map容器的一些方法说明

#include <stdio.h>#include <string>#include <map>#include <memory>using namespace std;int _tmain(int argc, _TCHAR* argv[]){map<int, int> dmap;dmap.insert(pair<int, int>(3, 1));dmap.insert(pair<int, int>(4, 1));dmap.insert(pair<int, int>(5, 1));dmap.insert(pair<int, int>(6, 1));//count函数计算map中指定key的元素有多少个printf("count key[%d], num: %d\n", 3, dmap.count(3));printf("count key[%d], num: %d\n", 9, dmap.count(9));//size函数计算map中的元素数量printf("%d\n", dmap.size());//max_size计算map中最多容纳的元素printf("%0x\n", dmap.max_size());//empty检查map是否为空printf("%d\n", dmap.empty());dmap.erase(dmap.begin(), dmap.end());printf("%d\n", dmap.empty());getchar();return 0;}

运行结果为：

count key[3], num: 1count key[9], num: 04aaaaaaa01

[]运算符：

#include <stdio.h>#include <string>#include <map>using namespace std;int _tmain(int argc, _TCHAR* argv[]){map<int, int> imap;map<int, string> smap;imap[1] = 1;smap[1] = "hello";printf("imap[%d] = %d\n", 1, imap[1]);printf("smap[%d] = %s\n", 1, smap[1].c_str());printf("imap[%d] = %d\n", 2, imap[2]);printf("smap[%d] = %s\n", 2, smap[2].c_str());getchar();return 0;}

运算结果为：

imap[1] = 1smap[1] = helloimap[2] = 0smap[2] =

#include <stdio.h>#include <string>#include <map>using namespace std;class A{public:A() { m_a = -1; }A(int a) { m_a = 100; }int m_a;};int _tmain(int argc, _TCHAR* argv[]){map<int, A> imap;A a = A(1);imap[1] = a;A b = imap[1];printf("%d\n", b.m_a);A c = imap[10];printf("%d\n", c.m_a);getchar();return 0;}

运算结果为：

100-1

结果分析：下标运算符可以在map中取得key对应的value，如果key不存在，会返回一个对应类型的默认值。

#include <stdio.h>#include <map>using namespace std;int _tmain(int argc, _TCHAR* argv[]){map<int, int> imap;int i = imap[1];map<int, int>::iterator iter = imap.begin();for (; iter != imap.end(); iter++)printf("%d, %d\n", iter->first, iter->second);getchar();return 0;}

运行结果为：

1, 0

分析：如果以key为键的元素并不在map中的话, [key]会导致在map中自动插入一个元素

find函数使用

#include <stdio.h>#include <string>#include <map>using namespace std;int _tmain(int argc, _TCHAR* argv[]){map<int, int> imap;imap[1] = 1;imap[2] = 4;imap[3] = 8;map<int, int>::iterator iter = imap.find(1);if (iter != imap.end())printf("iter->first: %d, iter->second: %d\n", iter->first, iter->second);elseprintf("iter end\n");iter = imap.find(10);if (iter != imap.end())printf("iter->first: %d, iter->second: %d\n", iter->first, iter->second);elseprintf("iter end\n");getchar();return 0;}

运行结果为：

iter->first: 1, iter->second: 1iter end

分析： find返回一个迭代器, 它指向查找的key对应的元素，如果key不存在，则返回map.end()

at函数

#include <stdio.h>#include <string>#include <map>#include <memory>using namespace std;int _tmain(int argc, _TCHAR* argv[]){map<int, int> imap;imap[1] = 1;imap[2] = 4;imap[3] = 8;int i = imap.at(1);      //返回1printf("%d\n", i);i = imap.at(10);         //出错，at方法中检测到访问不存在的key，会引发异常printf("%d\n", i);getchar();return 0;}

分析：at的用法和[]类似，不过[]访问不存在的key会返回类型空值，at则会程序崩溃

swap方法：

#include <stdio.h>#include <string>#include <map>using namespace std;int _tmain(int argc, _TCHAR* argv[]){map<int, int> imap1;map<int, int> imap2;imap1[1] = 1;imap1[2] = 2;imap2[3] = 3;imap2[4] = 4;imap1.swap(imap2);map<int, int>::iterator iter = imap1.begin();for (; iter != imap1.end(); ++iter)printf("iter->first: %d, iter->second: %d\n", iter->first, iter->second);printf("\n");iter = imap2.begin();for (; iter != imap2.end(); ++iter)printf("iter->first: %d, iter->second: %d\n", iter->first, iter->second);getchar();return 0;}

运行结果为：

iter->first: 3, iter->second: 3iter->first: 4, iter->second: 4iter->first: 1, iter->second: 1iter->first: 2, iter->second: 2

分析：swap可以对调两个map中的内容，但是有一个前提，就是两个map必须使用相同的模板类实例，上面程序如果map<double, double> imap2编译会出错，swap返回值为void

map的初始化和赋值

map<int, int> imap1;map<int, int> imap2(imap1);map<int, int> imap3(imap2.begin(), imap2.end());map<int, int> imap4 = imap1;map<int, int> imap5;imap5 = imap1;map<int, int> imap6({ pair<int, int>(1, 1), pair<int, int>(2, 2) }); //initializer_list方式初始化

排序：

#include <stdio.h>#include <string>#include <map>#include <vector>#include <algorithm>#include <memory>using namespace std;bool Comp(const pair<string, int>& a, const pair<string, int>& b){return a.second < b.second;}int _tmain(int argc, _TCHAR* argv[]){map<string, int> s_map;s_map["a"] = 19;s_map["b"] = 12;s_map["c"] = 10;//按key排序map<string, int>::iterator iter = s_map.begin();for (; iter != s_map.end(); ++iter)printf("%s: %d\n", iter->first.c_str(), iter->second);printf("\n");vector<pair<string, int>> s_vec(s_map.begin(), s_map.end());for (int i = 0; i < s_vec.size(); i++)printf("%s: %d\n", s_vec[i].first.c_str(), s_vec[i].second);printf("\n");//按值排序sort(s_vec.begin(), s_vec.end(), Comp);for (int i = 0; i < s_vec.size(); i++)printf("%s: %d\n", s_vec[i].first.c_str(), s_vec[i].second);printf("\n");getchar();return 0;}

运行结果如下：

a: 19b: 12c: 10a: 19b: 12c: 10c: 10b: 12a: 19