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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_mapint 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
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