如何遍历/迭代STL地图? [英] How to traverse/iterate STL Map?
问题描述
需要知道如何遍历stl地图。我不想用它的钥匙。我不在乎排序,只是一种方法来访问它包含的所有元素。有办法吗?
当然。这是用于遍历标准库集合的基本方法:
C ++ 03 / C ++ 11:
#include< cstdlib>
#include< map>
#include< string>
using namespace std;
int main()
{
typedef map 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;
}
}
如果需要修改元素,请使用 iterator
而不是 const_iterator
,而不是复制值从迭代器,获得一个引用和修改值通过:
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;
}
这是你通常用手遍历容器的方法。最大的区别是对于映射
类型 * it
是 code>而不是元素本身
C ++ 11
一个C ++ 11编译器(例如,最新的GCC, - std = c ++ 11
或MSVC),那么你也有其他选择。
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;
}
}
其次,您也可以使用lambdas。结合 decltype
,这可能会导致更清晰的代码(虽然有折衷):
#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还引入了范围基础的概念 for
循环,你可以认为它类似于其他语言。但是,一些编译器不完全支持这一点,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;
}
}
In need to know how to traverse an stl map. I don't want to use its key. I don't care about the ordering, just a way to access all elements it contains. Is there a way to do this?
Sure. This is the basic method used to traverse Standard Library collection:
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, use iterator
rather than const_iterator
, and 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";
}
This is how you typically traverse containers by hand. The big difference is that for a map
the type of *it
is a pair
rather than the element itself
C++11
If you have the benefit of a C++11 compiler (for example, latest GCC with --std=c++11
or MSVC), then you have other options as well.
First you can make use of the auto
keyword to get rid of all that nasty verbosity:
#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):
#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 for
loop, which you may recognize as similar to other languages. However, some compilers do not fully support this yet -- notably, 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;
}
}
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