C++ map begin()
C++ Map begin()函数
C++映射 begin()函数用于返回引用映射容器第一个元素的迭代器。
语法
iterator begin(); //until C++ 11
const_iterator begin() const; //until C++ 11
iterator begin() noexcept; //since C++ 11
const_iterator begin() const noexcept; //since C++ 11
参数
无
返回值
它返回指向迭代器第一个元素的迭代器地图。
示例1
让我们看一个简单的begin()函数示例。
#include <iostream>
#include <map>
using namespace std;
int main ()
{
map<char,string> mymap;
mymap['b'] = "Java";
mymap['a'] = "C++";
mymap['c'] = "SQL";
// show content:
for (map<char,string>::iterator it=mymap.begin(); it!=mymap.end(); ++it)
cout << it->first << " => " << it->second << '\n';
return 0;
}
输出:
a => C++
b => Java
c => SQL
在上面，begin()函数用于返回指向mymap映射中第一个元素的迭代器。
示例2
看到一个简单的示例，使用for-each循环遍历地图。
#include <iostream>
#include <map>
#include <string>
#include <iterator>
#include <algorithm>
using namespace std;
int main() {
map<string, int> m;
m["Room1"] = 100;
m["Room2"] = 200;
m["Room3"] = 300;
// Create a map iterator and point to beginning of map
map<string, int>::iterator it = m.begin();
// Iterate over a map using std::for_each and Lambda function
for_each(m.begin(), m.end(),[](pair<string, int> element){
// Accessing KEY from element
string word = element.first;
// Accessing VALUE from element.
int count = element.second;
cout<<word<<" = "<<count<<endl;
});
return 0;
}
输出:
Room1 = 100
Room2 = 200
Room3 = 300
在上面的示例中，我们使用STL算法std :: for-each遍历地图。它将在每个map元素上进行迭代，并调用我们提供的回调。
示例3
让我们看一个使用while循环在地图上进行迭代的简单示例。
#include <iostream>
#include <map>
#include <string>
int main()
{
using namespace std;
map<int,string> mymap = {
{ 100, "Nikita"},
{ 200, "Deep" },
{ 300, "Priya" },
{ 400, "Suman" },
{ 500, "Aman" }};
cout<<"Elements are:" <<endl;
map<int, string>::const_iterator it; // declare an iterator
it = mymap.begin(); // assign it to the start of the vector
while (it != mymap.end()) // while it hasn't reach the end
{
cout << it->first << " = " << it->second << "\n";
// print the value of the element it points to
++it; // and iterate to the next element
}
cout << endl;
}
输出:
Elements are:
100: Nikita
200: Deep
300: Priya
400: Suman
500: Aman
在上面，begin()函数用于返回指向mymap映射中第一个元素的迭代器。
示例4
看一个简单的例子:
#include <iostream>
#include <string>
#include <map>
using namespace std;
int main ()
{
map<int,int> mymap = {
{ 10, 10},
{ 20, 20 },
{ 30, 30 } };
cout<<"Elements are:" <<endl;
for (auto it = mymap.begin(); it != mymap.end(); ++it)
cout << it->first
<< " + "
<< it->second
<< " = "
<<it->first + it->second
<< '\n';
auto ite = mymap.begin();
cout << "The first element is: ";
cout << "{" << ite->first << ", "
<< ite->second << "}\n";
return 0;
}
输出:
Elements are:
10 + 10 = 20
20 + 20 = 40
30 + 30 = 60
The first element is: {10, 10}
在上面的示例中，begin()函数用于返回指向mymap映射中第一个元素的迭代器。
