使用valueForKeyPath查找NSMutableArray的平均值 [英] Find the average of an NSMutableArray with valueForKeyPath

问题描述

目前，我正试图找到一种简洁的方法来平均矩阵。显而易见的解决方案是对矩阵求和，然后除以元素的数量。但是，我在苹果开发者网站上遇到过一种方法，声称可以使用valueForKeyPath以更简单的方式完成此操作。这在以下链接：

Currently I'm trying to find a compact way to average a matrix. The obvious solution is to sum the matrix, then divide by the number of elements. I have, however, come across a method on the apple developer website that claims this can be done in a simpler way using valueForKeyPath. This is linked here:

http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/KeyValueCoding/Articles/CollectionOperators.html

以下是我正在努力尝试让它发挥作用的示例：

Here is the example I'm currently working on to try and get it to work:

-(void)arrayAverager
{
   NSMutableArray *myArray = [NSMutableArray arrayWithCapacity:25];
   [myArray addObject:[NSNumber numberWithInteger:myValue]];
   NSNumber *averageValue = [myArray valueForKeyPath:@"@avg.self"];
   NSLog(@"avg  = %@", averageValue);

} 

问题是：代替对数组进行平均，它只是打印出数组1中的元素。

The problem is: instead of averaging the array it merely prints out the elements in the array 1 by 1.

UPDATE

-(void) pixelAverager

{
    //Put x-coordinate value of all wanted pixels into an array
    NSMutableArray *xCoordinateArray = [NSMutableArray arrayWithCapacity:25];
    [xCoordinateArray addObject:[NSNumber numberWithInteger:xCoordinate]];
    NSLog(@"avg = %@", [xCoordinateArray valueForKeyPath:@"@avg.intValue"]);
 }

推荐答案

你需要使用 @ avg.floatValue （或 @ avg.doubleValue ，或者你有什么）。 @avg 运算符将平均点后面的名称指定的数组中对象的属性。文档在这一点上令人困惑，但这就是：

You need to use @avg.floatValue (or @avg.doubleValue, or what have you). The @avg operator will average the property of the objects in the array specified by the name after the dot. The documentation is confusing on this point, but that is what:

获取由键路径$ b指定的属性指定的值运营商右边的$ b

to get the values specified by the property specified by the key path to the right of the operator

正在说。由于您拥有 NSNumber 对象的集合，因此您可以使用 * value 访问者之一，例如： floatValue 获取每个对象的值。例如：

Is saying. Since you have a collection of NSNumber objects, you use one of the *value accessors, e.g. floatValue to get the value of each object. As an example:

#include <Foundation/Foundation.h>

int main(void) {
  NSMutableArray *ma = [NSMutableArray array];
  [ma addObject:[NSNumber numberWithFloat:1.0]];
  [ma addObject:[NSNumber numberWithFloat:2.0]];
  [ma addObject:[NSNumber numberWithFloat:3.0]];

  NSLog(@"avg = %@", [ma valueForKeyPath:@"@avg.floatValue"]);

  return 0;
}

编译并运行此代码会返回：

Compiling and running this code returns:

$ clang avg.m -framework Foundation -o avg
steve:~/code/tmp
$ ./avg 
2013-01-18 12:33:15.500 avg[32190:707] avg = 2
steve:~/code/tmp

这种方法的好处在于，只要所有对象都响应指定的方法，这个工作适用于任何集合，同源或其他， @avg 会有效。

The nice thing about this approach is that this work for any collection, homogenous or otherwise, as long as all objects respond to the specified method, @avg will work.

编辑

正如评论中所指出的， OP的问题在于他使用一个元素平均一个集合，因此它似乎只是打印集合的内容。对于 NSNumber 对象的集合， @ avg.self 的工作正常。

As pointed in the comments, the OP's problem is that he is averaging a collection with one element, and thus it appears to simply print the contents of the collection. For a collection of NSNumber objects, @avg.self works just fine.

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