C++ map begin()函数用于返回引用映射集合第一个元素的迭代器。
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
它返回一个指向映射第一个元素的迭代器。
我们来看一个begin()函数的简单示例。
#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
在上面的代码中,begin()函数用于返回指向mymap映射中第一个元素的迭代器。
让我们看一个简单的示例,使用for-each循环遍历映射。
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#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遍历映射。它将在每个map元素上进行迭代,并调用我们提供的回调。
让我们看一个使用while循环迭代映射的简单示例。
#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
在上面的代码中,begin()函数用于返回指向mymap映射中第一个元素的迭代器。
让我们看一个简单的例子:
#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。
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