Question

从数组的末尾（而不是开头）“切片”NSArray的最佳方法是什么（例如，找到包含NSArray的最后几个元素的子数组未知长度）？在Python中，您可以使用负索引来实现此目的，例如：

new_list = old_list[-5:-3]

在Objective-C中，最自然的方法是什么？

Answer 1

没有什么可以匹配Python的漂亮语法，但你可以这样做：

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

您还可以为NSArray写一个类别，例如：

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

如果你想走这条路，Python源肯定会回报学习。执行切片操作时，list object会创建slice object。切片对象上的相关方法是PySlice_GetIndicesEx。您只需要小心将这些索引转换为NSRange。正如该函数中的评论所警告的那样“这比你想象的要难得多”。（我稍后会试着解决这个问题。）

更新：我们在NSArray处有一个切片类别。索引计算逻辑非常直接来自我上面链接的Python代码。*如果你不必担心Python切片的步幅部分，它实际上比我想象的要容易得多。我通过一些测试运行它，它似乎与Python版本一样。

@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

从数组末尾切片NSArray

1 个答案: