filtering NSArray into a new NSArray in Objective-C

Question

I have an NSArray and I d like to create a new NSArray with objects from the original array that meet certain criteria  The criteria is decided by a function that returns a BOOL   I can create an NSMutableArray  iterate through the source array and copy over the objects that the filter function accepts and then create an immutable version of it   Is there a better way

User · Accepted Answer

NSArray and NSMutableArray provide methods to filter array contents  NSArray provides filteredArrayUsingPredicate  which returns a new array containing objects in the receiver that match the specified predicate  NSMutableArray adds filterUsingPredicate  which evaluates the receiver   s content against the specified predicate and leaves only objects that match  These methods are illustrated in the following example   NSMutableArray  array        NSMutableArray arrayWithObjects   Bill     Ben     Chris     Melissa   nil    NSPredicate  bPredicate        NSPredicate predicateWithFormat   SELF beginswith c   b     NSArray  beginWithB        array filteredArrayUsingPredicate bPredicate      beginWithB contains     Bill     Ben      NSPredicate  sPredicate        NSPredicate predicateWithFormat   SELF contains c   s      array filteredArrayUsingPredicate sPredicate      array now contains     Chris     Melissa

User · Answer

Checkout this library  https   github com BadChoice Collection  It comes with lots of easy array functions to never write a loop again  So you can just do   NSArray  youngHeroes    self heroes filter  BOOL Hero  object        return object age intValue  lt  20        or  NSArray  oldHeroes    self heroes reject  BOOL Hero  object        return object age intValue  lt  20

User · Answer

The Best and easy Way is to create this method And Pass Array And Value   -  NSArray    filter  NSArray   array where  NSString   key is  id value      NSMutableArray  temArr   NSMutableArray alloc  init       for NSDictionary  dic in self          if  dic key  isEqual value                temArr addObject dic       return temArr

User · Answer

There are loads of ways to do this  but by far the neatest is surely using  NSPredicate predicateWithBlock    NSArray  filteredArray    array filteredArrayUsingPredicate  NSPredicate predicateWithBlock  BOOL id object  NSDictionary  bindings        return  object shouldIKeepYou       Return YES for each object you want in filteredArray        I think that s about as concise as it gets   Swift  For those working with NSArrays in Swift  you may prefer this even more concise version  let filteredArray   array filter    0 shouldIKeepYou      filter is just a method on Array  NSArray is implicitly bridged to Swift   s Array   It takes one argument  a closure that takes one object in the array and returns a Bool  In your closure  just return true for any objects you want in the filtered array

User · Answer

Another category method you could use  -  NSArray    filteredArrayUsingBlock  BOOL     id obj  block       NSIndexSet  const filteredIndexes    self indexesOfObjectsPassingTest  BOOL  id  Nonnull obj  NSUInteger idx  BOOL   Nonnull stop                                           return block obj                                               return  self objectsAtIndexes filteredIndexes

User · Answer

Assuming that your objects are all of a similar type you could add a method as a category of their base class that calls the function you re using for your criteria  Then create an NSPredicate object that refers to that method   In some category define your method that uses your function   implementation BaseClass  SomeCategory  -  BOOL myMethod       return someComparisonFunction self  whatever      end   Then wherever you ll be filtering   -  NSArray   myFilteredObjects       NSPredicate  pred    NSPredicate predicateWithFormat   myMethod   TRUE        return  myArray filteredArrayUsingPredicate pred       Of course  if your function only compares against properties reachable from within your class it may just be easier to convert the function s conditions to a predicate string

User · Answer

NSPredicate is nextstep s way of constructing  condition  to filter a collection  NSArray  NSSet  NSDictionary    For example consider two arrays arr and filteredarr   NSPredicate  predicate    NSPredicate predicateWithFormat   SELF contains c        c     filteredarr    NSMutableArray arrayWithArray  arr filteredArrayUsingPredicate predicate      the filteredarr will surely have the items that contains the character c alone   to make it easy to remember those who little sql background it is    --select   from tbl where column1 like   a  --    1 select   from tbl  --  collection  2 column1 like   a   --  NSPredicate  predicate    NSPredicate predicateWithFormat   SELF contains c        c     3 select   from tbl where column1 like   a   --    NSMutableArray arrayWithArray  arr filteredArrayUsingPredicate predicate      I hope this helps

User · Answer

Assuming that your objects are all of a similar type you could add a method as a category of their base class that calls the function you re using for your criteria  Then create an NSPredicate object that refers to that method   In some category define your method that uses your function   implementation BaseClass  SomeCategory  -  BOOL myMethod       return someComparisonFunction self  whatever      end   Then wherever you ll be filtering   -  NSArray   myFilteredObjects       NSPredicate  pred    NSPredicate predicateWithFormat   myMethod   TRUE        return  myArray filteredArrayUsingPredicate pred       Of course  if your function only compares against properties reachable from within your class it may just be easier to convert the function s conditions to a predicate string

User · Answer

If you are OS X 10 6 iOS 4 0 or later  you re probably better off with blocks than NSPredicate  See - NSArray indexesOfObjectsPassingTest   or write your own category to add a handy -select  or -filter  method  example    Want somebody else to write that category  test it  etc   Check out BlocksKit  array docs   And there are many more examples to be found by  say  searching for e g   nsarray block category select

User · Answer

Based on an answer by Clay Bridges  here is an example of filtering using blocks  change yourArray to your array variable name and testFunc to the name of your testing function    yourArray    yourArray objectsAtIndexes  yourArray indexesOfObjectsPassingTest  BOOL id obj  NSUInteger idx  BOOL  stop        return  self testFunc obj

[objective-c] filtering NSArray into a new NSArray in Objective-C

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