std :: set和std :: vector之间有什么区别？ [英] What is the difference between std::set and std::vector?

问题描述

我现在正在学习STL。我阅读了关于 set 容器。我有问题，当你想使用 set ？在阅读描述集后，它看起来是无用的，因为我们可以用向量。你能说出向量 vs 设置容器的优点和cos。感谢

解决方案

订购设置。根据您提供的函子，它会保证保持在特定的顺序中。无论您添加或删除什么元素（除非您添加重复，这不允许在设置），它将始终是有序的。

A 向量只有您明确给出的顺序。 向量中的项目是您放置它们的位置。如果你把它们乱序，那么它们就出错了;您现在需要排序容器以将它们按顺序放回。

诚然， set 使用相对有限。通过适当的纪律，可以将项目插入向量并保持顺序。但是，如果您经常从容器中插入和删除项目，则向量将遇到许多问题。它会做很多复制/移动的元素等等，因为它实际上只是一个数组。

将一个项目插入一个向量与已经在向量中的项目数成正比。将项目插入集所需的时间与项目数量的 log 2 成正比。如果项目数量很大，这是一个巨大的区别。 log 10（100,000）为〜16;这是一个重大的速度改进。这同样适用于删除。

但是，如果您立即执行所有插入操作，则在初始化时，这没有问题。您可以将所有内容插入向量，对其排序（支付一次价格），然后对排序的向量使用标准算法来查找元素并遍历排序的列表。虽然对 set 的元素的迭代不是很慢，但是在向量上的迭代更快。 p>

所以有些情况下，一个排序的向量击败一个 set 。话虽如此，你真的不应该费心这种优化，除非你知道这是必要的。所以使用 set ，除非你有你所写的系统的经验（因此知道你需要的性能）或有分析数据在手，告诉你，您需要一个向量，而不是设置。

I am learning STL now. I read about set container. I have question when you want to use set? After reading description of set it looks like it is useless because we can substitute it by vector. Could you say pros and cos for vector vs set containers. Thanks

解决方案

A set is ordered. It is guaranteed to remain in a specific ordering, according to a functor that you provide. No matter what elements you add or remove (unless you add a duplicate, which is not allowed in a set), it will always be ordered.

A vector has exactly and only the ordering you explicitly give it. Items in a vector are where you put them. If you put them in out of order, then they're out of order; you now need to sort the container to put them back in order.

Admittedly, set has relatively limited use. With proper discipline, one could insert items into a vector and keep it ordered. However, if you are constantly inserting and removing items from the container, vector will run into many issues. It will be doing a lot of copying/moving of elements and so forth, since it is effectively just an array.

The time it takes to insert an item into a vector is proportional to the number of items already in the vector. The time it takes to insert an item into a set is proportional to the log₂ of the number of items. If the number of items is large, that's a huge difference. log₂(100,000) is ~16; that's a major speed improvement. The same goes for removal.

However, if you do all of your insertions at once, at initialization time, then there's no problem. You can insert everything into the vector, sort it (paying that price once), and then use standard algorithms for sorted vectors to find elements and iterate over the sorted list. And while iteration over the elements of a set isn't exactly slow, iterating over a vector is faster.

So there are cases where a sorted vector beats a set. That being said, you really shouldn't bother with the expense of this kind of optimization unless you know that it is necessary. So use a set unless you have experience with the kind of system you're writing (and thus know that you need that performance) or have profiling data in hand that tells you that you need a vector and not a set.

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