下面是如何声明一个字符串数组并遍历它们:

NSArray *langs = @[@"es", @"en", @"pt", @"it", @"fr"];

for (int i = 0; i < [langs count]; i++) {
  NSString *lang = (NSString*) [langs objectAtIndex:i];
  NSLog(@"%@, ",lang);
}

适用于OS X 10.4。X和之前:

 int i;
 for (i = 0; i < [myArray count]; i++) {
   id myArrayElement = [myArray objectAtIndex:i];
   ...do something useful with myArrayElement
 }

适用于OS X 10.5。x(或iPhone)及其他版本:

for (id myArrayElement in myArray) {
   ...do something useful with myArrayElement
}

10.5+/iOS通常首选的代码。

for (id object in array) {
    // do something with object
}

此构造用于枚举符合NSFastEnumeration协议的集合中的对象。这种方法具有速度优势，因为它将指向多个对象的指针(通过单个方法调用获得)存储在缓冲区中，并使用指针算术在缓冲区中遍历它们。这比每次在循环中调用-objectAtIndex要快得多。

同样值得注意的是，虽然技术上可以使用for-in循环遍历NSEnumerator，但我发现这实际上抵消了快速枚举的所有速度优势。原因是默认的NSEnumerator实现-countByEnumeratingWithState:objects:count:每次调用时只在缓冲区中放置一个对象。

我在radar://6296108中报告了这个(NSEnumerators的快速枚举是缓慢的)，但它被返回为Not To Be Fixed。原因是快速枚举预取一组对象，如果你只想枚举枚举器中的一个给定点(例如，直到找到一个特定的对象，或满足条件)，并在跳出循环后使用相同的枚举器，通常情况下会跳过几个对象。

如果你正在为OS X 10.6 / iOS 4.0及以上版本编码，你也可以选择使用基于块的api来枚举数组和其他集合:

[array enumerateObjectsUsingBlock:^(id object, NSUInteger idx, BOOL *stop) {
    // do something with object
}];

你也可以使用-enumerateObjectsWithOptions:usingBlock:并传递NSEnumerationConcurrent和/或NSEnumerationReverse作为选项参数。

---

10.4或更早版本

10.5之前的标准习惯用法是使用NSEnumerator和while循环，如下所示:

NSEnumerator *e = [array objectEnumerator];
id object;
while (object = [e nextObject]) {
  // do something with object
}

我建议保持简单。将自己绑定到数组类型上是不灵活的，而且无论如何，使用-objectAtIndex:所声称的速度提高对于在10.5+上使用快速枚举的改进来说是微不足道的。(快速枚举实际上在底层数据结构上使用指针算术，并消除了大部分方法调用开销。)过早的优化从来都不是一个好主意——它会导致更混乱的代码来解决问题，而不是你的瓶颈。

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! :-)

---

这三种方法是:

        //NSArray
    NSArray *arrData = @[@1,@2,@3,@4];

    // 1.Classical
    for (int i=0; i< [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
    }];

三是最快的执行方式。有了自动补全功能，就不用写迭代信封了。

测试结果和源代码如下(您可以在应用程序中设置迭代次数)。时间以毫秒为单位，每个条目是运行测试5-10次的平均结果。我发现它通常精确到2-3位有效数字，之后它会随着每次运行而变化。这使得误差范围小于1%。测试在iPhone 3G上运行，因为这是我感兴趣的目标平台。

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.

在这个测试中，我做了一个非常简单的随机访问性能比较C float*和NSNumbers的NSArray

我创建了一个简单的循环来总结每个数组的内容，并使用mach_absolute_time()计时。NSMutableArray平均需要400倍的时间!!(不是400%，只是400倍!长了40000% !)

标题:

/ / Array_Speed_TestViewController.h

//阵列速度测试

//由Mehmet Akten于05/02/2009创作。

//版权所有MSA视觉有限公司2009。版权所有。

#import <UIKit/UIKit.h>

@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

实现:

/ / Array_Speed_TestViewController.m

//阵列速度测试

//由Mehmet Akten于05/02/2009创作。

//版权所有MSA视觉有限公司2009。版权所有。

    #import "Array_Speed_TestViewController.h"
    #include <mach/mach.h>
    #include <mach/mach_time.h>

 @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(&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<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<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<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

来自:memo.tv

////////////////////

自块引入以来一直可用，这允许迭代具有块的数组。它的语法不像快速枚举那样好，但有一个非常有趣的特性:并发枚举。如果枚举顺序不重要，并且作业可以并行完成而不需要锁定，那么这可以在多核系统上提供相当大的加速。在并发枚举一节中详细介绍。

[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

快速枚举背后的思想是使用快速C数组访问来优化迭代。它不仅要比传统的NSEnumerator更快，而且Objective-C 2.0还提供了非常简洁的语法。

id object;
for (object in myArray) {
    [self doSomethingWith:object];
}

/////////////////

NSEnumerator

这是外部迭代的一种形式:[myArray objectEnumerator]返回一个对象。这个对象有一个方法nextObject，我们可以在循环中调用它，直到它返回nil

NSEnumerator *enumerator = [myArray objectEnumerator];
id object;
while (object = [enumerator nextObject]) {
    [self doSomethingWith:object];
}

/////////////////

, objectAtIndex:枚举

使用一个递增整数的for循环并使用[myArray objectAtIndex:index]查询对象是最基本的枚举形式。

NSUInteger count = [myArray count];
for (NSUInteger index = 0; index < count ; index++) {
    [self doSomethingWith:[myArray objectAtIndex:index]];
}

////////////// 来自:darkdust.net

下面是如何声明一个字符串数组并遍历它们:

NSArray *langs = @[@"es", @"en", @"pt", @"it", @"fr"];

for (int i = 0; i < [langs count]; i++) {
  NSString *lang = (NSString*) [langs objectAtIndex:i];
  NSLog(@"%@, ",lang);
}