做app的时候，总免不了要多次遍历数组或者字典。

究竟哪种遍历方式比较快呢？我做了如下测试：

首先定义测试用宏：

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#define MULogTimeintervalBegin(INFO) NSTimeInterval start = [NSDate timeIntervalSinceReferenceDate];\ NSTimeInterval duration = 0;\ NSLog (@ "MULogTimeintervalBegin:%@" , INFO)   #define MULogTimeintervalPauseAndLog(INFO) duration = [NSDate timeIntervalSinceReferenceDate] - start;\ start += duration;\ NSLog (@ "%@:%f" , INFO, duration);\ duration = 0 #define TESTSCALE 100000

接着编写测试代码：

NSarray：

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- ( void )testArray {      NSMutableArray * testArray = [ NSMutableArray arrayWithCapacity:TESTSCALE];      for ( NSInteger i = 1; i <= TESTSCALE; ++i) {          [testArray addObject:[ NSString stringWithFormat:@ "%ld" , i]];      }      NSLog (@ "init:%ld" , [testArray count]);            __block NSMutableString * sum = [ NSMutableString stringWithCapacity:TESTSCALE];            MULogTimeintervalBegin(@ "ArrayTest" );      NSUInteger count = [testArray count];      for ( NSInteger i = 0; i < count; ++i) {          [sum appendString:[testArray objectAtIndex:i]];      }      [sum setString:@ "" ];      MULogTimeintervalPauseAndLog(@ "for statement" );            for ( NSString * item in testArray) {          [sum appendString:item];      }      [sum setString:@ "" ];      MULogTimeintervalPauseAndLog(@ "for-in" );            [testArray enumerateObjectsUsingBlock:^( id obj, NSUInteger idx, BOOL *stop) {          [sum appendString:obj];      }];      [sum setString:@ "" ];      MULogTimeintervalPauseAndLog(@ "enumerateBlock" ); }

NSDictionary：

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- ( void )testDictionary {      NSMutableDictionary * testDic = [ NSMutableDictionary dictionaryWithCapacity:TESTSCALE];      for ( NSInteger i = 1; i <= TESTSCALE; ++i) {          [testDic setObject:@ "test" forKey:[ NSString stringWithFormat:@ "%ld" , i]];      }      NSLog (@ "init:%ld" , [testDic count]);            __block NSMutableString * sum = [ NSMutableString stringWithCapacity:TESTSCALE];            MULogTimeintervalBegin(@ "DictionaryTest" );      for ( NSString * object in [testDic allValues]) {          [sum appendString:object];      }      [sum setString:@ "" ];      MULogTimeintervalPauseAndLog(@ "for statement allValues" );            for ( id akey in [testDic allKeys]) {          [sum appendString:[testDic objectForKey:akey]];      }      [sum setString:@ "" ];      MULogTimeintervalPauseAndLog(@ "for statement allKeys" );            [testDic enumerateKeysAndObjectsUsingBlock:^( id key, id obj, BOOL *stop) {          [sum appendString:obj];      } ];      MULogTimeintervalPauseAndLog(@ "enumeration" ); }

下面是测试结果：

Test Case '-[LoopTestTests testArray]' started.

2012-08-02 17:14:22.061 otest[388:303] init:100000

2012-08-02 17:14:22.062 otest[388:303] MULogTimeintervalBegin:ArrayTest

2012-08-02 17:14:22.075 otest[388:303]for statement:0.013108

2012-08-02 17:14:22.083 otest[388:303]for-in:0.008186

2012-08-02 17:14:22.095 otest[388:303] enumerateBlock:0.012290

Test Case '-[LoopTestTests testArray]' passed (0.165 seconds).

Test Case '-[LoopTestTests testDictionary]' started.

2012-08-02 17:14:22.273 otest[388:303] init:100000

2012-08-02 17:14:22.274 otest[388:303] MULogTimeintervalBegin:DictionaryTest

2012-08-02 17:14:22.284 otest[388:303] for statement allValues:0.010566

2012-08-02 17:14:22.307 otest[388:303] for statement allKeys:0.022377

2012-08-02 17:14:22.330 otest[388:303] enumeration:0.023914

Test Case '-[LoopTestTests testDictionary]' passed (0.217 seconds).

可以看出对于数组来说，for-in方式遍历速度是最快的，普通风格的for和block方式速度差不多。对于字典来说，allValues方式遍历最快，allKeys和block差不多。

那么，为什么会这样呢？

NSArray：

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for ( NSInteger i = 0; i < count; ++i) {          [sum appendString:[testArray objectAtIndex:i]]; }

这里由于存在：[objectAtIndex:i]这样的取操作，所以速度会有所下降。

而

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for ( NSString * item in testArray) {          [sum appendString:item]; }

尽管也有取操作，但是绕开了oc的message机制，速度会快一点。也有可能是编译器为了for-in作了优化。

block为什么会慢一些这个有待研究。

NSDictionary：

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for ( id akey in [testDic allKeys]) {          [sum appendString:[testDic objectForKey:akey]]; }

这个就很明显了，第二种方法多了一次objectForKey的操作。block的话有待研究。

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google了一下，stackoverflow上面有类似的讨论：

大意是：for-in语法会对容器里面的元素的内存地址建立一个缓冲，遍历的时候从缓冲直接取得元素的地址而不是通过调用方法来获取，所以效率比较高。另外，这也是不能在循环体中修改容器元素的原因之一。