c++ std::unordered_map的使用
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。
unordered_map 类似与python 的dict(字典).
实例代码如下:
1 元素的访问与添加
#include "unordered_map.hpp"
#include 2 使用at单独访问元素值,改变单个元素值
// reference: http://www.cplusplus.com/reference/unordered_map/unordered_map/at/
// 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;
3 使用泛型指针访问元素
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;
}
4 测试bucket
// 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;
}
**5 count,查找**
```c
/// 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;
}
6 erase,删除元素
/ erase
std::unordered_map<std::string, std::string> mymap6={{"U.S.","Washington"},{"U.K.","London"},
{"France","Paris"},{"Russia","Moscow"},
{"China","Beijing"},{"Germany","Berlin"},
{"Japan","Tokyo"}};
// // populating container:
// mymap6["U.S."] = "Washington";
// mymap6["U.K."] = "London";
// mymap6["France"] = "Paris";
// mymap6["Russia"] = "Moscow";
// mymap6["China"] = "Beijing";
// mymap6["Germany"] = "Berlin";
// mymap6["Japan"] = "Tokyo";
// erase examples:
mymap6.erase(mymap6.begin()); // erasing by iterator
mymap6.erase("France"); // erasing by key
mymap6.erase(mymap6.find("China"), mymap6.end()); // erasing by range
// show content:
for (auto& x : mymap6){
std::cout << x.first << ": " << x.second << std::endl;}
7 查找元素
// 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;
8 插入元素
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;
}
9插入和查找,以及使用泛型指针
//
// 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 type itr->first stores the key part
// and itr->second stroes the value part
std::cout << itr->first << " " << itr->second << std::endl;
}
return 0;
}