C++ 如何遍历/迭代 STL 映射？

How can I traverse/iterate an STL map?

提问by atoMerz

I want to traverse an STL map. I don't want to use its key. I don't care about the ordering, I just look for a way to access all elements it contains. How can I do this?

我想遍历 STL 映射。我不想使用它的钥匙。我不在乎顺序，我只是在寻找一种方法来访问它包含的所有元素。我怎样才能做到这一点？

回答by John Dibling

Yes, you can traverse a Standard Library map. This is the basic method used to traverse a map, and serves as guidance to traverse any Standard Library collection:

是的，您可以遍历标准库map。这是用于遍历 a 的基本方法map，并作为遍历任何标准库集合的指南：

C++03/C++11:

C++03/C++11：

#include <cstdlib>
#include <map>
#include <string>
using namespace std;

int main()
{
    typedef map<int,string> MyMap;
    MyMap my_map;
    // ... magic

    for( MyMap::const_iterator it = my_map.begin(); it != my_map.end(); ++it )
    {
      int key = it->first;
      string value = it->second;
    }
}

If you need to modify the elements:

如果需要修改元素：

• Use iteratorrather than const_iterator.

• Instead of copying the values out of the iterator, get a reference and modify the values through that.

  for( MyMap::iterator it = my_map.begin(); it != my_map.end(); ++it ) { int key = it->first; string& value = it->second; if( value == "foo" ) value = "bar"; }

• 使用iterator而不是const_iterator.

• 不是从迭代器中复制值，而是获取引用并通过它修改值。

  for( MyMap::iterator it = my_map.begin(); it != my_map.end(); ++it ) { int key = it->first; 字符串&值=它->秒；if( value == "foo" ) value = "bar"; }

This is how you typically traverse Standard Library containers by hand. The big difference is that for a mapthe type of *itis a pairrather than the element itself

这就是您通常手动遍历标准库容器的方式。最大的区别在于，对于 amap的类型*it是 apair而不是元素本身

C++11

C++11

If you have the benefit of a C++11 compiler (for example, latest GCC with --std=c++11or MSVC), then you have other options as well.

如果您受益于 C++11 编译器（例如，最新的 GCC--std=c++11或 MSVC），那么您还有其他选择。

First you can make use of the autokeyword to get rid of all that nasty verbosity:

首先，您可以使用auto关键字来摆脱所有令人讨厌的冗长：

#include <cstdlib>
#include <map>
#include <string>
using namespace std;

int main()
{
    map<int,string> my_map;
    // ... magic

    for( auto it = my_map.begin(); it != my_map.end(); ++it )
    {
      int key = it->first;
      string& value = it->second;
    }
}

Second, you can also employ lambdas. In conjunction with decltype, this might result in cleaner code (though with tradeoffs):

其次，您还可以使用 lambda。与 结合使用decltype，这可能会产生更清晰的代码（尽管需要权衡）：

#include <cstdlib>
#include <map>
#include <string>
#include <algorithm>
using namespace std;

int main()
{
    map<int,string> my_map;
    // ... magic

    for_each(my_map.begin(), my_map.end(), [](decltype(*my_map.begin()) val)
    {
        string& value = val.second;
        int key = val.first;
    });
}

C++11 also instroduces the concept of a range-bases forloop, which you may recognize as similar to other languages. However, some compilers do not fully support this yet -- notably, MSVC.

C++11 还引入了范围基for循环的概念，您可能认为它与其他语言类似。然而，一些编译器还没有完全支持这一点——特别是 MSVC。

#include <cstdlib>
#include <map>
#include <string>
#include <algorithm>
using namespace std;

int main()
{
    map<int,string> my_map;
    // ... magic

    for(auto val : my_map )
    {
        string& value = val.second;
        int key = val.first;
    }
}

回答by fredoverflow

As with any STL container, the begin()and end()methods return iterators that you can use to iterate over the map. Dereferencing a map iterator yields a std::pair<const Key, Value>.

与任何 STL 容器一样，begin()和end()方法返回可用于迭代映射的迭代器。取消引用映射迭代器会产生一个std::pair<const Key, Value>.

回答by vitaut

You can traverse STL mapin the same way as any other STL container: using iterators, e.g.

您可以以与任何其他 STL 容器相同的方式遍历STL 映射：使用迭代器，例如

for (std::map<key, value>::const_iterator
     i = myMap.begin(), end = myMap.end(); i != end; ++i)
{
    // *i is a key-value pair
}

回答by honk

C++17

C++17

Since C++17you can use range-based for loopstogether with structured bindingsfor iterating over a map. The resulting code, e.g. for printing all elements of a map, is short and well readable:

从C++17 开始，您可以使用基于范围的 for 循环和结构化绑定来迭代映射。生成的代码（例如用于打印地图的所有元素）简短且易读：

std::map<int, std::string> m{ {3, "a"}, {5, "b"}, {9, "c"} };

for (const auto &[k, v] : m)
    std::cout << "m[" << k << "] = " << v << std::endl;

Output:

输出：

m[3] = a
m[5] = b
m[9] = c

m[3] = a
m[5] = b
m[9] = c

Code on Coliru

Coliru 上的代码

回答by rashedcs

You can iterate map by using auto iterator.

您可以使用自动迭代器迭代地图。

Code Snippet:

代码片段：

#include<bits/stdc++.h>
using namespace std;

int main()
{
      ios::sync_with_stdio(false);
      map<string, int> mp;

      mp["a"]=500;
      mp["b"]=200;
      mp["d"]=300;
      mp["c"]=400;

      for(auto it=mp.begin(); it != mp.end(); it++)
      {
         cout<<it->first <<" : "<<it->second<<endl;
      }
      return 0;
}

回答by Harsh Sharma

Using forwith autofor C++11 and above usage

使用forwith autofor C++11 及以上用法

map<int,int> map_variable; //you can use any data type for keys, as well as value

for(auto &x:map_variable)
{ 
    cout<<x.first ;// gives the key
    cout<<x.second; //gives the value
}

The newer format of forusing autowas introduced in C++11

在 C++11 中引入了新的forusing格式auto

To give it functionality like some higher level languages like python

赋予它像 Python 等高级语言一样的功能

Where there was already an implementation of such type of iteration

已经实现了这种类型的迭代的地方

P.S. : map variable keeps values sorted, so when iterating you will get keys in sorted order

PS：映射变量保持值排序，因此在迭代时，您将按排序顺序获得键