C++ multimap begin()
C++ multimap begin()函数
C++ multimap begin()函数用于返回引用 multimap 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
复杂度
常量。
迭代器有效性
没有变化。
数据争用
已访问容器。
异常安全
此函数从不抛出异常。
示例1
让我们看一下begin()函数的简单示例:
#include <iostream>
#include <map>
using namespace std;
int main ()
{
multimap<char,string> mymultimap;
mymultimap = {
{'a',"Java"},
{'b', "C++"},
{'b', "Python"},
{'a', "Android"}
};
// show content:
for (multimap<char,string>::iterator it=mymultimap.begin(); it!=mymultimap.end(); ++it)
cout << it->first << " => " << it->second << '\n';
return 0;
}
输出:
a => Java
a => Android
b => C++
b => Python
在上面的示例中，begin()函数用于返回指向mymultimap set 映射中第一个元素的迭代器。
示例2
让我们看一个简单的示例，使用for-each循环遍历multimap:
#include <iostream>
#include <map>
#include <string>
#include <iterator>
#include <algorithm>
using namespace std;
int main() {
multimap<string, int> m;
m= {
{"Room1", 100},
{"Room2", 200},
{"Room1", 300},
{"Room1", 100}
};
// Create a multimap iterator and point to beginning of multimap
multimap<string, int>::iterator it = m.begin();
// Iterate over a multimap 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
Room1 = 300
Room1 = 100
Room2 = 200
在上面的示例中，我们对每个映射使用STL算法来迭代 multimap 。它将在每个multimap元素上进行迭代，并调用我们提供的回调。
示例3
让我们看一个简单的示例，使用while来迭代multimap。循环:
#include <iostream>
#include <map>
#include <string>
int main()
{
using namespace std;
multimap<int,string> mymultimap = {
{ 100, "Nikita"},
{ 200, "Deep" },
{ 300, "Priya" },
{ 200, "Suman" },
{ 100, "Aman" }};
cout<<"Elements are: "<<endl;
multimap<int, string>::const_iterator it; // declare an iterator
it = mymultimap.begin(); // assign it to the start of the vector
while (it != mymultimap.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
100 = Aman
200 = Deep
200 = Suman
300 = Priya
在上面，begin()函数用于返回指向mymultimap set 映射中第一个元素的迭代器。
示例4
让我们看一个简单的例子:
#include <map>
#include <iostream>
int main()
{
using namespace std;
multimap <int, int> m1;
multimap <int, int> :: iterator m1_Iter;
typedef pair <int, int> Int_Pair;
m1.insert ( Int_Pair ( 0, 0 ) );
m1.insert ( Int_Pair ( 1, 1 ) );
m1.insert ( Int_Pair ( 2, 4 ) );
m1_Iter = m1.begin ();
cout << "The first element of m1 is " << m1_Iter-> first << endl;
m1_Iter = m1.begin ();
m1.erase ( m1_Iter );
m1_Iter = m1.begin();
cout << "First element of m1 is now " << m1_Iter-> first << endl;
}
输出:
The first element of m1 is 0
First element of m1 is now 1
在上面的示例中，begin()函数用于返回指向mymultimap set 映射中第一个元素的迭代器。
