std::map partial match for the key

Question

I have an std::map and I want to search for a key using a substring. For exampe

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

typedef std::map<std::string, std::string> TStrStrMap;
typedef std::pair<std::string, std::string> TStrStrPair;

int main(int argc, char *argv[])
{
        TStrStrMap tMap;

        tMap.insert(TStrStrPair("John", "AA"));
        tMap.insert(TStrStrPair("Mary", "BBB"));
        tMap.insert(TStrStrPair("Mother", "A"));
        tMap.insert(TStrStrPair("Marlon", "C"));


        return 0;
}

I want to search for the position that holds the substring "Marl" and not "Marlon". Is it possible? How?

EDIT: no boost libraries!

Answer 1

When your substring is a prefix as in your example, you can use lower_bound to search for "Marl".

    map<string,string>::const_iterator m = tMap.lower_bound("Marl");
    cerr << (*m).second << endl;

This does not work for non-prefix substrings: in the general case, searching a map is not much different from searching other containers.

Answer 2

You can't efficiently search for substring, but you can for prefix:

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

typedef map<string, string> TStrStrMap;
typedef pair<string, string> TStrStrPair;

TStrStrMap::const_iterator FindPrefix(const TStrStrMap& map, const string& search_for) {
    TStrStrMap::const_iterator i = map.lower_bound(search_for);
    if (i != map.end()) {
        const string& key = i->first;
        if (key.compare(0, search_for.size(), search_for) == 0) // Really a prefix?
            return i;
    }
    return map.end();
}

void Test(const TStrStrMap& map, const string& search_for) {
    cout << search_for;
    auto i = FindPrefix(map, search_for);
    if (i != map.end())
        cout << '\t' << i->first << ", " << i->second;
    cout << endl;
}

int main(int argc, char *argv[])
{
    TStrStrMap tMap;

    tMap.insert(TStrStrPair("John", "AA"));
    tMap.insert(TStrStrPair("Mary", "BBB"));
    tMap.insert(TStrStrPair("Mother", "A"));
    tMap.insert(TStrStrPair("Marlon", "C"));

    Test(tMap, "Marl");
    Test(tMap, "Mo");
    Test(tMap, "ther");
    Test(tMap, "Mad");
    Test(tMap, "Mom");
    Test(tMap, "Perr");
    Test(tMap, "Jo");

    return 0;
}

This prints:

Marl    Marlon, C
Mo      Mother, A
ther
Mad
Mom
Perr
Jo      John, AA

Answer 3

To search for a substring of a key in a map you have no choice but to either use a new map on a special kind of key type or to search your map in O(n). std::map uses (by default) operator<() for ordering keys and for searching, and that compare function for std::string is a plain lexicographical compare.

If you create a new map on a special key type that has operator<() compare on basis of a substring take note that this will also affect the decision of whether a new element to insert would be a duplicate. In other words, such a map will only have elements that are not substrings of each other.

The O(n) search practically means you use std::find() over the map, with a custom predicate that takes a std::pair<std::string,std::string> and returns true if the second element of the pair is a substring of the first.

Answer 4

typedef TStrStrMap::value_type map_value_type;

struct key_contains_substring
   : std::binary_function<map_value_type, std::string, bool>
{
    bool operator()(const map_value_type& map_value, const std::string& substr)
    {
         return std::search(map_value.first.begin(), map_value.first.end(),
                    substr.begin(), substr.end() != map_value.first.end());  
    }
};

...


TStrStrMap::const_iterator it = std::find_if(tMap.begin(), tMap.end(), 
        std::bind2nd(key_contains_substring(), "Marl");