从数组的末尾切片NSArray

如何解决从数组的末尾切片NSArray

| 从数组的末尾（而不是从开头）对“ 0”进行“切片”的最佳方法是什么（例如，找到包含长度未知的“ 0”的最后几个元素的子数组）？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中最自然的方法是什么？

解决方法

没有什么可以匹配Python的漂亮语法了，但是您可以这样做：

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

您还可以为NSArray编写一个类别，例如：

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

如果您想走这条路，Python来源肯定会回报您的学习。执行切片操作时，列表对象将创建切片对象。切片对象的相关方法为PySlice_GetIndicesEx。您只需要小心地将这些索引变成NSRange。正如该函数中的注释警告说：“这比您想象的要难。” （稍后，我将尝试破解）。更新：这里我们在NSArray上有一个切片类别。索引计算逻辑与我上面链接的Python代码几乎完全相同。*如果您不必担心Python切片的跨步部分，它实际上比起初想的要容易得多。我已经通过一些测试运行了它，它的工作方式似乎与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