. If its not add it
* to the set and to the returning array. If the value is already a member, skip over it.
*/
for (object in sortedArray){
if (![valuesAdded member:object]){
[valuesAdded addObject:object];
[filteredArray addObject:object];
}
}
return filteredArray;
}

Check two NSArrays for containing each other's objects (NSManagedObject)

I'm stuck at following problem for quite some time now:
I've got two NSArrays, both containing NSManagedObject subclass-objects.
They're fed by different sources but the objects in them still have the same properties/values.
What I want to do now is check if array A contains objects from array B and vice versa.
Unfortunately NSArray's containsObject-method doesn't seem to work here.
I think it uses id-testing for the equality check on each object, doesn't it?
So, does anybody have a clue, what to try?
I even tried to encapsulate my objects in NSSets, using member: as my comparison-method but this didn't work out as well, especially because "you must not override" isEqual etc. for NSManagedObject subclasses.
Here's a code snippet:
//manufacturers is an array, parsed out of some xml here...
for(Manufacturer *manu in [fetchedResultsController fetchedObjects])
{
if(![manufacturers containsObject:manu])
{
NSLog(#"Deleting %#", manu.name);
[self.mContext deleteObject:manu];
}
}
for(Manufacturer *manu in manufacturers)
{
if(![[fetchedResultsController fetchedObjects] containsObject:manu])
{
NSLog(#"Adding %#", manu.name);
[newArray addObject:manu];
}
}
Thanks in advance for any hint ;)
I'm not sure if this works, but you could try to match the dictionaries you get with dictionaryWithValuesForKeys:.
Something like this:
NSArray *keysToCompare = [NSArray arrayWithObjects:#"FooAttribute", #"BarAttribute", nil];
// create an array with the dictionary representation of the managedObject
NSMutableArray *fetchedObjectsDictionaries = [NSMutableArray arrayWithCapacity:[[fetchedResultsController fetchedObjects] count]];
for (NSManagedObject *object in [fetchedResultsController fetchedObjects]) {
NSDictionary *dictionaryRepresentation = [object dictionaryWithValuesForKeys:keysToCompare];
[fetchedObjectsDictionaries addObject:dictionaryRepresentation];
}
// another array with dictionaries for managedObjects
NSMutableArray *manufacturersDictionaries = [NSMutableArray arrayWithCapacity:[manufacturers count]];
for (NSManagedObject *object in manufacturers) {
NSDictionary *dictionaryRepresentation = [object dictionaryWithValuesForKeys:keysToCompare];
[manufacturersDictionaries addObject:dictionaryRepresentation];
}
// compare those dictionaries
for (NSInteger i = 0; i < [fetchedObjectsDictionaries count]; i++) {
NSDictionary *dictionary = [fetchedObjectsDictionaries objectAtIndex:i];
if (![manufacturersDictionaries containsObject:dictionary]) {
// get the corresponding managedObject
NSManagedObject *object = [[fetchedResultsController fetchedObjects] objectAtIndex:i];
[newArray addObject:object];
}
}
if that won't work you can write your own isEqualToManufacturer: method and enumerate trough the arrays manually.
There would be 3 types of equality you can check for: same memory address, managed object id equality, and value equality. Your current code already checks to see if the objects share the same memory address and this is most likely not what you are interested in. This leaves two possible options. Using the managed object id equality method you can check if the manufacturers point to the same row in the database. Using the value equality you can check if two manufacturers are equal based on the shared values. Below is a way to check for NSManagedObjectID equality.
for(Manufacturer *manu in [fetchedResultsController fetchedObjects])
{
id databaseIDTest = ^(Manufacturer * checkManu, NSUInteger idx, BOOL *stop){
return [[checkManu objectID] isEqual:[manu objectID]];
};
if([manufacturers indexOfObjectPassingTest:databaseIDTest] == NSIndexNotFound)
{
NSLog(#"Deleting %#", manu.name);
[self.mContext deleteObject:manu];
}
}
for(Manufacturer *manu in manufacturers)
{
id databaseIDTest = ^(Manufacturer * checkManu, NSUInteger idx, BOOL *stop){
return [[checkManu objectID] isEqual:[manu objectID]];
};
NSArray * fetchedObjects = [fetchedResultsController fetchedObjects];
if([fetchedObjects indexOfObjectPassingTest:databaseIDTest] == NSIndexNotFound)
{
NSLog(#"Adding %#", manu.name);
[newArray addObject:manu];
}
}
You need to override -isEqual: since that's what -[NSArray containsObject:] calls into:
- (BOOL)isEqual:(id)other;
{
if (![other isKindOfClass:[Manufacturer class]]) {
return NO;
}
Manufacturer *otherManufacturer = other;
return ([self.name isEqual:otherManufacturer.name] &&
...
);
}
Checking for containment inside an NSSet is cheaper (and may make sense if you run into performance problems). It only works if you have a relatively decent -hash implementation, but it's easy to implement like this:
- (NSUInteger)hash;
{
return [self.name hash] + [self.foo hash] + ...;
}
Don't go trough too much trouble with the hash, just use 2 - 3 values that are most likely to uniquely identify the object.

How do I get the index of an object in an NSArray using string value?

I want to get the index of an object within the NSMutableArray of categories.
The category object has an attribute "category_title" and I want to be able to get the index by passing the value of category_title.
I have looked through the docs and can't find a simple way to go about this.
NSArray does not guarantee that you can only store one copy of a given object, so you have to make sure that you handle that yourself (or use NSOrderedSet).
That said, there are a couple approaches here. If your category objects implement isEqual: to match category_title, then you can just use -indexOfObject:.
If you can't do that (because the category objects use a different definition of equality), use -indexOfObjectPassingTest:. It takes a block in which you can do whatever test you want to define your "test" - in this case, testing category_title string equality.
Note that these are all declared for NSArray, so you won't see them if you are only looking at the NSMutableArray header/documentation.
EDIT: Code sample. This assumes objects of class CASCategory with an NSString property categoryTitle (I can't bring myself to put underscores in an ivar name :-):
CASCategory *cat1 = [[CASCategory alloc] init];
[cat1 setCategoryTitle:#"foo"];
CASCategory *cat2 = [[CASCategory alloc] init];
[cat2 setCategoryTitle:#"bar"];
CASCategory *cat3 = [[CASCategory alloc] init];
[cat3 setCategoryTitle:#"baz"];
NSMutableArray *array = [NSMutableArray arrayWithObjects:cat1, cat2, cat3, nil];
[cat1 release];
[cat2 release];
[cat3 release];
NSUInteger barIndex = [array indexOfObjectPassingTest:^BOOL(id obj, NSUInteger idx, BOOL *stop) {
if ([[(CASCategory *)obj categoryTitle] isEqualToString:#"bar"]) {
*stop = YES;
return YES;
}
return NO;
}];
if (barIndex != NSNotFound) {
NSLog(#"The title of category at index %lu is %#", barIndex, [[array objectAtIndex:barIndex] categoryTitle]);
}
else {
NSLog(#"Not found");
}
Not sure that I understand the question but something like this might work (assuming the Mutable Array contains objects of Class "Category"):
int indx;
bool chk;
for (Category *aCategory in theArray)
{
chk = ([[aCategory category_title] isEqualToString:#"valOfCategoryTitle"])
if ( chk )
indx = [theArray indexOfObject:aCategory];
}
Try this code much more simpler:-
int f = [yourArray indexOfObject:#"yourString"];

How do I find (not remove) duplicates in an NSDictionary of NSArrays?

The title pretty much says it all, but just to clarify: I have an NSMutableDictonary containing several NSMutableArrays. What I would like to do is find any value that is present in multiple arrays (there will not be any duplicates in a single array) and return that value. Can someone please help? Thanks in advance!
Edit: For clarity's sake I will specify some of my variables:
linesMutableDictionary contains a list of Line objects (which are a custom NSObject subclass of mine)
pointsArray is an array inside each Line object and contains the values I am trying to search through.
Basically I am trying to find out which lines share common points (the purpose of my app is geometry based)
- (NSValue*)checkForDupes:(NSMutableDictionary*)dict {
NSMutableArray *derp = [NSMutableArray array];
for (NSString *key in [dict allKeys]) {
Line *temp = (Line*)[dict objectForKey:key];
for (NSValue *val in [temp pointsArray]) {
if ([derp containsObject:val])
return val;
}
[derp addObjectsFromArray:[temp pointsArray]];
}
return nil;
}
this should work
If by duplicates you mean returning YES to isEqual: you could first make an NSSet of all the elements (NSSet cannot, by definition, have duplicates):
NSMutableSet* allElements = [[NSMutableSet alloc] init];
for (NSArray* array in [dictionary allValues]) {
[allElements addObjectsFromArray:array];
}
Now you loop through the elements and check if they are in multiple arrays
NSMutableSet* allDuplicateElements = [[NSMutableSet alloc] init];
for (NSObject* element in allElements) {
NSUInteger count = 0;
for (NSArray* array in [dictionary allValues]) {
if ([array containsObject:element]) count++;
if (count > 1) {
[allDuplicateElements addObject:element];
break;
}
}
}
Then you have your duplicate elements and don't forget to release allElements and allDuplicateElements.

Putting single attribute from group of entity objects into an array

If I have an NSArray of custom objects (in this case Core Data objects), how would I put all the items of a particular attribute in another NSArray. Is there a way I can use blocks?
For instance, if the class is People, and one of the attributes is age, and there are five objects in the array of people, then the final NSArray would just show the ages:
{ 12, 45, 23, 43, 32 }
Order is not important.
EDIT I have added a blocks based implementation too alongwith the selector based implementation.
What you are looking for is something equivalent to the "map" function from the functional world (something which, unfortunately, is not supported by Cocoa out of the box):
#interface NSArray (FunctionalUtils)
- (NSArray *)mapWithSelector:(SEL)selector;
- (NSArray *)mapWithBlock:(id (^)(id obj))block;
#end
And the implementation:
#implementation NSArray (FunctionalUtils)
- (NSArray *)mapWithSelector:(SEL)selector {
NSMutableArray *result = [[NSMutableArray alloc] initWithCapacity:[self count]];
for (id object in self) {
[result addObject:[object performSelector:selector]];
}
return [result autorelease];
}
- (NSArray *)mapWithBlock:(id (^)(id obj))block {
NSMutableArray *result = [[NSMutableArray alloc] initWithCapacity:[self count]];
for (id object in self) {
[result addObject:block(object)];
}
return [result autorelease];
}
#end
Now, when you need to "map" from people to their ages, you can just do this:
ages = [people mapWithSelector:#selector(age)];
OR
ages = [people mapWithBlock:^(Person *p) { return [p age]; }];
The result, ages, will be a new NSArray containing just the ages of the people. In general, this will work for any sort of simple mapping operations that you might need.
One caveat: Since it returns an NSArray, the elements inside ages should be NSNumber, not a plain old integer. So for this to work, your -age selector should return an NSNumber, not an int or NSInteger.
Assuming that each object has a method called "age" that returns an NSNumber *, you should be able to do something like the following:
-(NSArray *)createAgeArray:(NSArray *)peopleArray {
NSMutableArray *result = [[NSMutableArray alloc] initWithCapacity:[peopleArray count]];
[peopleArray enumerateObjectsUsingBlock:^(id person, NSUInteger i, BOOL *stop) {
[result addObject:[person age]];
}];
return [result autorelease];
}