如何获得未知大小NSArray的前X个元素？

In objectiveC I have an NSArray, let's call it NSArray* largeArray, and I want to get a new NSArray* smallArray with just the first x objects

在objectiveC中,我有一个NSArray,让我们称之为NSArray * largeArray,我想得到一个新的NSArray * smallArray只带有前x个对象

...OR in the event that largeArray is already size <= x I just want a copy of the largeArray. So truncating any objects after index x.

...或者如果largeArray已经是<= x,我只想要一个largeArray的副本。因此在索引x之后截断任何对象。

This approach:

NSArray *smallArray = [largeArray subarrayWithRange:NSMakeRange(0, x)];

Was the answer to this very similar question. But it fails with an error if largeArray is already small.

是这个非常相似的问题的答案。但是如果largeArray已经很小,它就会失败并出现错误。

3 个解决方案

#1

You could do this...

你可以这样做......

NSArray *smallArray = [largeArray subarrayWithRange:NSMakeRange(0, MIN(x, largeArray.count))];

That will take the first x elements or the full array if it's smaller than x.

如果它小于x,那将采用前x个元素或完整数组。

If largeArray.count is 100.

如果largeArray.count是100。

If x = 110 then it will take the first 100 results. If x = 90 then it will take the first 90 results.

如果x = 110则将获得前100个结果。如果x = 90,那么它将获得前90个结果。

Yep, that works :D

是的,有效:D

#2

Fogmeister's answer is perfectly good, but, in situations where the array is often already small enough, that answer will be mildly inefficient since it always makes a copy. Here is a more efficient version:

Fogmeister的答案是非常好的,但是,在阵列通常已经足够小的情况下,这个答案将会轻微低效,因为它总是会复制。这是一个更有效的版本:

NSAarray *smallArray = largeArray;
if (smallArray.count > MAX_NUM_ITEMS)
    smallArray = [smallArray subarrayWithRange:NSMakeRange(0, MAX_NUM_ITEMS)];

When the array is already within the limit this version will just make a reference to the existing array.

当数组已经在限制范围内时,此版本将仅引用现有数组。

#3

-5

Here's the obvious long-hand way of doing it:

这是明显的长手方式:

NSMutableArray* smallMutableArray;
if ([largeArray count] <= x) {
    smallMutableArray = [largeArray copy];
} else {
    for (int i=0; i<x; i++) {
       [smallMutableArray addObject:[largeArray objectAtIndex:i]];
    }
}

#1