C++ std::set 与向量或映射的优点
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advantages of std::set vs vectors or maps
提问by brunodd
This may be a stupid question, I am quite new to C++ and programming in general. I wish to understand the use of several STL containers and with that in mind, I was wondering what the advantages are of using std::set vs for example using vectors or maps? I can't seem to find an explicit answer to this question. I noticed that sets use maps, but then why not always use maps or always use sets. Instead 2 quite similar containers are provided. Thanks in advance.
这可能是一个愚蠢的问题，我对 C++ 和一般编程很陌生。我想了解几个 STL 容器的使用，考虑到这一点，我想知道使用 std::set 与例如使用向量或映射的优点是什么？我似乎无法找到这个问题的明确答案。我注意到集合使用地图，但为什么不总是使用地图或总是使用集合。而是提供了 2 个非常相似的容器。提前致谢。
回答by David Rodríguez - dribeas
Both std::setand std::mapare associative containers. The difference is that std::sets contain only the key, while in std::mapthere is an associated value. Choosing one over the other depends mainly on what the task at hand is. If you want to build a dictionary of all the words that appear in a text, you could use a std::set<std::string>, but if you also want to count how many times each word appeared (i.e. associate a value to the key) then you would need an std::map<std::string,int>. If you don't need to associate that count, it does not make sense to have the intthat is unnecessary.
这两个std::set和std::map是相关联的容器。区别在于std::sets 只包含键，而 instd::map有一个关联的值。选择一个主要取决于手头的任务是什么。如果您想为文本中出现的所有单词构建一个字典，您可以使用 a std::set<std::string>，但如果您还想计算每个单词出现的次数（即，将一个值与键关联），那么您需要一个std::map<std::string,int>. 如果您不需要关联该计数，那么拥有该计数是没有意义的int。
回答by Angel Koh
a set is useful for storing unique things like an enum for "typeOfFruits"
集合对于存储独特的东西很有用，比如“typeOfFruits”的枚举
std::set<typeOfFruits> fruits;
fruits.insert (banana);
fruits.insert (apple);
fruits.insert (pineapple);
//it's fast to know if my store sells a type of fruit.
if (fruits.find (pear) == fruits.end())
{ std::cout<<"i don't have pear"; }
a map is useful for storing unique things, plus a 'value'
地图对于存储独特的东西很有用，再加上一个“价值”
std::map<typeOfFruits, double /*unit price*/> fruits;
fruits[banana] = 1.05;
fruits[apple] = 0.85;
fruits[pineapple] = 3.05;
//repeating pineapple will replace the old price (value)
fruits[pineapple] = 3.35;
//it's fast to know how much a fruit costs.
std::map<typeOfFruits, double /*unit price*/> itr = fruits.find(pineapple);
if (itr != fruits.end())
{ std::cout<<"pineapples costs: $" <<itr->second; }
a vector is useful for storing things where the sequence is ordered (push_back()). imagine you are scanning your fruits in a checkout, and the program tracks this scanning.
向量可用于存储序列已排序的内容（push_back()）。假设您正在结帐时扫描水果，并且程序会跟踪此扫描。
std::vector<typeOfFruits> fruits;
fruits.push_back(apple);
fruits.push_back(apple);
fruits.push_back(apple);
fruits.push_back(banana);
fruits.push_back(banana);
fruits.push_back(pineapple);
//i scanned 3 apples, 2 bananas and 1 pineapple.
回答by user2554481
No body has mentioned the facts that std::setis actually immutable. You should not change the value of any element in it. std::setdoes not track over changes so when you edit a element in it you go behind its back and are likely to change its inner ordering. This is a risky behavior. Therefore use std::mapif you want to edit elements after you put them into the container. Make sure you use keyto induce ordering and everything you need to change afterwards into value.
没有任何机构提到过std::set实际上不可改变的事实。您不应更改其中任何元素的值。std::set不会跟踪更改，因此当您编辑其中的元素时，您会落后于它，并且可能会更改其内部顺序。这是一种危险的行为。因此，std::map如果您想在将元素放入容器后对其进行编辑，请使用。确保您使用key来诱导排序以及之后需要更改的所有内容value。
回答by lucas92
vectoris faster for insertions and deletions at the back of the container. You can access the elements via the operator [].dequeueis similar tovectorbut it features front insertion and deletion.setonly has the key whilemaphas apair. Both of these containers are faster for insertion and deletion in the middle of the container. You can also access elements via find with the STL algorithms.
vector在容器后面插入和删除的速度更快。您可以通过运算符 [] 访问元素。dequeue类似于vector但它具有前端插入和删除功能。set只有钥匙而map有pair. 这两个容器在容器中间插入和删除都更快。您还可以使用 STL 算法通过 find 访问元素。
回答by Scott Jones
It comes down to the complexity guarantees that are most desired for your application, with respect to insertion, removal, retrieval, etc. I highly recommend Scott Meyers' Effective STL.
它归结为您的应用程序最需要的复杂性保证，涉及插入、删除、检索等。我强烈推荐Scott Meyers 的 Effective STL。
