C++11中std::unordered_map的使用
unordered map is an associative container that contains key-value pairs with unique keys. Search, insertion, and removal of elements have average constant-time complexity. Internally, the elements are not sorted in any particular order,but organized into buckets. Which bucket an element is placed into depends entirely on the hash of its key. This allows fast access to individual elements, since once the hash is computed, it refers to the exact bucket the element is placed into.
unordered_map containers are faster than map containers to access individual elements by their key, although they are generally less efficient for range iteration through a subset of their elements.
unordered_map's generally use more memory. A map just has a few house-keeping pointers then memory for each object. Contrarily, unordered_map's have a big array (these can get quite big in some implementations) and then additional memory for each object. If you need to be memory-aware, a map should prove better, because it lacks the large array.
unordered_map is an associated container that stores elements formed by combination of key value and a mapped value. The key value is used to uniquely identify the element and mapped value is the content associated with the key. Both key and value can be of any type predefined or user-defined. Internally unordered_map is implemented using Hash Table,the key provided to map are hashed into indices of hash table that is why performance of data structure depends on hash function a lot but on an average the cost of look-up from hash table is O(1). In worst case unordered_map may require O(n) time but practically it is much faster and outperforms tree based maps.
std::map对应的数据结构是红黑树。红黑树是一种近似于平衡的二叉查找树,里面的数据是有序的。在红黑树上做查找、插入、删除操作的时间复杂度为O(logN)。而std::unordered_map对应哈希表,哈希表的特点就是查找效率高,时间复杂度为常数级别O(1), 而额外空间复杂度则要高出许多。所以对于需要高效率查询的情况,使用std::unordered_map容器,但是std::unordered_map对于迭代器遍历效率并不高。而如果对内存大小比较敏感或者数据存储要求有序的话,则可以用std::map容器。
下面是从其他文章中copy的测试代码,详细内容介绍可以参考对应的reference:
#include "unordered_map.hpp" #include <iostream> #include <string> #include <unordered_map> ////////////////////////////////////////////////// // reference: http://en.cppreference.com/w/cpp/container/unordered_map int test_unordered_map1() { // Create an unordered_map of three strings (that map to strings) std::unordered_map<std::string, std::string> u = { { "RED", "#FF0000" }, { "GREEN", "#00FF00" }, { "BLUE", "#0000FF" } }; // Iterate and print keys and values of unordered_map for (const auto& n : u) { std::cout << "Key:[" << n.first << "] Value:[" << n.second << "]\n"; } // Add two new entries to the unordered_map u["BLACK"] = "#000000"; u["WHITE"] = "#FFFFFF"; // Output values by key std::cout << "The HEX of color RED is:[" << u["RED"] << "]\n"; std::cout << "The HEX of color BLACK is:[" << u["BLACK"] << "]\n"; std::cout << "The u's size: " << u.size() << std::endl; return 0; } ///////////////////////////////////////////////////// // reference: http://www.cplusplus.com/reference/unordered_map/unordered_map/at/ typedef std::unordered_map<std::string, std::string> stringmap; stringmap merge(stringmap a, stringmap b) { stringmap temp(a); temp.insert(b.begin(), b.end()); return temp; } int test_unordered_map2() { ////////// at/size std::unordered_map<std::string, int> mymap = { { "Mars", 3000 }, { "Saturn", 60000 }, { "Jupiter", 70000 } }; mymap.at("Mars") = 3396; mymap.at("Saturn") += 272; mymap.at("Jupiter") = mymap.at("Saturn") + 9638; for (auto& x : mymap) { std::cout << x.first << ": " << x.second << std::endl; } std::cout << "mymap.size() is " << mymap.size() << std::endl; /////////// begin std::unordered_map<std::string, std::string> mymap2 = { { "Australia", "Canberra" }, { "U.S.", "Washington" }, { "France", "Paris" } }; std::cout << "mymap2 contains:"; for (auto it = mymap2.begin(); it != mymap2.end(); ++it) std::cout << " " << it->first << ":" << it->second; std::cout << std::endl; std::cout << "mymap2's buckets contain:\n"; for (unsigned i = 0; i < mymap2.bucket_count(); ++i) { std::cout << "bucket #" << i << " contains:"; for (auto local_it = mymap2.begin(i); local_it != mymap2.end(i); ++local_it) std::cout << " " << local_it->first << ":" << local_it->second; std::cout << std::endl; } ////////////// bucket std::unordered_map<std::string, std::string> mymap3 = { { "us", "United States" }, { "uk", "United Kingdom" }, { "fr", "France" }, { "de", "Germany" } }; for (auto& x : mymap3) { std::cout << "Element [" << x.first << ":" << x.second << "]"; std::cout << " is in bucket #" << mymap3.bucket(x.first) << std::endl; } /////////////// count std::unordered_map<std::string, double> mymap4 = { { "Burger", 2.99 }, { "Fries", 1.99 }, { "Soda", 1.50 } }; for (auto& x : { "Burger", "Pizza", "Salad", "Soda" }) { if (mymap4.count(x)>0) std::cout << "mymap4 has " << x << std::endl; else std::cout << "mymap4 has no " << x << std::endl; } ///////////////// erase std::unordered_map<std::string, std::string> mymap5; // populating container: mymap5["U.S."] = "Washington"; mymap5["U.K."] = "London"; mymap5["France"] = "Paris"; mymap5["Russia"] = "Moscow"; mymap5["China"] = "Beijing"; mymap5["Germany"] = "Berlin"; mymap5["Japan"] = "Tokyo"; // erase examples: mymap5.erase(mymap5.begin()); // erasing by iterator mymap5.erase("France"); // erasing by key mymap5.erase(mymap5.find("China"), mymap5.end()); // erasing by range // show content: for (auto& x : mymap5) std::cout << x.first << ": " << x.second << std::endl; ////////////////////// find std::unordered_map<std::string, double> mymap6 = { { "mom", 5.4 }, { "dad", 6.1 }, { "bro", 5.9 } }; std::string input; std::cout << "who? "; getline(std::cin, input); std::unordered_map<std::string, double>::const_iterator got = mymap6.find(input); if (got == mymap6.end()) std::cout << "not found"; else std::cout << got->first << " is " << got->second; std::cout << std::endl; //////////////////// insert std::unordered_map<std::string, double> myrecipe, mypantry = { { "milk", 2.0 }, { "flour", 1.5 } }; std::pair<std::string, double> myshopping("baking powder", 0.3); myrecipe.insert(myshopping); // copy insertion myrecipe.insert(std::make_pair<std::string, double>("eggs", 6.0)); // move insertion myrecipe.insert(mypantry.begin(), mypantry.end()); // range insertion myrecipe.insert({ { "sugar", 0.8 }, { "salt", 0.1 } }); // initializer list insertion std::cout << "myrecipe contains:" << std::endl; for (auto& x : myrecipe) std::cout << x.first << ": " << x.second << std::endl; std::cout << std::endl; //////////////////// = stringmap first = { { "AAPL", "Apple" }, { "MSFT", "Microsoft" } }; // init list stringmap second = { { "GOOG", "Google" }, { "ORCL", "Oracle" } }; // init list stringmap third = merge(first, second); // move first = third; // copy std::cout << "first contains:"; for (auto& elem : first) std::cout << " " << elem.first << ":" << elem.second; std::cout << std::endl; return 0; } ////////////////////////////////////////////////////// // reference: http://www.geeksforgeeks.org/unordered_map-in-stl-and-its-applications/ int test_unordered_map3() { // key will be of string type and mapped value will be of double type std::unordered_map<std::string, double> umap; // inserting values by using [] operator umap["PI"] = 3.14; umap["root2"] = 1.414; umap["root3"] = 1.732; umap["log10"] = 2.302; umap["loge"] = 1.0; // inserting value by insert function umap.insert(std::make_pair("e", 2.718)); std::string key = "PI"; // If key not found in map iterator to end is returned if (umap.find(key) == umap.end()) { std::cout << key << " not found\n\n"; } else {// If key found then iterator to that key is returned std::cout << "Found " << key << "\n\n"; } key = "lambda"; if (umap.find(key) == umap.end()) std::cout << key << " not found\n"; else std::cout << "Found " << key << std::endl; // iterating over all value of umap std::unordered_map<std::string, double>::iterator itr; std::cout << "\nAll Elements : \n"; for (itr = umap.begin(); itr != umap.end(); itr++) { // itr works as a pointer to pair<string, double> type itr->first stores the key part // and itr->second stroes the value part std::cout << itr->first << " " << itr->second << std::endl; } return 0; }
GitHub:https://github.com/fengbingchun/Messy_Test
C++11中std::unordered_map的使用的更多相关文章
- C++11中std::function的使用
class template std::function is a general-purpose polymorphic function wrapper. Instances of std::fu ...
- C++11中std::move的使用
std::move is used to indicate that an object t may be "moved from", i.e. allowing the effi ...
- C++11中std::bind的使用
std::bind: Each argument may either be bound to a value or be a placeholder: (1).If bound to a value ...
- C++11中std::forward的使用 (转)
std::forward argument: Returns an rvalue reference to arg if arg is not an lvalue reference; If arg ...
- C++11中std::forward的使用
std::forward argument: Returns an rvalue reference to arg if arg is not an lvalue reference; If arg ...
- C++11中std::move、std::forward、左右值引用、移动构造函数的测试
关于C++11新特性之std::move.std::forward.左右值引用网上资料已经很多了,我主要针对测试性能做一个测试,梳理一下这些逻辑,首先,左值比较熟悉,右值就是临时变量,意味着使用一次就 ...
- 用C++11的std::async代替线程的创建
c++11中增加了线程,使得我们可以非常方便的创建线程,它的基本用法是这样的: void f(int n); std::thread t(f, n + 1); t.join(); 但是线程毕竟是属于比 ...
- C++11 使用 std::async创建异步程序
c++11中增加了线程,使得我们可以非常方便的创建线程,它的基本用法是这样的: void f(int n); std::thread t(f, n + 1); t.join(); 但是线程毕竟是属于比 ...
- (原创)用C++11的std::async代替线程的创建
c++11中增加了线程,使得我们可以非常方便的创建线程,它的基本用法是这样的: void f(int n); std::thread t(f, n + ); t.join(); 但是线程毕竟是属于比较 ...
随机推荐
- java实现MsOffice文档向pdf文档转化
本篇文档实现功能,将word和ppt文档的文件转化成pdf格式的文档 应用到jacob 第一步:下载压缩包 (1)jacob官网下载jacob压缩包 (2)网址:http://sourceforge. ...
- C语言使用pthread多线程编程(windows系统)二
我们进行多线程编程,可以有多种选择,可以使用WindowsAPI,如果你在使用GTK,也可以使用GTK实现了的线程库,如果你想让你的程序有更多的移植性你最好是选择POSIX中的Pthread函数库,我 ...
- CentOS 6.0 VNC远程桌面配置方法(转帖)
问题:新装开发机,安装VNC软件后,按照下面文档配置后,无法用VNC view连接,关闭防火墙后可以连上 解决方法:说明问题出在防火墙配置上,除了允许15900端口外,还有其他要设,经过排查后,加上如 ...
- TimeHelp 获取时间戳转换类
using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace Shar ...
- npm查看全局安装过的包
在使用node的时候,用npm安装了很多软件,过一段时间没有使用就会忘记,怎么查看自己全局安装过的包,用命令 npm list -g --depth 0 在百度里搜不到结果的,我在google里老外的 ...
- Mac 下使用brew install 报错: Cowardly refusing to `sudo brew install'
Mac 下使用brew install 报错: localhost:infer-osx-v0.6.0 admin$ sudo brew install opam Error: Cowardly ref ...
- ReportViewer 2010 打印预览,用鼠标快速切换显示比例时报错:存储空间不足,不能处理此命令
CreateCompatibleDIB 存储空间不足 无法处理此命令 安装 ReportViewer 2010 sp1 即可.
- 使用 Lombok 简化项目中无谓的Java代码
在写使用Java时,难免会有一些模板代码要写,不然get/set,toString, hashCode, close 资源,定义构造函数等等.代码会显得很冗余,很长.Lombok项目可以是我们摆脱这些 ...
- VS2015 Git 源代码管理工具使用记录
1. 首先到源代码托管平台申请个账户:https://git.oschina.net/ 2.创建流程图: 2.1 开始创建项目: 2.2 3. 4.
- Java 字符串 String
什么是Java中的字符串 在 Java 中,字符串被作为 String 类型的对象处理. String 类位于 java.lang 包中.默认情况下,该包被自动导入所有的程序. 创建 String 对 ...