C++ std::map<std::string, int> 获取键以特定字符串开头的值
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C++ std::map<std::string, int> get values whose key starts with a particular string
提问by Dejwi
I'm using std::map in such a way:
我以这样的方式使用 std::map ：
#include <map>
#include <string>
#include <iostream>
using namespace std;
int main(int argc, char* argv[])
{
map<string, int> my_map;
my_map.insert(pair<string, int>("Ab", 1));
my_map.insert(pair<string, int>("Abb", 2));
my_map.insert(pair<string, int>("Abc", 3));
my_map.insert(pair<string, int>("Abd", 4));
my_map.insert(pair<string, int>("Ac", 5));
my_map.insert(pair<string, int>("Ad", 5));
cout<<my_map.lower_bound("Ab")->second<<endl;
cout<<my_map.upper_bound("Ab")->second<<endl;
return 0;
}
I'd like to get all values whose key starts with a particular string (for example "Ab"). I can easily get the begin iterator using map::lower_bound. But how can I get an upper bound? Do I have to iterate the whole set starting at lower bound and check every key if it still starts with "Ab"?
我想获取其键以特定字符串开头的所有值（例如“Ab”）。我可以使用 map::lower_bound 轻松获取开始迭代器。但是我怎样才能得到一个上限呢？我是否必须从下限开始迭代整个集合并检查每个键是否仍以“Ab”开头？
回答by Matt Stokes
I found a similar answer check out this page: (map complex find operation)
我在这个页面找到了一个类似的答案：（地图复杂的查找操作）
Code Exert:
代码执行：
template<typename Map> typename Map::const_iterator
find_prefix(Map const& map, typename Map::key_type const& key)
{
typename Map::const_iterator it = map.upper_bound(key);
while (it != map.begin())
{
--it;
if(key.substr(0, it->first.size()) == it->first)
return it;
}
return map.end(); // map contains no prefix
}
It looks as if in this example you iterate from the upper_bound backwards till the beginning looking for the specific substring
看起来好像在这个例子中你从 upper_bound 向后迭代直到开始寻找特定的子字符串
This example is slightly different but should server as a good building block
这个例子略有不同，但应该作为一个很好的构建块
回答by lucas92
class BeginWithKey
{
public:
BeginWithKey(const string key);
bool operator()(const string& s,const int x);
private:
const string& key_;
};
BeginWithKey::BeginWithKey(const string key):key_(key)
{
}
bool BeginWithKey::operator()(const string& s, const int& rh)
{
bool begin = true;
for(int i = 0; i < key_.size() && begin; ++i)
begin = (s[i] == key_[i]);
return !begin;
}
int main()
{
//your code
//copying the map object
map<string, int> copy = my_map;
//removing the strings not beginning with abc
BeginWithKey func("abc");
remove_if(copy.begin(), copy.end(), func);
return 0;
}
The code will work with any string key.
该代码适用于任何字符串键。
回答by Roee Gavirel
you can use Boost filter iteratorwhich give you a "begin" and a "end" iterator from normal iterators when they given a predicate (a bool function which says which values to include)
您可以使用Boost 过滤器迭代器，当它们给出谓词时，它会为您提供来自普通迭代器的“开始”和“结束”迭代器（一个布尔函数，它说明要包含哪些值）
For example:
例如：
template <class Predicate>
boost::filter_iterator<Predicate, map<string,int>::const_iterator> begin(Predicate predicate) const
{
return boost::make_filter_iterator(predicate, my_map.begin(), my_map.end());
}
template <class Predicate>
boost::filter_iterator<Predicate, map<string,int>::const_iterator> end(Predicate predicate) const
{
return boost::make_filter_iterator(predicate, my_map.end(), my_map.end());
}
struct isMatch
{
isMatch(const std::string prefix) {m_prefix = prefix;};
bool operator()(std::string value)
{
return value.find_first_of(m_prefix) == 0;
};
std::string m_prefix;
};
//using:
isMatch startWithAb("Ab");
auto myBegin = boost::filter_iterator<startWithAb> begin();
auto myEnd = boost::filter_iterator<startWithAb> end();
