从数组末尾切片 NSArray

“切片”的最佳方法是什么NSArray从数组的末尾而不是开头开始（例如，查找包含 a 的最后几个元素的子数组）NSArray长度未知）？在 Python 中，您可以使用负索引来完成此操作，例如：

new_list = old_list[-5:-3]

在 Objective-C 中执行此操作最自然的方法是什么？

没有什么可以与 Python 的良好语法相匹配，但你可以这样做：

NSUInteger count = [myArray count];
NSArray * slice = [myArray subarrayWithRange:(NSRange){count-n, n}];

您还可以为以下内容编写一个类别NSArray， 就像是：

@interface NSArray (jrdioko_slice)
- (NSArray *) jrdioko_sliceFrom:(NSInteger)start to:(NSInteger)stop;
@end

如果你想走这条路，Python源码肯定会值得学习。 A列表对象 http://svn.python.org/view/%2acheckout%2a/python/tags/r265/Objects/listobject.c?content-type=text%2Fplain创建一个切片对象 http://svn.python.org/view/%2acheckout%2a/python/tags/r265/Objects/sliceobject.c?content-type=text%2Fplain当执行切片操作时。切片对象的相关方法是PySlice_GetIndicesEx。你只需要小心地将这些索引变成NSRange。正如该函数中的评论所警告的那样“这比您想象的更难做到正确”。 （稍后我会尝试解决这个问题。）

更新：这里我们有一个切片类别NSArray。索引计算逻辑几乎直接来自我上面链接的 Python 代码。*如果您不必担心 Python 切片的步幅部分，它实际上比我一开始想象的要容易得多。我已经通过一些测试运行了它，它的工作原理似乎与 Python 版本相同。

@interface NSArray (WSS_Slice)
- (NSArray *)WSS_arrayBySlicingFrom:(NSInteger)start to:(NSInteger)stop;
@end

// Python allows skipping any of the indexes of a slice and supplies default
// values. Skipping an argument to a method is not possible, so (ab)use 
// NSNotFound as "not specified" index value. The other way to do this would
// be with varargs, which might be even handier if one decided to implement
// the stride functionality.
enum {
    WSS_SliceNoIndex = NSNotFound
};

@implementation NSArray (WSS_Slice)

- (NSArray *)WSS_arrayBySlicingFrom:(NSInteger)start to:(NSInteger)stop {
    // There's an important caveat here: specifying the parameters as 
    // NSInteger allows negative indexes, but limits the method's 
    // (theoretical) use: the maximum size of an NSArray is NSUIntegerMax, 
    // which is quite a bit larger than NSIntegerMax. 
    NSUInteger count = [self count];

    // Due to this caveat, bail if the array is too big.
    if( count >= NSIntegerMax ) return nil;

    // Define default start and stop
    NSInteger defaultStart = 0;
    NSInteger defaultStop = count;

    // Set start to default if not specified
    if( start == WSS_SliceNoIndex ){
        start = defaultStart;
    }
    else {
        // If start is negative, change it to the correct positive index.
        if( start < 0 ) start += count;
        // Correct for out-of-bounds index:
        // If it's _still_ negative, set it to 0
        if( start < 0 ) start = 0;
        // If it's past the end, set it to just include the last item
        if( start > count ) start = count;
    }

    // Perform all the same calculations on stop
    if( stop == WSS_SliceNoIndex ){
        stop = defaultStop;
    }
    else {
        if( stop < 0 ) stop += count;
        if( stop < 0 ) stop = 0;
        if( stop > count ) stop = count;
    }

    // Calculate slice length with corrected indexes
    NSInteger sliceLength = stop - start;

    // If no slice, return a new empty array
    if( sliceLength <= 0 ){
        return [NSArray array];
    }
    else {
        return [self subarrayWithRange:(NSRange){start, sliceLength}];
    }

}
@end

_{*Therefore I think I need to include a link to the Python License http://docs.python.org/release/2.6.5/license and also note that this may still be “Copyright © 2001-2010 Python Software Foundation; All Rights Reserved”, because although this looks to me like a separately-copyrightable derivative work, I ain't a lawyer.}