C ++映射 operator [] strong>函数用于通过给定的键值 strong>访问映射中的元素。
它类似于 at（） strong>函数。它们之间的唯一区别是，如果映射中不存在所访问的密钥，则抛出异常；反之，如果密钥中不存在该密钥，则 operator [] strong>会将密钥插入映射中。地图。
句法
考虑键值 k strong>，语法为：
mapped_type& operator[] (const key_type& k); //until C++ 11 mapped_type& operator[] (const key_type& k); //since C++ 11 mapped_type& operator[] (key_type&& k); //since C++ 11
范围
k strong>：访问其映射值的元素的键值。
返回值
它使用键值返回对元素映射值的引用。
例子1
让我们看一个访问元素的简单示例。
#include <iostream>
#include <map>
using namespace std;
int main()
{
map<char, int> m = {
{a, 1},
{b, 2},
{c, 3},
{d, 4},
{e, 5},
};
cout << "Map contains following elements" << endl;
cout << "m[a] = " << m[a] << endl;
cout << "m[b] = " << m[b] << endl;
cout << "m[c] = " << m[c] << endl;
cout << "m[d] = " << m[d] << endl;
cout << "m[e] = " << m[e] << endl;
return 0;
}
输出 strong>：
Map contains following elements m[a] = 1 m[b] = 2 m[c] = 3 m[d] = 4 m[e] = 5
在上面，operator []函数用于访问map的元素。
例子2
让我们看一个简单的示例，使用它们的键值添加元素。
#include <iostream>
#include <string>
#include <map>
using namespace std;
int main ()
{
map<int,string> mymap = {
{ 101, "" },
{ 102, "" },
{ 103, ""} };
mymap[101] = "Java";
mymap[102] = "T";
mymap[103] = "Point";
// prints value associated with key 101, i.e. Java
cout<<mymap[101];
// prints value associated with key 102, i.e T
cout<<mymap[102];
// prints value associated with key 103, i.e Point
cout<<mymap[103];
return 0;
}
输出 strong>：
JavaTPoint
在上面的示例中，operator []用于在初始化后使用关联的键值添加元素。
例子3
让我们看一个简单的示例，以更改与键值关联的值。
#include <iostream> #include <string> #include <map>using namespace std;
int main () { map<int,string> mymap = { { 100, "Nikita"}, { 200, "Deep" }, { 300, "Priya" }, { 400, "Suman" }, { 500, "Aman" }};
cout<<"Elements are:" <<endl; for (auto& x: mymap) { cout << x.first << ": " << x.second << ; }
mymap[100] = "Nidhi"; // changes the value associated with key 100 to Nidhi mymap[300] = "Pinku"; // changes the value associated with key 300 to Pinku mymap[500] = "Arohi"; // changes the value associated with key 500 to Arohi
cout<<"\nElements after make changes are:" <<endl; for (auto& x: mymap) { cout << x.first << ": " << x.second << ; }
return 0; }
输出 strong>：
Elements are: 100: Nikita 200: Deep 300: Priya 400: Suman 500: AmanElements after make changes are: 100: Nidhi 200: Deep 300: Pinku 400: Suman 500: Arohi
在上面的示例中，operator []函数用于更改与其键值关联的值。
例子4
让我们看一个简单的例子来区分operator []和at（）。
#include <iostream>
#include <string>
#include <map>
using namespace std;
int main ()
{
map<char,string> mp = {
{ a,"Java"},
{ b, "C++" },
{ c, "Python" }};
cout<<endl<<mp[a];
cout<<endl<<mp[b];
cout<<endl<<mp[c];
mp[d] = "SQL";
/* since there is no key with value d in the map,
it insert a key-value pair in map with key d and value = "SQL" */
cout<<endl<<mp[d];
try {
mp.at(z);
// since there is no key with value z in the map, it throws an exception
} catch(const out_of_range &e) {
cout<<endl<<"\nOut of Range Exception at "<<e.what();
}
return 0;
}
输出 strong>：
Java C++ Python SQLOut of Range Exception at map::at
在上面的示例中，当我们使用at（）函数时，由于在映射中不存在带有值z的键，并且当我们使用operator []并在键值d中添加元素时，由于没有键，它会引发out_of_range异常在地图中值为" d"时，它将在地图中插入具有键" d"和值为" SQL"的键-值对。
