Objective c 按值atIndex对嵌套NSArray的NSMutableArray排序

我有一个NSMutableArray，其中包含许多NSArray。在每个NSArray中的特定（静态）索引处，有一个值，我希望按该值对NSMutableArray进行排序（从大到小）。现在我正在尝试使用NSSortDescriptor，但不知道如何通过KVC获取和比较特定索引的值。详细说明：

#define INDEX_OF_DESIRED_STRING 2

NSArray *a1 = [NSArray arrayWithObjects:@"test", @"jjj", @"3454", nil]; 
NSArray *a2 = [NSArray arrayWithObjects:@"test1", @"jjj", @"12", nil]; 
NSArray *a3 = [NSArray arrayWithObjects:@"test2", @"jjj", @"232333", nil]; 
NSArray *a4 = [NSArray arrayWithObjects:@"test3", @"jjj", @".122", nil]; 

NSMutableArray *mutableA = [[NSMutableArray alloc] initWithObjects:a1, a2, a3, a4, nil]; 

// Then I'd sort with something like this... although this of course
// does not take the arrays into account. Sorts as if it were only made of strings

NSSortDescriptor *sd = [NSSortDescriptor sortDescriptorWithKey:@"floatValue"
                                                 ascending:NO];

[mutableA sortUsingDescriptors:[NSArray arrayWithObject:sd]]; 

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尝试使用比较器块进行排序：

NSArray *a1 = [NSArray arrayWithObjects:@"test", @"jjj", @"3454", nil]; 
NSArray *a2 = [NSArray arrayWithObjects:@"test1", @"jjj", @"12", nil]; 
NSArray *a3 = [NSArray arrayWithObjects:@"test2", @"jjj", @"232333", nil]; 
NSArray *a4 = [NSArray arrayWithObjects:@"test3", @"jjj", @".122", nil]; 

NSMutableArray *mutableA = [[NSMutableArray alloc] initWithObjects:a1, a2, a3, a4, nil];

NSLog(@"mutableA before sorting: %@", mutableA);

[mutableA sortUsingComparator:^NSComparisonResult(id obj1, id obj2) {
    NSArray *array1 = (NSArray *)obj1;
    NSArray *array2 = (NSArray *)obj2;
    NSString *num1String = [array1 objectAtIndex:INDEX_OF_DESIRED_STRING];
    NSString *num2String = [array2 objectAtIndex:INDEX_OF_DESIRED_STRING];

    return [num1String compare:num2String];
}];

NSLog(@"mutableA after sorting: %@", mutableA);

---

比较器块比它可能的更详细，但我想让它清楚发生了什么。

如果将数值存储为NSNumbers而不是NSStrings，会发生什么？回答很好。谢谢AndrewSuper。谢谢你能详细解释一下它是如何分类的吗？