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# 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,

See map.

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

#if 0

{

for (auto i = -1; i < 8; ++i) {

std::cout << i << std::endl;

std::map<int,std::function<void()>> m;

m.emplace(0, [](){

std::cout << "too small" << std::endl;

});

m.emplace(2, [](){

std::cout << "zero" << std::endl;

});

m.emplace(5, [](){

std::cout << "two" << std::endl;

});

m.emplace(7, [](){

std::cout << "five" << std::endl;

});

auto it = m.upper_bound(i);

if (it == m.end()) {

std::cout << "too large" << std::endl;

} else {

it->second();

}

std::cout << std::endl;

}

#endif

}

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