NSArray的排序函数

sortedArrayHint

下面是官方的解释， 只能看懂这个东西是可以作为其他函数的参数用于加速排序的。

Analyzes the array and returns a “hint” that speeds the sorting of the array when the hint is supplied tosortedArrayUsingFunction:context:hint:. (read-only)

Declaration

OBJECTIVE-C

@property(readonly, copy) NSData *sortedArrayHint

Import Statement

Availability

Available in iOS 2.0 and later.

以下是更多的官方解释。https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/Collections/Articles/Arrays.html

The most complex of these methods is sortedArrayUsingFunction:context:hint:. The hinted sort is most efficient when you have a large array (N entries) that you sort once and then change only slightly (P additions and deletions, where P is much smaller than N). You can reuse the work you did in the original sort by conceptually doing a merge sort between the N “old” items and the P “new” items. To obtain an appropriate hint, you use sortedArrayHint when the original array has been sorted, and keep hold of it until you need it (when you want to re-sort the array after it has been modified).

用法翻译：

sortedArrayHint 用于规模比较大的数组排序，而且是已经用其他函数排好序一次，后面对数组进行了少量的增加删除后再次排序，才可以使用。 可以在第一次排好序之后保留sortedArrayHint ，如果数组有小规模的变化（增加，删除）， 下一次重新排序使用sortedArrayUsingFunction:context:hint:将会极大的提高效率。如果变化比较大，那么将不能有效的提高效率，可以采用其他排序函数。

• sortedArrayUsingFunction:context:

• 这个函数比较简单，只是有些人看不懂函数指针 NSInteger (*)(id, id, void) comparator，
• 这个函数指针的意思就是 这个比较函数要使用一个 返回值为NSInteger 函数参数 为两个待比较对象，void*是一个自定义的对象，如果这个函数支持额外的比较参数，比如说 字符串忽略大小写比较等等。 void*的实参由context 实参传入。另外注意这个返回值虽然定义为NSInteger,但是却限定返回值只能为
• 升序 NSOrderedAscending，相等NSOrderedSame， 降序NSOrderedDescending

• Returns a new array that lists the receiving array’s elements in ascending order as defined by the comparison function comparator.

  Declaration

  OBJECTIVE-C

  - (NSArray *)sortedArrayUsingFunction:(NSInteger (*)(id,
                                                id,
                                                void *))comparator
                              context:(void *)context


  Discussion

  The new array contains references to the receiving array’s elements, not copies of them.

  The comparison function is used to compare two elements at a time and should return NSOrderedAscendingif the first element is smaller than the second, NSOrderedDescending if the first element is larger than the second, and NSOrderedSame if the elements are equal. Each time the comparison function is called, it’s passed context as its third argument. This allows the comparison to be based on some outside parameter, such as whether character sorting is case-sensitive or case-insensitive.

  //2.找出数组中最长的字符串(10分)
  //
  //数组中的元素都是字符串，返回其中最长字符串的地址
  
  NSInteger compareLengthOfStr(NSString* str1,NSString* str2,void* option)
  {
      if ([str1 length] < [str2length]) {
          returnNSOrderedDescending;
      }
      else if([str1 length] == [str2 length]) {
          returnNSOrderedSame;
      }
      else
      {
          returnNSOrderedAscending;
      }
  }
  
  -(NSString *)longestStringInArray:(NSArray *)arr
  {
      NSArray * sortedArray = [arrsortedArrayUsingFunction:compareLengthOfStrcontext:nil];
      NSString* returnStr = [sortedArray[0] retain];
      return [returnStr autorelease];
  }