C++ std::map<std::string, int> 获取键以特定字符串开头的值

Question

I'm using std::map in such a way:

#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;
}

http://ideone.com/5YPQmj

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"?

Answer 1

I found a similar answer check out this page: ( map complex find operation )

Code Exert:

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

This example is slightly different but should server as a good building block

Answer 2

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)
{
    if(s.length() < key_.length())
        return false;

    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.

Answer 3

you can use Boost filter iterator which 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)

For example:

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();