114.STL中的map
114.STL中的map
1.map的简介
map是C++STL中的一个关联式容器，它提供一对一的hash，它类似于Python中的字典，也有着键值对（Key-Value）这一说。我们可以通过键（Key）来找到值（Value），但需要注意的是，每个键（Key）只能在map中出现一次哦！
map可以储存多种类型的数据，它主要用于一对一映射的情况，map内部的实现是通过自建一颗红黑树，这颗树可以对数据进行自动排序。所以在map内部所有的数据都是有序的，这个功能以后可以方便我们解决很多问题。
使用map时需要包含头文件：
#include<map>
2.map的定义及初始化
定义：map<数据类型1，数据类型2>变量名
例：
//map的定义
map<int,int> m1; //定义一个Key数据类型为int，Value数据类型为int的map容器m1
map<double,double> m2; //定义一个Key数据类型为double，Value数据类型为double的map容器m2
map<int,string> m3; //定义一个Key数据类型为int，Value数据类型为string的map容器m3
3.迭代器
begin 返回指向容器起始位置的迭代器（iterator）
end 返回指向容器末尾位置的迭代器
cbegin 返回指向容器起始位置的常迭代器（const_iterator）
cend 返回指向容器末尾位置的常迭代器
代码：
//unordered_map::bucket_count
#include <iostream>
#include <string>
#include <unordered_map>
using namespace std;
int main()
{
unordered_map<string, string> mymap =
{
{"house", "maison"},
{"apple", "pomme"},
{"tree", "arbre"},
{"book", "livre"},
{"door", "porte"},
{"grapefruit", "pamplemousse"}
};
/************begin和end迭代器***************/
cout << "mymap contains:";
for (auto it = mymap.begin(); it != mymap.end(); ++it)
cout << " " << it->first << ":" << it->second;
cout << endl;
/************bucket操作***************/
unsigned n = mymap.bucket_count();
cout << "mymap has " << n << " buckets.\n";
for (unsigned i = 0; i < n; ++i)
{
cout << "bucket #" << i << "'s size:" << mymap.bucket_size(i) << " contains: ";
for (auto it = mymap.begin(i); it != mymap.end(i); ++it)
cout << "[" << it->first << ":" << it->second << "] ";
cout << "\n";
}
cout << "\nkey:'apple' is in bucket #" << mymap.bucket("apple") << endl;
cout << "\nkey:'computer' is in bucket #" << mymap.bucket("computer") << endl;
return 0;
}
输出：
mymap contains: house:maison apple:pomme tree:arbre book:livre door:porte grapefruit:pamplemousse
mymap has 8 buckets.
bucket #0's size:1 contains: [book:livre]
bucket #1's size:1 contains: [door:porte]
bucket #2's size:1 contains: [grapefruit:pamplemousse]
bucket #3's size:1 contains: [house:maison]
bucket #4's size:0 contains:
bucket #5's size:1 contains: [tree:arbre]
bucket #6's size:0 contains:
bucket #7's size:1 contains: [apple:pomme]
key:'apple' is in bucket #7
key:'computer' is in bucket #0
4.Capacity操作
size 返回有效元素个数
max_size 返回 unordered_map 支持的最大元素个数
empty 判断是否为空
5.元素访问
operator[] 访问元素
at 访问元素
#include<iostream>
#include<map>
using namespace std;
int main()
{
map<int, string> m1;
m1[1] = "ctx";
m1[2] = "cxt";
m1[3] = "txc";
m1[10] = "txt";
m1[5] = "666";
map<int, string>::iterator it;
for (it = m1.begin(); it != m1.end(); it++)
{
cout << it->first << " " << it->second << endl;
}
//it->为Key，it->second为Value
return 0;
}
输出：
1 ctx
2 cxt
3 txc
5 666
10 txt
6.查找
unordered_map<string, double>::const_iterator got = myrecipe.find("coffee");
if (got == myrecipe.end())
cout << "not found";
else
cout << "found " << got->first << " is " << got->second << "\n\n";
7.元素修改
insert 插入元素
erase 删除元素
swap 交换内容
clear 清空内容
emplace 构造及插入一个元素
emplace_hint 按提示构造及插入一个元素
插入：
unordered_map < string, double> myrecipe, mypantry = { {"milk",2.0},{"flour",1.5} };
/****************插入*****************/
pair<string, double> myshopping("baking powder", 0.3);
myrecipe.insert(myshopping); // 复制插入
myrecipe.insert(make_pair<string, double>("eggs", 6.0)); // 移动插入
myrecipe.insert(mypantry.begin(), mypantry.end()); // 范围插入
myrecipe.insert({ {"sugar",0.8},{"salt",0.1} }); // 初始化数组插入(可以用二维一次插入多个元素，也可以用一维插入一个元素)
myrecipe["coffee"] = 10.0; //数组形式插入
myrecipe.emplace("apple", 5.0);//构造及插入一个元素
删除：
myrecipe.erase(myrecipe.begin()); //通过位置
myrecipe.erase("milk"); //通过key
display(myrecipe, "After erase myrecipe contains:");
交换：
myrecipe.swap(mypantry);
display(myrecipe, "After swap with mypantry, myrecipe contains:");
修改：
/****************修改*****************/
myrecipe.at("coffee") = 9.0;
myrecipe["milk"] = 3.0;
display(myrecipe, "After modify myrecipe contains:");
清空：
myrecipe.clear();
display(myrecipe, "After clear, myrecipe contains:");
完整代码：
// unordered_map::insert
#include <iostream>
#include <string>
#include <unordered_map>
using namespace std;
void display(unordered_map<string, double> myrecipe, string str)
{
cout << str << endl;
for (auto& x : myrecipe)
cout << x.first << ": " << x.second << endl;
cout << endl;
}
int main()
{
unordered_map < string, double> myrecipe, mypantry = { {"milk",2.0},{"flour",1.5} };
/****************插入*****************/
pair<string, double> myshopping("baking powder", 0.3);
myrecipe.insert(myshopping); // 复制插入
myrecipe.insert(make_pair<string, double>("eggs", 6.0)); // 移动插入
myrecipe.insert(mypantry.begin(), mypantry.end()); // 范围插入
myrecipe.insert({ {"sugar",0.8},{"salt",0.1} }); // 初始化数组插入(可以用二维一次插入多个元素，也可以用一维插入一个元素)
myrecipe["coffee"] = 10.0; //数组形式插入
myrecipe.emplace("apple", 5.0);
display(myrecipe, "myrecipe contains:");
/****************查找*****************/
unordered_map<string, double>::const_iterator got = myrecipe.find("coffee");
if (got == myrecipe.end())
cout << "not found";
else
cout << "found " << got->first << " is " << got->second << "\n\n";
/****************修改*****************/
myrecipe.at("coffee") = 9.0;
myrecipe["milk"] = 3.0;
display(myrecipe, "After modify myrecipe contains:");
/****************擦除*****************/
myrecipe.erase(myrecipe.begin()); //通过位置
myrecipe.erase("milk"); //通过key
display(myrecipe, "After erase myrecipe contains:");
/****************交换*****************/
myrecipe.swap(mypantry);
display(myrecipe, "After swap with mypantry, myrecipe contains:");
/****************清空*****************/
myrecipe.clear();
display(myrecipe, "After clear, myrecipe contains:");
return 0;
}
输出：
found coffee is 10
After modify myrecipe contains:
baking powder: 0.3
eggs: 6
flour: 1.5
apple: 5
milk: 3
sugar: 0.8
salt: 0.1
coffee: 9
After erase myrecipe contains:
eggs: 6
flour: 1.5
apple: 5
sugar: 0.8
salt: 0.1
coffee: 9
After swap with mypantry, myrecipe contains:
milk: 2
flour: 1.5
After clear, myrecipe contains:
8.操作
find 通过给定主键查找元素,没找到：返回unordered_map::end
count 返回匹配给定主键的元素的个数
equal_range 返回值匹配给定搜索值的元素组成的范围
9.Buckets
bucket_count 返回槽（Bucket）数
max_bucket_count 返回最大槽数
bucket_size 返回槽大小
bucket 返回元素所在槽的序号
load_factor 返回载入因子，即一个元素槽（Bucket）的最大元素数
max_load_factor 返回或设置最大载入因子
rehash 设置槽数
reserve 请求改变容器容量
参考：[详细介绍C++STL：unordered_map](详细介绍C++STL：unordered_map - 朤尧 - 博客园 (cnblogs.com))
