How do I iterate over an NSArray

Question

I m looking for the standard idiom to iterate over an NSArray  My code needs to be suitable for OS X 10 4

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Do this  -  for  id object in array                statement

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The three ways are              NSArray     NSArray  arrData      1  2  3  4           1 Classical     for  int i 0  i lt   arrData count   i             NSLog     d      i arrData i                  2 Fast iteration     for  id element in arrData           NSLog       element                 3 Blocks      arrData enumerateObjectsUsingBlock   id obj  NSUInteger idx  BOOL  stop             NSLog     lu      idx obj               Set stop to YES in case you want to break the iteration            Is the fastest way in execution  and 3  with autocompletion forget about writing iteration envelope

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The generally-preferred code for 10 5  iOS   for  id object in array           do something with object     This construct is used to enumerate objects in a collection which conforms to the NSFastEnumeration protocol  This approach has a speed advantage because it stores pointers to several objects  obtained via a single method call  in a buffer and iterates through them by advancing through the buffer using pointer arithmetic  This is much faster than calling -objectAtIndex  each time through the loop   It s also worth noting that while you technically can use a for-in loop to step through an NSEnumerator  I have found that this nullifies virtually all of the speed advantage of fast enumeration  The reason is that the default NSEnumerator implementation of -countByEnumeratingWithState objects count  places only one object in the buffer on each call   I reported this in radar   6296108  Fast enumeration of NSEnumerators is sluggish  but it was returned as Not To Be Fixed  The reason is that fast enumeration pre-fetches a group of objects  and if you want to enumerate only to a given point in the enumerator  e g  until a particular object is found  or condition is met  and use the same enumerator after breaking out of the loop  it would often be the case that several objects would be skipped   If you are coding for OS X 10 6   iOS 4 0 and above  you also have the option of using block-based APIs to enumerate arrays and other collections    array enumerateObjectsUsingBlock   id object  NSUInteger idx  BOOL  stop           do something with object       You can also use -enumerateObjectsWithOptions usingBlock  and pass NSEnumerationConcurrent and or NSEnumerationReverse as the options argument     10 4 or earlier  The standard idiom for pre-10 5 is to use an NSEnumerator and a while loop  like so   NSEnumerator  e    array objectEnumerator   id object  while  object    e nextObject          do something with object     I recommend keeping it simple  Tying yourself to an array type is inflexible  and the purported speed increase of using -objectAtIndex  is insignificant to the improvement with fast enumeration on 10 5  anyway   Fast enumeration actually uses pointer arithmetic on the underlying data structure  and removes most of the method call overhead   Premature optimization is never a good idea     it results in messier code to solve a problem that isn t your bottleneck anyway   When using -objectEnumerator  you very easily change to another enumerable collection  like an NSSet  keys in an NSDictionary  etc    or even switch to -reverseObjectEnumerator to enumerate an array backwards  all with no other code changes  If the iteration code is in a method  you could even pass in any NSEnumerator and the code doesn t even have to care about what it s iterating  Further  an NSEnumerator  at least those provided by Apple code  retains the collection it s enumerating as long as there are more objects  so you don t have to worry about how long an autoreleased object will exist   Perhaps the biggest thing an NSEnumerator  or fast enumeration  protects you from is having a mutable collection  array or otherwise  change underneath you without your knowledge while you re enumerating it  If you access the objects by index  you can run into strange exceptions or off-by-one errors  often long after the problem has occurred  that can be horrific to debug  Enumeration using one of the standard idioms has a  fail-fast  behavior  so the problem  caused by incorrect code  will manifest itself immediately when you try to access the next object after the mutation has occurred  As programs get more complex and multi-threaded  or even depend on something that third-party code may modify  fragile enumeration code becomes increasingly problematic  Encapsulation and abstraction FTW    -

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Add each method in your NSArray category  you gonna need it a lot  Code taken from ObjectiveSugar  -  void each  void     id object  block        self enumerateObjectsUsingBlock   id obj  NSUInteger idx  BOOL  stop            block obj

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Here is how you declare an array of strings and iterate over them   NSArray  langs       es     en     pt     it     fr     for  int i   0  i  lt   langs count   i        NSString  lang    NSString    langs objectAtIndex i     NSLog         lang

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The results of the test and source code are below  you can set the number of iterations in the app   The time is in milliseconds  and each entry is an average result of running the test 5-10 times  I found that generally it is accurate to 2-3 significant digits and after that it would vary with each run  That gives a margin of error of less than 1   The test was running on an iPhone 3G as that s the target platform I was interested in   numberOfItems   NSArray  ms     C Array  ms     Ratio 100             0 39            0 0025          156 191             0 61            0 0028          218 3 256           12 5            0 026           481 4 789           16              0 037           432 6 794           21              0 050           420 10 919          36              0 081           444 19 731          64              0 15            427 22 030          75              0 162           463 32 758          109             0 24            454 77 969          258             0 57            453 100 000         390             0 73            534   The classes provided by Cocoa for handling data sets  NSDictionary  NSArray  NSSet etc   provide a very nice interface for managing information  without having to worry about the bureaucracy of memory management  reallocation etc  Of course this does come at a cost though  I think it s pretty obvious that say using an NSArray of NSNumbers is going to be slower than a C Array of floats for simple iterations  so I decided to do some tests  and the results were pretty shocking  I wasn t expecting it to be this bad  Note  these tests are conducted on an iPhone 3G as that s the target platform I was interested in   In this test I do a very simple random access performance comparison between a C float  and NSArray of NSNumbers  I create a simple loop to sum up the contents of each array and time them using mach absolute time    The NSMutableArray takes on average 400 times longer    not 400 percent  just 400 times longer  thats 40 000  longer     Header       Array Speed TestViewController h      Array Speed Test      Created by Mehmet Akten on 05 02 2009       Copyright MSA Visuals Ltd  2009  All rights reserved    import  lt UIKit UIKit h gt    interface Array Speed TestViewController   UIViewController        int                     numberOfItems              number of items in array      float                    cArray                    normal c array      NSMutableArray           nsArray                   ns array      double                  machTimerMillisMult        multiplier to convert mach absolute time   to milliseconds        IBOutlet    UISlider     sliderCount       IBOutlet    UILabel      labelCount        IBOutlet    UILabel      labelResults       - IBAction  doNSArray  id sender   - IBAction  doCArray  id sender   - IBAction  sliderChanged  id sender     end   Implementation       Array Speed TestViewController m      Array Speed Test      Created by Mehmet Akten on 05 02 2009       Copyright MSA Visuals Ltd  2009  All rights reserved        import  Array Speed TestViewController h       include  lt mach mach h gt       include  lt mach mach time h gt     implementation Array Speed TestViewController        Implement viewDidLoad to do additional setup after loading the view  typically from a nib   -  void viewDidLoad        NSLog   viewDidLoad           super viewDidLoad         cArray        NULL       nsArray       NULL           read initial slider value setup accordingly       self sliderChanged sliderCount            get mach timer unit size and calculater millisecond factor      mach timebase info data t info       mach timebase info  amp info        machTimerMillisMult    double info numer     double info denom   1000000 0        NSLog   machTimerMillisMult    f   machTimerMillisMult            pass in results of mach absolute time       this converts to milliseconds and outputs to the label  - void displayResult  uint64 t duration        double millis   duration   machTimerMillisMult        NSLog   displayResult   f milliseconds   millis         NSString  str     NSString alloc  initWithFormat    f milliseconds   millis         labelResults setText str         str release             process using NSArray  - IBAction  doNSArray  id sender        NSLog   doNSArray       sender         uint64 t startTime   mach absolute time         float total   0       for int i 0  i lt numberOfItems  i               total      nsArray objectAtIndex i  floatValue                self displayResult mach absolute time   - startTime             process using C Array  - IBAction  doCArray  id sender        NSLog   doCArray       sender         uint64 t start   mach absolute time         float total   0       for int i 0  i lt numberOfItems  i               total    cArray i                self displayResult mach absolute time   - start            allocate NSArray and C Array   - void  allocateArrays        NSLog   allocateArrays             allocate c array      if cArray  delete cArray       cArray   new float numberOfItems            allocate NSArray       nsArray release        nsArray     NSMutableArray alloc  initWithCapacity numberOfItems             fill with random values      for int i 0  i lt numberOfItems  i                  add number to c array          cArray i    random     1 0f  RAND MAX 1                add number to NSArray          NSNumber  number     NSNumber alloc  initWithFloat cArray i              nsArray addObject number             number release                    callback for when slider is changed  - IBAction  sliderChanged  id sender        numberOfItems   sliderCount value       NSLog   sliderChanged       i   sender  numberOfItems         NSString  str     NSString alloc  initWithFormat    i items   numberOfItems         labelCount setText str         str release          self allocateArrays           cleanup  -  void dealloc         nsArray release        if cArray  delete cArray         super dealloc         end   From   memo tv                        Available since the introduction of blocks  this allows to iterate an array with blocks  Its syntax isn t as nice as fast enumeration  but there is one very interesting feature  concurrent enumeration  If enumeration order is not important and the jobs can be done in parallel without locking  this can provide a considerable speedup on a multi-core system  More about that in the concurrent enumeration section    myArray enumerateObjectsUsingBlock   id object  NSUInteger index  BOOL  stop         self doSomethingWith object        myArray enumerateObjectsWithOptions NSEnumerationConcurrent usingBlock   id obj  NSUInteger idx  BOOL  stop         self doSomethingWith object                     NSFastEnumerator  The idea behind fast enumeration is to use fast C array access to optimize iteration  Not only is it supposed to be faster than traditional NSEnumerator  but Objective-C 2 0 also provides a very concise syntax   id object  for  object in myArray         self doSomethingWith object                          NSEnumerator  This is a form of external iteration   myArray objectEnumerator  returns an object  This object has a method nextObject that we can call in a loop until it returns nil  NSEnumerator  enumerator    myArray objectEnumerator   id object  while  object    enumerator nextObject          self doSomethingWith object                          objectAtIndex  enumeration  Using a for loop which increases an integer and querying the object using  myArray objectAtIndex index  is the most basic form of enumeration   NSUInteger count    myArray count   for  NSUInteger index   0  index  lt  count   index           self doSomethingWith  myArray objectAtIndex index                       From   darkdust net

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For OS X 10 4 x and previous    int i   for  i   0  i  lt   myArray count   i         id myArrayElement    myArray objectAtIndex i         do something useful with myArrayElement      For OS X 10 5 x  or iPhone  and beyond   for  id myArrayElement in myArray          do something useful with myArrayElement

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For Swift   let arrayNumbers    11  12  13  14  15  16  17  18  19  20      1 for  index  value  in arrayNumbers enumerated         print index  value            do somthing with array value and index       2 for value in arrayNumbers       print value            do somthing with array value

[objective-c] How do I iterate over an NSArray?

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