cpp-cheat/cpp/map.cpp

/*
# map

    http://www.cplusplus.com/reference/map/map/

    Also comes in an unordered version `unordered_map`.

    Ordered.

    Also comes in an multiple value input version `multimap`.

    Does not require a hash function. Usually implemented as a self balancing tree such as a rb tree.

# hashmap

    There seems to be no explicit hashmap container, only a generic map interface,

    However unordered_map is likely to be hashmap based.

    A nonstandard `hash_map` already provided with gcc and msvc++.
    It is placed in the `std::` namespace, but it is *not* ISO.
*/

#include "common.hpp"

template <class K, class V>
std::string map_to_str(std::map<K,V> map) {
    std::stringstream result;
    for (auto& pair : map) {
        result << pair.first << ":" << pair.second << ", ";
    }
    return result.str();
}

int main() {
    /*
    Create initialized map.
    */
    {
        std::map<int,std::string> m{
            {0, "zero"},
            {1, "one"},
        };
        assert(m[0] == "zero");
        assert(m[1] == "one");
    }

    /*
    emplace

        Put a value pair into the map without creating the pair explicitly.

        Only inserts if not already present.

        Needs gcc 4.8: http://stackoverflow.com/questions/15812276/stdset-has-no-member-emplace
    */
    {
        std::map<int,std::string> m;
        assert((m.emplace(0, "zero").second));
        assert((m.emplace(1, "one").second));
        assert(!(m.emplace(1, "one2").second));
        assert(m[0] == "zero");
        assert(m[1] == "one");
    }

    /*
    # operator[]

        get value from a given key.
    */
    {
        std::map<int,std::string> m{
            {0, "zero"},
            {1, "one"},
        };
        assert(m[0] == "zero");
        assert(m[1] == "one");

        // Returns a reference that can override the value.
        m[1] = "one2";
        assert(m[1] == "one2");

        // WARNING: if the key does not exist, it is inserted with a value with default constructor.
        // This can be avoided by using `find` instead of `[]`.
        // Inserts `(2,"")` because `""` is the value for the default String constructor.
        // http://stackoverflow.com/questions/10124679/what-happens-if-i-read-a-maps-value-where-the-key-does-not-exist
        {
            assert(m[2] == "");
            assert(m.size() == 3);

            // This behaviour is however very convenient for nested containers.
            {
                std::map<int,std::map<int,int>> m;
                // Create the empty map at m[0], and immediately add a (0,0) pair to it.
                m[0][0] = 0;
                // map at m[0] already exists, now just add a new (1, 1) pair to it.
                m[0][1] = 1;
                m[1][0] = 2;
                assert(m[0][0] == 0);
                assert(m[0][1] == 1);
                assert(m[1][0] == 2);
            }
        }
    }

    /*
    # find

    # check if in map

        Similar to `std::set` find with respect to the keys:
        returns an iterator pointing to the pair which has given key, not the value.

        If not found, returns `map::end()`

        This is preferable to `[]` since it does not insert non-existent elements.
    */
    {
        std::map<int,std::string> m{
            {0, "zero"},
            {1, "one"},
        };

        auto found = m.find(0);
        assert(found != m.end());
        assert(found->second == "zero");

        assert(m.find(2) == m.end());
        assert(m.size() == 2);

        /*
        Get default provided value if key not present

        TODO: any less verbose way than finding and check != end? Like:

            m.get(key, default)

        I know I can define a helper, but come on...
        */
        {
            std::map<int,int> m{};
            int default_ = 42;
            int result;
            auto f = m.find(1);
            if (f == m.end()) {
                result = default_;
            } else {
                result = f->second;
            }
        }
    }

    /*
    # insert

        Insert pair into map.

        The return value is similar to that of a set insertion with respect to the key.

        Just use emplace instead, less verbose as it was added after perfect forwarding.

        http://stackoverflow.com/questions/17172080/insert-vs-emplace-vs-operator-in-c-map
    */
    {
        std::map<int,std::string> m;
        std::pair<std::map<int,std::string>::iterator,bool> ret;

        ret = m.insert(std::make_pair(0, "zero"));
        assert(ret.first == m.find(0));
        assert(ret.second == true);

        ret = m.insert(std::make_pair(1, "one"));
        assert(ret.first == m.find(1));
        assert(ret.second == true);

        //key already present
        ret = m.insert(std::make_pair(1, "one2"));
        assert(m[1] == "one");
        assert(ret.first == m.find(1));
        assert(ret.second == false);
    }

    /*
    # iterate

        Map is ordered:
        http://stackoverflow.com/questions/7648756/is-the-order-of-iterating-through-stdmap-known-and-guaranteed-by-the-standard

        It is iterated in key `<` order.

        So this basically requires implementations to use balanced
        trees intead of hashmap.

        Iteration returns key value pairs.
    */
    {
        std::map<int,std::string> m{
            {1, "one"},
            {0, "zero"},
        };

        int i = 0;
        int is[] = {0, 1};
        for (auto& im : m) {
            assert(im.first == is[i]);
            //cout << im->second << endl;
            ++i;
        }
        assert(i == 2);
        assert(map_to_str(m) == "0:zero, 1:one, ");
    }

    /*
    # erase

        Remove element from map.
    */
    {
        // key version. Returns number of elements removed (0 or 1).
        {
            std::map<int,std::string> m{
                {0, "zero"},
                {1, "one"},
            };
            int ret;
            ret = m.erase(1);
            assert(ret = 1);
            assert((m == std::map<int,std::string>{{0, "zero"}}));

            ret = m.erase(1);
            assert(ret == 0);
        }

        // iterator version. Returns iterator to next element.
        // Does not invalidate other iterators.
        // http://stackoverflow.com/questions/6438086/iterator-invalidation-rules
        {
            std::map<int,std::string> m{
                {0, "zero"},
                {1, "one"},
            };
            auto itNext = m.find(1);
            auto it = m.find(0);
            assert(m.erase(it) == itNext);
            assert((m == std::map<int,std::string>{{1, "one"}}));
        }
    }

    // # Range switch case.
    // http://stackoverflow.com/questions/9432226/how-do-i-select-a-range-of-values-in-a-switch-statement/42331563#42331563
    {
		auto result = std::map<int, int>{
			{-1, -1},
			{ 0,  0},
			{ 1,  0},
			{ 2,  2},
			{ 3,  2},
			{ 4,  2},
			{ 5,  5},
			{ 6,  5},
			{ 7,  7},
		};
		std::map<int,std::function<void()>> m;
		int x;
		m.emplace(0, [&](){
			x = -1;
		});
		m.emplace(2, [&](){
			x = 0;
		});
		m.emplace(5, [&](){
			x = 2;
		});
		m.emplace(7, [&](){
			x = 5;
		});
        for (auto i = -1; i < 8; ++i) {
            auto it = m.upper_bound(i);
            if (it == m.end()) {
            	x = 7;
            } else {
                it->second();
            }
            assert(x == result[i]);
        }
    }
}