I am trying to create an unordered_map to map pairs with integers:
#include <unordered_map>
using namespace std;
using Vote = pair<string, string>;
using Unordered_map = unordered_map<Vote, int>;
I have a class where I have declared an Unordered_map as a private member.
However, I am getting the following error when I try to compile it:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/type_traits:948:38: Implicit instantiation of undefined template 'std::__1::hash, std::__1::basic_string > >'
I am not getting this error if I use a regular map like map<pair<string, string>, int> instead of an unordered_map.
Is it not possible to use pair as key in unordered maps?
You need to provide a suitable hash function for your key type. A simple example:
#include <unordered_map>
#include <functional>
#include <string>
#include <utility>
// Only for pairs of std::hash-able types for simplicity.
// You can of course template this struct to allow other hash functions
struct pair_hash {
template <class T1, class T2>
std::size_t operator () (const std::pair<T1,T2> &p) const {
auto h1 = std::hash<T1>{}(p.first);
auto h2 = std::hash<T2>{}(p.second);
// Mainly for demonstration purposes, i.e. works but is overly simple
// In the real world, use sth. like boost.hash_combine
return h1 ^ h2;
}
};
using Vote = std::pair<std::string, std::string>;
using Unordered_map = std::unordered_map<Vote, int, pair_hash>;
int main() {
Unordered_map um;
}
This will work, but not have the best hash-properties†. You might want to have a look at something like boost.hash_combine for higher quality results when combining the hashes. This is also discussed in greater detail – including the aforementioned solution from boost – in this answer.
For real world use: Boost also provides the function set hash_value which already provides a hash function for std::pair, as well as std::tuple and most standard containers.
†More precisely, it will produce too many collisions. E.g., every symmetric pair will hash to 0 and pairs that differ only by permutation will have the same hash. This is probably fine for your programming exercise, but might seriously hurt performance of real world code.
My preferred way of solving this problem is to define a key function that transforms your pair into a unique integer (or any hashable data type). This key is not the hash key. It is the unique ID of the pair of data that will then be optimally hashed by the unordered_map. For example, you wanted to define an unordered_map of the type
unordered_map<pair<int,int>,double> Map;
And you want to use Map[make_pair(i,j)]=value or Map.find(make_pair(i,j)) to operate on the map. Then you'll have to tell the system how to hash a pair of integers make_pair(i,j). Instead of that, we can define
inline size_t key(int i,int j) {return (size_t) i << 32 | (unsigned int) j;}
and then change the type of the map to
unordered_map<size_t,double> Map;
We can now use Map[key(i,j)]=value or Map.find(key(i,j)) to operate on the map. Every make_pair now becomes calling the inline key function.
This method guarantees that the key will be optimally hashed, because now the hashing part is done by the system, which will always choose the internal hash table size to be prime to make sure every bucket is equally likely. But you have to make yourself 100% sure that the key is unique for every pair, i.e., no two distinct pairs can have the same key, or there can be very difficult bugs to find.
If using pair is not a strict requirement, you can simply use map twice.
#include <unordered_map>
using namespace std;
using Unordered_map = unordered_map<string, unordered_map<string, int>>;
Unordered_map um;
um["Region1"]["Candidate1"] = 10;
cout << um["Region1"]["Candidate1"]; // 10
For pair key, we can use boost pair hash function:
#include <iostream>
#include <boost/functional/hash.hpp>
#include <unordered_map>
using namespace std;
int main() {
unordered_map<pair<string, string>, int, boost::hash<pair<string, string>>> m;
m[make_pair("123", "456")] = 1;
cout << m[make_pair("123", "456")] << endl;
return 0;
}
Similarly we can use boost hash for vectors,
#include <iostream>
#include <boost/functional/hash.hpp>
#include <unordered_map>
#include <vector>
using namespace std;
int main() {
unordered_map<vector<string>, int, boost::hash<vector<string>>> m;
vector<string> a({"123", "456"});
m[a] = 1;
cout << m[a] << endl;
return 0;
}
Reference: C++ Standard Library: A tutorial and reference, Second version Chapter 7.9.2: Creating and Controlling unordered Container
All solutions I found in Google use XOR to generate hashcode of pair, which is totally bad. see why-is-xor-the-default-way-to-combine-hashes. However, the book has given us the best solution, using hash_combine, which is taken from Boost. The solution is much better than XOR when I tested it in Online Judge(Atcoder). I organized the code as a template as follow. You can copy and paste it as much as you can. And it is convenient to change it to fit any custom struct/class.
Update: add hash template for tuple.
#include <functional>
namespace hash_tuple {
template <typename TT> struct hash {
size_t operator()(TT const &tt) const { return std::hash<TT>()(tt); }
};
// from boost (functional/hash):
// see http://www.boost.org/doc/libs/1_35_0/doc/html/hash/combine.html template
template <class T> inline void hash_combine(std::size_t &seed, T const &v) {
seed ^= hash_tuple::hash<T>()(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}
// Recursive template code derived from Matthieu M.
template <class Tuple, size_t Index = std::tuple_size<Tuple>::value - 1>
struct HashValueImpl {
void operator()(size_t &seed, Tuple const &tuple) const {
HashValueImpl<Tuple, Index - 1>{}(seed, tuple);
hash_combine(seed, std::get<Index>(tuple));
}
};
template <class Tuple> struct HashValueImpl<Tuple, 0> {
void operator()(size_t &seed, Tuple const &tuple) const {
hash_combine(seed, std::get<0>(tuple));
}
};
template <typename... TT> struct hash<std::tuple<TT...>> {
size_t operator()(std::tuple<TT...> const &tt) const {
size_t seed = 0;
HashValueImpl<std::tuple<TT...>>{}(seed, tt);
return seed;
}
};
// auxiliary generic functions to create a hash value using a seed
template <typename T> inline void hash_val(std::size_t &seed, const T &val) {
hash_combine(seed, val);
}
template <typename T, typename... Types>
inline void hash_val(std::size_t &seed, const T &val, const Types &... args) {
hash_combine(seed, val);
hash_val(seed, args...);
}
template <typename... Types>
inline std::size_t hash_val(const Types &... args) {
std::size_t seed = 0;
hash_val(seed, args...);
return seed;
}
struct pair_hash {
template <class T1, class T2>
std::size_t operator()(const std::pair<T1, T2> &p) const {
return hash_val(p.first, p.second);
}
};
} // namespace hash_tuple
#include <bits/stdc++.h>
int main() {
using ll = long long;
// std::unordered_map<std::pair<ll, ll>, ll, hash_tuple::pair_hash>
// hashmapPair; std::unordered_set<std::pair<ll, ll>, hash_tuple::pair_hash>
// hashsetPair;
std::unordered_map<std::pair<ll, ll>, ll, hash_tuple::pair_hash>
hashmapPair;
hashmapPair[{0, 0}] = 10;
std::unordered_set<std::pair<ll, ll>, hash_tuple::pair_hash> hashsetPair;
hashsetPair.insert({1, 1});
using TI = std::tuple<ll, ll, ll, ll>;
std::unordered_map<TI, ll, hash_tuple::hash<TI>> hashmapTuple;
hashmapTuple[{0, 1, 2, 3}] = 10;
std::unordered_set<TI, hash_tuple::hash<TI>> hashsetTuple;
hashsetTuple.emplace(0, 1, 2, 3);
return 0;
}
As your compilation error indicates, there is no valid instantiation of std::hash<std::pair<std::string, std::string>> in your std namespace.
According to my compiler:
Error C2338 The C++ Standard doesn't provide a hash for this
type. c:\program files (x86)\microsoft visual studio
14.0\vc\include\xstddef 381
You can provide your own specialization for std::hash<Vote> as follows:
#include <string>
#include <unordered_map>
#include <functional>
using namespace std;
using Vote = pair<string, string>;
using Unordered_map = unordered_map<Vote, int>;
namespace std
{
template<>
struct hash<Vote>
{
size_t operator()(Vote const& v) const
{
// ... hash function here ...
}
};
}
int main()
{
Unordered_map m;
}
In the comments on the answer by Baum mit Augen, the user Joe Black asked for an example on using a lambda expressions instead of defining a hash function. I agree with the opinion of Baum mit Augen, that this might harm readability, especially if you want to implement a more universal solution. Therefore, I'd like to keep my example short by focusing on a specific solution for std::pair<std::string, std::string>, as presented by the OP. The example also uses a handcrafted combination of std::hash<std::string> function calls:
using Vote = std::pair<std::string, std::string>;
auto hash = [](const Vote& v){
return std::hash<std::string>()(v.first) * 31 + std::hash<std::string>()(v.second);
};
using Unordered_map = std::unordered_map<Vote, int, decltype(hash)>;
Unordered_map um(8, hash);
Code on Ideone
I've simplified #YoungForest's answer to only work with pairs (= not with arbitrary length tuples) as was requested by the OP. I've also minimized the boilerplate code:
#include <functional>
#include <iostream>
#include <unordered_map>
#include <utility> # pair
using namespace std;
// from boost (functional/hash):
// see http://www.boost.org/doc/libs/1_35_0/doc/html/hash/combine.html template
template <class T> inline void hash_combine(size_t &seed, T const &v) {
seed ^= hash<T>()(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}
struct pair_hash {
template <class T1, class T2>
size_t operator()(const pair<T1, T2> &p) const {
size_t seed = 0;
hash_combine(seed, p.first);
hash_combine(seed, p.second);
return seed;
}
};
int main() {
unordered_map<pair<int, int>, int, pair_hash> d;
d[{1, 2}] = 3;
cout << d.find({1, 2})->second << endl;
return 0;
}
It uses the same logic as in the boost library (that is better than the xor version).
There is a hack to such problems
Use a std:unordered_map of string
Look at the following example-
I am required to hash the endpoint(corner) of a rectangle
Error Approach
unordered_map<pair<int, int>, int> M; //ERROR
pair<int, int> p;
M[p]++;
Hack
unordered_map<string, int> M;
pair<int, int> p;
string s = to_string(p.first) + "_" + to_string(p.second);
M[s]++;
Such hack even works if you are required to create a hash of decimal or double as a key :)
Related
I am trying to create an unordered_map to map pairs with integers:
#include <unordered_map>
using namespace std;
using Vote = pair<string, string>;
using Unordered_map = unordered_map<Vote, int>;
I have a class where I have declared an Unordered_map as a private member.
However, I am getting the following error when I try to compile it:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/type_traits:948:38: Implicit instantiation of undefined template 'std::__1::hash, std::__1::basic_string > >'
I am not getting this error if I use a regular map like map<pair<string, string>, int> instead of an unordered_map.
Is it not possible to use pair as key in unordered maps?
You need to provide a suitable hash function for your key type. A simple example:
#include <unordered_map>
#include <functional>
#include <string>
#include <utility>
// Only for pairs of std::hash-able types for simplicity.
// You can of course template this struct to allow other hash functions
struct pair_hash {
template <class T1, class T2>
std::size_t operator () (const std::pair<T1,T2> &p) const {
auto h1 = std::hash<T1>{}(p.first);
auto h2 = std::hash<T2>{}(p.second);
// Mainly for demonstration purposes, i.e. works but is overly simple
// In the real world, use sth. like boost.hash_combine
return h1 ^ h2;
}
};
using Vote = std::pair<std::string, std::string>;
using Unordered_map = std::unordered_map<Vote, int, pair_hash>;
int main() {
Unordered_map um;
}
This will work, but not have the best hash-properties†. You might want to have a look at something like boost.hash_combine for higher quality results when combining the hashes. This is also discussed in greater detail – including the aforementioned solution from boost – in this answer.
For real world use: Boost also provides the function set hash_value which already provides a hash function for std::pair, as well as std::tuple and most standard containers.
†More precisely, it will produce too many collisions. E.g., every symmetric pair will hash to 0 and pairs that differ only by permutation will have the same hash. This is probably fine for your programming exercise, but might seriously hurt performance of real world code.
My preferred way of solving this problem is to define a key function that transforms your pair into a unique integer (or any hashable data type). This key is not the hash key. It is the unique ID of the pair of data that will then be optimally hashed by the unordered_map. For example, you wanted to define an unordered_map of the type
unordered_map<pair<int,int>,double> Map;
And you want to use Map[make_pair(i,j)]=value or Map.find(make_pair(i,j)) to operate on the map. Then you'll have to tell the system how to hash a pair of integers make_pair(i,j). Instead of that, we can define
inline size_t key(int i,int j) {return (size_t) i << 32 | (unsigned int) j;}
and then change the type of the map to
unordered_map<size_t,double> Map;
We can now use Map[key(i,j)]=value or Map.find(key(i,j)) to operate on the map. Every make_pair now becomes calling the inline key function.
This method guarantees that the key will be optimally hashed, because now the hashing part is done by the system, which will always choose the internal hash table size to be prime to make sure every bucket is equally likely. But you have to make yourself 100% sure that the key is unique for every pair, i.e., no two distinct pairs can have the same key, or there can be very difficult bugs to find.
If using pair is not a strict requirement, you can simply use map twice.
#include <unordered_map>
using namespace std;
using Unordered_map = unordered_map<string, unordered_map<string, int>>;
Unordered_map um;
um["Region1"]["Candidate1"] = 10;
cout << um["Region1"]["Candidate1"]; // 10
For pair key, we can use boost pair hash function:
#include <iostream>
#include <boost/functional/hash.hpp>
#include <unordered_map>
using namespace std;
int main() {
unordered_map<pair<string, string>, int, boost::hash<pair<string, string>>> m;
m[make_pair("123", "456")] = 1;
cout << m[make_pair("123", "456")] << endl;
return 0;
}
Similarly we can use boost hash for vectors,
#include <iostream>
#include <boost/functional/hash.hpp>
#include <unordered_map>
#include <vector>
using namespace std;
int main() {
unordered_map<vector<string>, int, boost::hash<vector<string>>> m;
vector<string> a({"123", "456"});
m[a] = 1;
cout << m[a] << endl;
return 0;
}
Reference: C++ Standard Library: A tutorial and reference, Second version Chapter 7.9.2: Creating and Controlling unordered Container
All solutions I found in Google use XOR to generate hashcode of pair, which is totally bad. see why-is-xor-the-default-way-to-combine-hashes. However, the book has given us the best solution, using hash_combine, which is taken from Boost. The solution is much better than XOR when I tested it in Online Judge(Atcoder). I organized the code as a template as follow. You can copy and paste it as much as you can. And it is convenient to change it to fit any custom struct/class.
Update: add hash template for tuple.
#include <functional>
namespace hash_tuple {
template <typename TT> struct hash {
size_t operator()(TT const &tt) const { return std::hash<TT>()(tt); }
};
// from boost (functional/hash):
// see http://www.boost.org/doc/libs/1_35_0/doc/html/hash/combine.html template
template <class T> inline void hash_combine(std::size_t &seed, T const &v) {
seed ^= hash_tuple::hash<T>()(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}
// Recursive template code derived from Matthieu M.
template <class Tuple, size_t Index = std::tuple_size<Tuple>::value - 1>
struct HashValueImpl {
void operator()(size_t &seed, Tuple const &tuple) const {
HashValueImpl<Tuple, Index - 1>{}(seed, tuple);
hash_combine(seed, std::get<Index>(tuple));
}
};
template <class Tuple> struct HashValueImpl<Tuple, 0> {
void operator()(size_t &seed, Tuple const &tuple) const {
hash_combine(seed, std::get<0>(tuple));
}
};
template <typename... TT> struct hash<std::tuple<TT...>> {
size_t operator()(std::tuple<TT...> const &tt) const {
size_t seed = 0;
HashValueImpl<std::tuple<TT...>>{}(seed, tt);
return seed;
}
};
// auxiliary generic functions to create a hash value using a seed
template <typename T> inline void hash_val(std::size_t &seed, const T &val) {
hash_combine(seed, val);
}
template <typename T, typename... Types>
inline void hash_val(std::size_t &seed, const T &val, const Types &... args) {
hash_combine(seed, val);
hash_val(seed, args...);
}
template <typename... Types>
inline std::size_t hash_val(const Types &... args) {
std::size_t seed = 0;
hash_val(seed, args...);
return seed;
}
struct pair_hash {
template <class T1, class T2>
std::size_t operator()(const std::pair<T1, T2> &p) const {
return hash_val(p.first, p.second);
}
};
} // namespace hash_tuple
#include <bits/stdc++.h>
int main() {
using ll = long long;
// std::unordered_map<std::pair<ll, ll>, ll, hash_tuple::pair_hash>
// hashmapPair; std::unordered_set<std::pair<ll, ll>, hash_tuple::pair_hash>
// hashsetPair;
std::unordered_map<std::pair<ll, ll>, ll, hash_tuple::pair_hash>
hashmapPair;
hashmapPair[{0, 0}] = 10;
std::unordered_set<std::pair<ll, ll>, hash_tuple::pair_hash> hashsetPair;
hashsetPair.insert({1, 1});
using TI = std::tuple<ll, ll, ll, ll>;
std::unordered_map<TI, ll, hash_tuple::hash<TI>> hashmapTuple;
hashmapTuple[{0, 1, 2, 3}] = 10;
std::unordered_set<TI, hash_tuple::hash<TI>> hashsetTuple;
hashsetTuple.emplace(0, 1, 2, 3);
return 0;
}
As your compilation error indicates, there is no valid instantiation of std::hash<std::pair<std::string, std::string>> in your std namespace.
According to my compiler:
Error C2338 The C++ Standard doesn't provide a hash for this
type. c:\program files (x86)\microsoft visual studio
14.0\vc\include\xstddef 381
You can provide your own specialization for std::hash<Vote> as follows:
#include <string>
#include <unordered_map>
#include <functional>
using namespace std;
using Vote = pair<string, string>;
using Unordered_map = unordered_map<Vote, int>;
namespace std
{
template<>
struct hash<Vote>
{
size_t operator()(Vote const& v) const
{
// ... hash function here ...
}
};
}
int main()
{
Unordered_map m;
}
In the comments on the answer by Baum mit Augen, the user Joe Black asked for an example on using a lambda expressions instead of defining a hash function. I agree with the opinion of Baum mit Augen, that this might harm readability, especially if you want to implement a more universal solution. Therefore, I'd like to keep my example short by focusing on a specific solution for std::pair<std::string, std::string>, as presented by the OP. The example also uses a handcrafted combination of std::hash<std::string> function calls:
using Vote = std::pair<std::string, std::string>;
auto hash = [](const Vote& v){
return std::hash<std::string>()(v.first) * 31 + std::hash<std::string>()(v.second);
};
using Unordered_map = std::unordered_map<Vote, int, decltype(hash)>;
Unordered_map um(8, hash);
Code on Ideone
I've simplified #YoungForest's answer to only work with pairs (= not with arbitrary length tuples) as was requested by the OP. I've also minimized the boilerplate code:
#include <functional>
#include <iostream>
#include <unordered_map>
#include <utility> # pair
using namespace std;
// from boost (functional/hash):
// see http://www.boost.org/doc/libs/1_35_0/doc/html/hash/combine.html template
template <class T> inline void hash_combine(size_t &seed, T const &v) {
seed ^= hash<T>()(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}
struct pair_hash {
template <class T1, class T2>
size_t operator()(const pair<T1, T2> &p) const {
size_t seed = 0;
hash_combine(seed, p.first);
hash_combine(seed, p.second);
return seed;
}
};
int main() {
unordered_map<pair<int, int>, int, pair_hash> d;
d[{1, 2}] = 3;
cout << d.find({1, 2})->second << endl;
return 0;
}
It uses the same logic as in the boost library (that is better than the xor version).
There is a hack to such problems
Use a std:unordered_map of string
Look at the following example-
I am required to hash the endpoint(corner) of a rectangle
Error Approach
unordered_map<pair<int, int>, int> M; //ERROR
pair<int, int> p;
M[p]++;
Hack
unordered_map<string, int> M;
pair<int, int> p;
string s = to_string(p.first) + "_" + to_string(p.second);
M[s]++;
Such hack even works if you are required to create a hash of decimal or double as a key :)
I am trying to create an unordered_map to map pairs with integers:
#include <unordered_map>
using namespace std;
using Vote = pair<string, string>;
using Unordered_map = unordered_map<Vote, int>;
I have a class where I have declared an Unordered_map as a private member.
However, I am getting the following error when I try to compile it:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/type_traits:948:38: Implicit instantiation of undefined template 'std::__1::hash, std::__1::basic_string > >'
I am not getting this error if I use a regular map like map<pair<string, string>, int> instead of an unordered_map.
Is it not possible to use pair as key in unordered maps?
You need to provide a suitable hash function for your key type. A simple example:
#include <unordered_map>
#include <functional>
#include <string>
#include <utility>
// Only for pairs of std::hash-able types for simplicity.
// You can of course template this struct to allow other hash functions
struct pair_hash {
template <class T1, class T2>
std::size_t operator () (const std::pair<T1,T2> &p) const {
auto h1 = std::hash<T1>{}(p.first);
auto h2 = std::hash<T2>{}(p.second);
// Mainly for demonstration purposes, i.e. works but is overly simple
// In the real world, use sth. like boost.hash_combine
return h1 ^ h2;
}
};
using Vote = std::pair<std::string, std::string>;
using Unordered_map = std::unordered_map<Vote, int, pair_hash>;
int main() {
Unordered_map um;
}
This will work, but not have the best hash-properties†. You might want to have a look at something like boost.hash_combine for higher quality results when combining the hashes. This is also discussed in greater detail – including the aforementioned solution from boost – in this answer.
For real world use: Boost also provides the function set hash_value which already provides a hash function for std::pair, as well as std::tuple and most standard containers.
†More precisely, it will produce too many collisions. E.g., every symmetric pair will hash to 0 and pairs that differ only by permutation will have the same hash. This is probably fine for your programming exercise, but might seriously hurt performance of real world code.
My preferred way of solving this problem is to define a key function that transforms your pair into a unique integer (or any hashable data type). This key is not the hash key. It is the unique ID of the pair of data that will then be optimally hashed by the unordered_map. For example, you wanted to define an unordered_map of the type
unordered_map<pair<int,int>,double> Map;
And you want to use Map[make_pair(i,j)]=value or Map.find(make_pair(i,j)) to operate on the map. Then you'll have to tell the system how to hash a pair of integers make_pair(i,j). Instead of that, we can define
inline size_t key(int i,int j) {return (size_t) i << 32 | (unsigned int) j;}
and then change the type of the map to
unordered_map<size_t,double> Map;
We can now use Map[key(i,j)]=value or Map.find(key(i,j)) to operate on the map. Every make_pair now becomes calling the inline key function.
This method guarantees that the key will be optimally hashed, because now the hashing part is done by the system, which will always choose the internal hash table size to be prime to make sure every bucket is equally likely. But you have to make yourself 100% sure that the key is unique for every pair, i.e., no two distinct pairs can have the same key, or there can be very difficult bugs to find.
If using pair is not a strict requirement, you can simply use map twice.
#include <unordered_map>
using namespace std;
using Unordered_map = unordered_map<string, unordered_map<string, int>>;
Unordered_map um;
um["Region1"]["Candidate1"] = 10;
cout << um["Region1"]["Candidate1"]; // 10
For pair key, we can use boost pair hash function:
#include <iostream>
#include <boost/functional/hash.hpp>
#include <unordered_map>
using namespace std;
int main() {
unordered_map<pair<string, string>, int, boost::hash<pair<string, string>>> m;
m[make_pair("123", "456")] = 1;
cout << m[make_pair("123", "456")] << endl;
return 0;
}
Similarly we can use boost hash for vectors,
#include <iostream>
#include <boost/functional/hash.hpp>
#include <unordered_map>
#include <vector>
using namespace std;
int main() {
unordered_map<vector<string>, int, boost::hash<vector<string>>> m;
vector<string> a({"123", "456"});
m[a] = 1;
cout << m[a] << endl;
return 0;
}
Reference: C++ Standard Library: A tutorial and reference, Second version Chapter 7.9.2: Creating and Controlling unordered Container
All solutions I found in Google use XOR to generate hashcode of pair, which is totally bad. see why-is-xor-the-default-way-to-combine-hashes. However, the book has given us the best solution, using hash_combine, which is taken from Boost. The solution is much better than XOR when I tested it in Online Judge(Atcoder). I organized the code as a template as follow. You can copy and paste it as much as you can. And it is convenient to change it to fit any custom struct/class.
Update: add hash template for tuple.
#include <functional>
namespace hash_tuple {
template <typename TT> struct hash {
size_t operator()(TT const &tt) const { return std::hash<TT>()(tt); }
};
// from boost (functional/hash):
// see http://www.boost.org/doc/libs/1_35_0/doc/html/hash/combine.html template
template <class T> inline void hash_combine(std::size_t &seed, T const &v) {
seed ^= hash_tuple::hash<T>()(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}
// Recursive template code derived from Matthieu M.
template <class Tuple, size_t Index = std::tuple_size<Tuple>::value - 1>
struct HashValueImpl {
void operator()(size_t &seed, Tuple const &tuple) const {
HashValueImpl<Tuple, Index - 1>{}(seed, tuple);
hash_combine(seed, std::get<Index>(tuple));
}
};
template <class Tuple> struct HashValueImpl<Tuple, 0> {
void operator()(size_t &seed, Tuple const &tuple) const {
hash_combine(seed, std::get<0>(tuple));
}
};
template <typename... TT> struct hash<std::tuple<TT...>> {
size_t operator()(std::tuple<TT...> const &tt) const {
size_t seed = 0;
HashValueImpl<std::tuple<TT...>>{}(seed, tt);
return seed;
}
};
// auxiliary generic functions to create a hash value using a seed
template <typename T> inline void hash_val(std::size_t &seed, const T &val) {
hash_combine(seed, val);
}
template <typename T, typename... Types>
inline void hash_val(std::size_t &seed, const T &val, const Types &... args) {
hash_combine(seed, val);
hash_val(seed, args...);
}
template <typename... Types>
inline std::size_t hash_val(const Types &... args) {
std::size_t seed = 0;
hash_val(seed, args...);
return seed;
}
struct pair_hash {
template <class T1, class T2>
std::size_t operator()(const std::pair<T1, T2> &p) const {
return hash_val(p.first, p.second);
}
};
} // namespace hash_tuple
#include <bits/stdc++.h>
int main() {
using ll = long long;
// std::unordered_map<std::pair<ll, ll>, ll, hash_tuple::pair_hash>
// hashmapPair; std::unordered_set<std::pair<ll, ll>, hash_tuple::pair_hash>
// hashsetPair;
std::unordered_map<std::pair<ll, ll>, ll, hash_tuple::pair_hash>
hashmapPair;
hashmapPair[{0, 0}] = 10;
std::unordered_set<std::pair<ll, ll>, hash_tuple::pair_hash> hashsetPair;
hashsetPair.insert({1, 1});
using TI = std::tuple<ll, ll, ll, ll>;
std::unordered_map<TI, ll, hash_tuple::hash<TI>> hashmapTuple;
hashmapTuple[{0, 1, 2, 3}] = 10;
std::unordered_set<TI, hash_tuple::hash<TI>> hashsetTuple;
hashsetTuple.emplace(0, 1, 2, 3);
return 0;
}
As your compilation error indicates, there is no valid instantiation of std::hash<std::pair<std::string, std::string>> in your std namespace.
According to my compiler:
Error C2338 The C++ Standard doesn't provide a hash for this
type. c:\program files (x86)\microsoft visual studio
14.0\vc\include\xstddef 381
You can provide your own specialization for std::hash<Vote> as follows:
#include <string>
#include <unordered_map>
#include <functional>
using namespace std;
using Vote = pair<string, string>;
using Unordered_map = unordered_map<Vote, int>;
namespace std
{
template<>
struct hash<Vote>
{
size_t operator()(Vote const& v) const
{
// ... hash function here ...
}
};
}
int main()
{
Unordered_map m;
}
In the comments on the answer by Baum mit Augen, the user Joe Black asked for an example on using a lambda expressions instead of defining a hash function. I agree with the opinion of Baum mit Augen, that this might harm readability, especially if you want to implement a more universal solution. Therefore, I'd like to keep my example short by focusing on a specific solution for std::pair<std::string, std::string>, as presented by the OP. The example also uses a handcrafted combination of std::hash<std::string> function calls:
using Vote = std::pair<std::string, std::string>;
auto hash = [](const Vote& v){
return std::hash<std::string>()(v.first) * 31 + std::hash<std::string>()(v.second);
};
using Unordered_map = std::unordered_map<Vote, int, decltype(hash)>;
Unordered_map um(8, hash);
Code on Ideone
I've simplified #YoungForest's answer to only work with pairs (= not with arbitrary length tuples) as was requested by the OP. I've also minimized the boilerplate code:
#include <functional>
#include <iostream>
#include <unordered_map>
#include <utility> # pair
using namespace std;
// from boost (functional/hash):
// see http://www.boost.org/doc/libs/1_35_0/doc/html/hash/combine.html template
template <class T> inline void hash_combine(size_t &seed, T const &v) {
seed ^= hash<T>()(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}
struct pair_hash {
template <class T1, class T2>
size_t operator()(const pair<T1, T2> &p) const {
size_t seed = 0;
hash_combine(seed, p.first);
hash_combine(seed, p.second);
return seed;
}
};
int main() {
unordered_map<pair<int, int>, int, pair_hash> d;
d[{1, 2}] = 3;
cout << d.find({1, 2})->second << endl;
return 0;
}
It uses the same logic as in the boost library (that is better than the xor version).
There is a hack to such problems
Use a std:unordered_map of string
Look at the following example-
I am required to hash the endpoint(corner) of a rectangle
Error Approach
unordered_map<pair<int, int>, int> M; //ERROR
pair<int, int> p;
M[p]++;
Hack
unordered_map<string, int> M;
pair<int, int> p;
string s = to_string(p.first) + "_" + to_string(p.second);
M[s]++;
Such hack even works if you are required to create a hash of decimal or double as a key :)
I am trying to create an unordered_map to map pairs with integers:
#include <unordered_map>
using namespace std;
using Vote = pair<string, string>;
using Unordered_map = unordered_map<Vote, int>;
I have a class where I have declared an Unordered_map as a private member.
However, I am getting the following error when I try to compile it:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/type_traits:948:38: Implicit instantiation of undefined template 'std::__1::hash, std::__1::basic_string > >'
I am not getting this error if I use a regular map like map<pair<string, string>, int> instead of an unordered_map.
Is it not possible to use pair as key in unordered maps?
You need to provide a suitable hash function for your key type. A simple example:
#include <unordered_map>
#include <functional>
#include <string>
#include <utility>
// Only for pairs of std::hash-able types for simplicity.
// You can of course template this struct to allow other hash functions
struct pair_hash {
template <class T1, class T2>
std::size_t operator () (const std::pair<T1,T2> &p) const {
auto h1 = std::hash<T1>{}(p.first);
auto h2 = std::hash<T2>{}(p.second);
// Mainly for demonstration purposes, i.e. works but is overly simple
// In the real world, use sth. like boost.hash_combine
return h1 ^ h2;
}
};
using Vote = std::pair<std::string, std::string>;
using Unordered_map = std::unordered_map<Vote, int, pair_hash>;
int main() {
Unordered_map um;
}
This will work, but not have the best hash-properties†. You might want to have a look at something like boost.hash_combine for higher quality results when combining the hashes. This is also discussed in greater detail – including the aforementioned solution from boost – in this answer.
For real world use: Boost also provides the function set hash_value which already provides a hash function for std::pair, as well as std::tuple and most standard containers.
†More precisely, it will produce too many collisions. E.g., every symmetric pair will hash to 0 and pairs that differ only by permutation will have the same hash. This is probably fine for your programming exercise, but might seriously hurt performance of real world code.
My preferred way of solving this problem is to define a key function that transforms your pair into a unique integer (or any hashable data type). This key is not the hash key. It is the unique ID of the pair of data that will then be optimally hashed by the unordered_map. For example, you wanted to define an unordered_map of the type
unordered_map<pair<int,int>,double> Map;
And you want to use Map[make_pair(i,j)]=value or Map.find(make_pair(i,j)) to operate on the map. Then you'll have to tell the system how to hash a pair of integers make_pair(i,j). Instead of that, we can define
inline size_t key(int i,int j) {return (size_t) i << 32 | (unsigned int) j;}
and then change the type of the map to
unordered_map<size_t,double> Map;
We can now use Map[key(i,j)]=value or Map.find(key(i,j)) to operate on the map. Every make_pair now becomes calling the inline key function.
This method guarantees that the key will be optimally hashed, because now the hashing part is done by the system, which will always choose the internal hash table size to be prime to make sure every bucket is equally likely. But you have to make yourself 100% sure that the key is unique for every pair, i.e., no two distinct pairs can have the same key, or there can be very difficult bugs to find.
If using pair is not a strict requirement, you can simply use map twice.
#include <unordered_map>
using namespace std;
using Unordered_map = unordered_map<string, unordered_map<string, int>>;
Unordered_map um;
um["Region1"]["Candidate1"] = 10;
cout << um["Region1"]["Candidate1"]; // 10
For pair key, we can use boost pair hash function:
#include <iostream>
#include <boost/functional/hash.hpp>
#include <unordered_map>
using namespace std;
int main() {
unordered_map<pair<string, string>, int, boost::hash<pair<string, string>>> m;
m[make_pair("123", "456")] = 1;
cout << m[make_pair("123", "456")] << endl;
return 0;
}
Similarly we can use boost hash for vectors,
#include <iostream>
#include <boost/functional/hash.hpp>
#include <unordered_map>
#include <vector>
using namespace std;
int main() {
unordered_map<vector<string>, int, boost::hash<vector<string>>> m;
vector<string> a({"123", "456"});
m[a] = 1;
cout << m[a] << endl;
return 0;
}
Reference: C++ Standard Library: A tutorial and reference, Second version Chapter 7.9.2: Creating and Controlling unordered Container
All solutions I found in Google use XOR to generate hashcode of pair, which is totally bad. see why-is-xor-the-default-way-to-combine-hashes. However, the book has given us the best solution, using hash_combine, which is taken from Boost. The solution is much better than XOR when I tested it in Online Judge(Atcoder). I organized the code as a template as follow. You can copy and paste it as much as you can. And it is convenient to change it to fit any custom struct/class.
Update: add hash template for tuple.
#include <functional>
namespace hash_tuple {
template <typename TT> struct hash {
size_t operator()(TT const &tt) const { return std::hash<TT>()(tt); }
};
// from boost (functional/hash):
// see http://www.boost.org/doc/libs/1_35_0/doc/html/hash/combine.html template
template <class T> inline void hash_combine(std::size_t &seed, T const &v) {
seed ^= hash_tuple::hash<T>()(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}
// Recursive template code derived from Matthieu M.
template <class Tuple, size_t Index = std::tuple_size<Tuple>::value - 1>
struct HashValueImpl {
void operator()(size_t &seed, Tuple const &tuple) const {
HashValueImpl<Tuple, Index - 1>{}(seed, tuple);
hash_combine(seed, std::get<Index>(tuple));
}
};
template <class Tuple> struct HashValueImpl<Tuple, 0> {
void operator()(size_t &seed, Tuple const &tuple) const {
hash_combine(seed, std::get<0>(tuple));
}
};
template <typename... TT> struct hash<std::tuple<TT...>> {
size_t operator()(std::tuple<TT...> const &tt) const {
size_t seed = 0;
HashValueImpl<std::tuple<TT...>>{}(seed, tt);
return seed;
}
};
// auxiliary generic functions to create a hash value using a seed
template <typename T> inline void hash_val(std::size_t &seed, const T &val) {
hash_combine(seed, val);
}
template <typename T, typename... Types>
inline void hash_val(std::size_t &seed, const T &val, const Types &... args) {
hash_combine(seed, val);
hash_val(seed, args...);
}
template <typename... Types>
inline std::size_t hash_val(const Types &... args) {
std::size_t seed = 0;
hash_val(seed, args...);
return seed;
}
struct pair_hash {
template <class T1, class T2>
std::size_t operator()(const std::pair<T1, T2> &p) const {
return hash_val(p.first, p.second);
}
};
} // namespace hash_tuple
#include <bits/stdc++.h>
int main() {
using ll = long long;
// std::unordered_map<std::pair<ll, ll>, ll, hash_tuple::pair_hash>
// hashmapPair; std::unordered_set<std::pair<ll, ll>, hash_tuple::pair_hash>
// hashsetPair;
std::unordered_map<std::pair<ll, ll>, ll, hash_tuple::pair_hash>
hashmapPair;
hashmapPair[{0, 0}] = 10;
std::unordered_set<std::pair<ll, ll>, hash_tuple::pair_hash> hashsetPair;
hashsetPair.insert({1, 1});
using TI = std::tuple<ll, ll, ll, ll>;
std::unordered_map<TI, ll, hash_tuple::hash<TI>> hashmapTuple;
hashmapTuple[{0, 1, 2, 3}] = 10;
std::unordered_set<TI, hash_tuple::hash<TI>> hashsetTuple;
hashsetTuple.emplace(0, 1, 2, 3);
return 0;
}
As your compilation error indicates, there is no valid instantiation of std::hash<std::pair<std::string, std::string>> in your std namespace.
According to my compiler:
Error C2338 The C++ Standard doesn't provide a hash for this
type. c:\program files (x86)\microsoft visual studio
14.0\vc\include\xstddef 381
You can provide your own specialization for std::hash<Vote> as follows:
#include <string>
#include <unordered_map>
#include <functional>
using namespace std;
using Vote = pair<string, string>;
using Unordered_map = unordered_map<Vote, int>;
namespace std
{
template<>
struct hash<Vote>
{
size_t operator()(Vote const& v) const
{
// ... hash function here ...
}
};
}
int main()
{
Unordered_map m;
}
In the comments on the answer by Baum mit Augen, the user Joe Black asked for an example on using a lambda expressions instead of defining a hash function. I agree with the opinion of Baum mit Augen, that this might harm readability, especially if you want to implement a more universal solution. Therefore, I'd like to keep my example short by focusing on a specific solution for std::pair<std::string, std::string>, as presented by the OP. The example also uses a handcrafted combination of std::hash<std::string> function calls:
using Vote = std::pair<std::string, std::string>;
auto hash = [](const Vote& v){
return std::hash<std::string>()(v.first) * 31 + std::hash<std::string>()(v.second);
};
using Unordered_map = std::unordered_map<Vote, int, decltype(hash)>;
Unordered_map um(8, hash);
Code on Ideone
I've simplified #YoungForest's answer to only work with pairs (= not with arbitrary length tuples) as was requested by the OP. I've also minimized the boilerplate code:
#include <functional>
#include <iostream>
#include <unordered_map>
#include <utility> # pair
using namespace std;
// from boost (functional/hash):
// see http://www.boost.org/doc/libs/1_35_0/doc/html/hash/combine.html template
template <class T> inline void hash_combine(size_t &seed, T const &v) {
seed ^= hash<T>()(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}
struct pair_hash {
template <class T1, class T2>
size_t operator()(const pair<T1, T2> &p) const {
size_t seed = 0;
hash_combine(seed, p.first);
hash_combine(seed, p.second);
return seed;
}
};
int main() {
unordered_map<pair<int, int>, int, pair_hash> d;
d[{1, 2}] = 3;
cout << d.find({1, 2})->second << endl;
return 0;
}
It uses the same logic as in the boost library (that is better than the xor version).
There is a hack to such problems
Use a std:unordered_map of string
Look at the following example-
I am required to hash the endpoint(corner) of a rectangle
Error Approach
unordered_map<pair<int, int>, int> M; //ERROR
pair<int, int> p;
M[p]++;
Hack
unordered_map<string, int> M;
pair<int, int> p;
string s = to_string(p.first) + "_" + to_string(p.second);
M[s]++;
Such hack even works if you are required to create a hash of decimal or double as a key :)
I am trying to create an unordered_map to map pairs with integers:
#include <unordered_map>
using namespace std;
using Vote = pair<string, string>;
using Unordered_map = unordered_map<Vote, int>;
I have a class where I have declared an Unordered_map as a private member.
However, I am getting the following error when I try to compile it:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/type_traits:948:38: Implicit instantiation of undefined template 'std::__1::hash, std::__1::basic_string > >'
I am not getting this error if I use a regular map like map<pair<string, string>, int> instead of an unordered_map.
Is it not possible to use pair as key in unordered maps?
You need to provide a suitable hash function for your key type. A simple example:
#include <unordered_map>
#include <functional>
#include <string>
#include <utility>
// Only for pairs of std::hash-able types for simplicity.
// You can of course template this struct to allow other hash functions
struct pair_hash {
template <class T1, class T2>
std::size_t operator () (const std::pair<T1,T2> &p) const {
auto h1 = std::hash<T1>{}(p.first);
auto h2 = std::hash<T2>{}(p.second);
// Mainly for demonstration purposes, i.e. works but is overly simple
// In the real world, use sth. like boost.hash_combine
return h1 ^ h2;
}
};
using Vote = std::pair<std::string, std::string>;
using Unordered_map = std::unordered_map<Vote, int, pair_hash>;
int main() {
Unordered_map um;
}
This will work, but not have the best hash-properties†. You might want to have a look at something like boost.hash_combine for higher quality results when combining the hashes. This is also discussed in greater detail – including the aforementioned solution from boost – in this answer.
For real world use: Boost also provides the function set hash_value which already provides a hash function for std::pair, as well as std::tuple and most standard containers.
†More precisely, it will produce too many collisions. E.g., every symmetric pair will hash to 0 and pairs that differ only by permutation will have the same hash. This is probably fine for your programming exercise, but might seriously hurt performance of real world code.
My preferred way of solving this problem is to define a key function that transforms your pair into a unique integer (or any hashable data type). This key is not the hash key. It is the unique ID of the pair of data that will then be optimally hashed by the unordered_map. For example, you wanted to define an unordered_map of the type
unordered_map<pair<int,int>,double> Map;
And you want to use Map[make_pair(i,j)]=value or Map.find(make_pair(i,j)) to operate on the map. Then you'll have to tell the system how to hash a pair of integers make_pair(i,j). Instead of that, we can define
inline size_t key(int i,int j) {return (size_t) i << 32 | (unsigned int) j;}
and then change the type of the map to
unordered_map<size_t,double> Map;
We can now use Map[key(i,j)]=value or Map.find(key(i,j)) to operate on the map. Every make_pair now becomes calling the inline key function.
This method guarantees that the key will be optimally hashed, because now the hashing part is done by the system, which will always choose the internal hash table size to be prime to make sure every bucket is equally likely. But you have to make yourself 100% sure that the key is unique for every pair, i.e., no two distinct pairs can have the same key, or there can be very difficult bugs to find.
If using pair is not a strict requirement, you can simply use map twice.
#include <unordered_map>
using namespace std;
using Unordered_map = unordered_map<string, unordered_map<string, int>>;
Unordered_map um;
um["Region1"]["Candidate1"] = 10;
cout << um["Region1"]["Candidate1"]; // 10
For pair key, we can use boost pair hash function:
#include <iostream>
#include <boost/functional/hash.hpp>
#include <unordered_map>
using namespace std;
int main() {
unordered_map<pair<string, string>, int, boost::hash<pair<string, string>>> m;
m[make_pair("123", "456")] = 1;
cout << m[make_pair("123", "456")] << endl;
return 0;
}
Similarly we can use boost hash for vectors,
#include <iostream>
#include <boost/functional/hash.hpp>
#include <unordered_map>
#include <vector>
using namespace std;
int main() {
unordered_map<vector<string>, int, boost::hash<vector<string>>> m;
vector<string> a({"123", "456"});
m[a] = 1;
cout << m[a] << endl;
return 0;
}
Reference: C++ Standard Library: A tutorial and reference, Second version Chapter 7.9.2: Creating and Controlling unordered Container
All solutions I found in Google use XOR to generate hashcode of pair, which is totally bad. see why-is-xor-the-default-way-to-combine-hashes. However, the book has given us the best solution, using hash_combine, which is taken from Boost. The solution is much better than XOR when I tested it in Online Judge(Atcoder). I organized the code as a template as follow. You can copy and paste it as much as you can. And it is convenient to change it to fit any custom struct/class.
Update: add hash template for tuple.
#include <functional>
namespace hash_tuple {
template <typename TT> struct hash {
size_t operator()(TT const &tt) const { return std::hash<TT>()(tt); }
};
// from boost (functional/hash):
// see http://www.boost.org/doc/libs/1_35_0/doc/html/hash/combine.html template
template <class T> inline void hash_combine(std::size_t &seed, T const &v) {
seed ^= hash_tuple::hash<T>()(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}
// Recursive template code derived from Matthieu M.
template <class Tuple, size_t Index = std::tuple_size<Tuple>::value - 1>
struct HashValueImpl {
void operator()(size_t &seed, Tuple const &tuple) const {
HashValueImpl<Tuple, Index - 1>{}(seed, tuple);
hash_combine(seed, std::get<Index>(tuple));
}
};
template <class Tuple> struct HashValueImpl<Tuple, 0> {
void operator()(size_t &seed, Tuple const &tuple) const {
hash_combine(seed, std::get<0>(tuple));
}
};
template <typename... TT> struct hash<std::tuple<TT...>> {
size_t operator()(std::tuple<TT...> const &tt) const {
size_t seed = 0;
HashValueImpl<std::tuple<TT...>>{}(seed, tt);
return seed;
}
};
// auxiliary generic functions to create a hash value using a seed
template <typename T> inline void hash_val(std::size_t &seed, const T &val) {
hash_combine(seed, val);
}
template <typename T, typename... Types>
inline void hash_val(std::size_t &seed, const T &val, const Types &... args) {
hash_combine(seed, val);
hash_val(seed, args...);
}
template <typename... Types>
inline std::size_t hash_val(const Types &... args) {
std::size_t seed = 0;
hash_val(seed, args...);
return seed;
}
struct pair_hash {
template <class T1, class T2>
std::size_t operator()(const std::pair<T1, T2> &p) const {
return hash_val(p.first, p.second);
}
};
} // namespace hash_tuple
#include <bits/stdc++.h>
int main() {
using ll = long long;
// std::unordered_map<std::pair<ll, ll>, ll, hash_tuple::pair_hash>
// hashmapPair; std::unordered_set<std::pair<ll, ll>, hash_tuple::pair_hash>
// hashsetPair;
std::unordered_map<std::pair<ll, ll>, ll, hash_tuple::pair_hash>
hashmapPair;
hashmapPair[{0, 0}] = 10;
std::unordered_set<std::pair<ll, ll>, hash_tuple::pair_hash> hashsetPair;
hashsetPair.insert({1, 1});
using TI = std::tuple<ll, ll, ll, ll>;
std::unordered_map<TI, ll, hash_tuple::hash<TI>> hashmapTuple;
hashmapTuple[{0, 1, 2, 3}] = 10;
std::unordered_set<TI, hash_tuple::hash<TI>> hashsetTuple;
hashsetTuple.emplace(0, 1, 2, 3);
return 0;
}
As your compilation error indicates, there is no valid instantiation of std::hash<std::pair<std::string, std::string>> in your std namespace.
According to my compiler:
Error C2338 The C++ Standard doesn't provide a hash for this
type. c:\program files (x86)\microsoft visual studio
14.0\vc\include\xstddef 381
You can provide your own specialization for std::hash<Vote> as follows:
#include <string>
#include <unordered_map>
#include <functional>
using namespace std;
using Vote = pair<string, string>;
using Unordered_map = unordered_map<Vote, int>;
namespace std
{
template<>
struct hash<Vote>
{
size_t operator()(Vote const& v) const
{
// ... hash function here ...
}
};
}
int main()
{
Unordered_map m;
}
In the comments on the answer by Baum mit Augen, the user Joe Black asked for an example on using a lambda expressions instead of defining a hash function. I agree with the opinion of Baum mit Augen, that this might harm readability, especially if you want to implement a more universal solution. Therefore, I'd like to keep my example short by focusing on a specific solution for std::pair<std::string, std::string>, as presented by the OP. The example also uses a handcrafted combination of std::hash<std::string> function calls:
using Vote = std::pair<std::string, std::string>;
auto hash = [](const Vote& v){
return std::hash<std::string>()(v.first) * 31 + std::hash<std::string>()(v.second);
};
using Unordered_map = std::unordered_map<Vote, int, decltype(hash)>;
Unordered_map um(8, hash);
Code on Ideone
I've simplified #YoungForest's answer to only work with pairs (= not with arbitrary length tuples) as was requested by the OP. I've also minimized the boilerplate code:
#include <functional>
#include <iostream>
#include <unordered_map>
#include <utility> # pair
using namespace std;
// from boost (functional/hash):
// see http://www.boost.org/doc/libs/1_35_0/doc/html/hash/combine.html template
template <class T> inline void hash_combine(size_t &seed, T const &v) {
seed ^= hash<T>()(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}
struct pair_hash {
template <class T1, class T2>
size_t operator()(const pair<T1, T2> &p) const {
size_t seed = 0;
hash_combine(seed, p.first);
hash_combine(seed, p.second);
return seed;
}
};
int main() {
unordered_map<pair<int, int>, int, pair_hash> d;
d[{1, 2}] = 3;
cout << d.find({1, 2})->second << endl;
return 0;
}
It uses the same logic as in the boost library (that is better than the xor version).
There is a hack to such problems
Use a std:unordered_map of string
Look at the following example-
I am required to hash the endpoint(corner) of a rectangle
Error Approach
unordered_map<pair<int, int>, int> M; //ERROR
pair<int, int> p;
M[p]++;
Hack
unordered_map<string, int> M;
pair<int, int> p;
string s = to_string(p.first) + "_" + to_string(p.second);
M[s]++;
Such hack even works if you are required to create a hash of decimal or double as a key :)
I need to define an unordered_map like this unordered_map<pair<int, int>, *Foo>, what is the syntax for defining and passing a hash and equal functions to this map?
I've tried passing to it this object:
class pairHash{
public:
long operator()(const pair<int, int> &k) const{
return k.first * 100 + k.second;
}
};
and no luck:
unordered_map<pair<int, int>, int> map = unordered_map<pair<int, int>, int>(1,
*(new pairHash()));
I have no Idea what is the size_type_Buskets means so I gave it 1.
What is the right way to do it?
Thanks.
This is an unfortunate omission in C++11; Boost has the answer in terms of hash_combine. Feel free to just paste it from them! Here's how I hash pairs:
template <class T>
inline void hash_combine(std::size_t & seed, const T & v)
{
std::hash<T> hasher;
seed ^= hasher(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}
namespace std
{
template<typename S, typename T> struct hash<pair<S, T>>
{
inline size_t operator()(const pair<S, T> & v) const
{
size_t seed = 0;
::hash_combine(seed, v.first);
::hash_combine(seed, v.second);
return seed;
}
};
}
You can use hash_combine as the basis for many other things, like tuples and ranges, so you could hash an entire (ordered) container, for example, as long as each member is individually hashable.
Now you can just declare a new map:
std::unordered_map<std::pair<int, int>, my_mapped_type> mymap;
If you want to use your homebrew hasher (which hasn't got good statistical properties), you have to specify the template parameters explicitly:
std::unordered_map<std::pair<int,int>, int, pairHash> yourmap;
Note that there's no need to specify a copy of a hasher object, as the default is to default-construct one for you.
The return type of the hash function should be size_t, not long (although this is not the cause of the error). The syntax you've shown for providing a custom hash function is incorrect.
You'll also need to provide an equal predicate to make the above work properly.
#include <unordered_map>
#include <utility>
using namespace std;
class pairHash{
public:
size_t operator()(const pair<int, int> &k) const{
return k.first * 100 + k.second;
}
};
struct pairEquals : binary_function<const pair<int,int>&, const pair<int,int>&, bool> {
result_type operator()( first_argument_type lhs, second_argument_type rhs ) const
{
return (lhs.first == rhs.first) && (lhs.second == rhs.second);
}
};
int main()
{
unordered_map<pair<int, int>, int, pairHash, pairEquals> myMap;
myMap[make_pair(10,20)] = 100;
myMap.insert( make_pair(make_pair(100,200), 1000) );
}
EDIT:
You don't need to define the equal predicate since operator== is defined for std::pair and it does exactly what I've done in pairEquals. You'll only need the pairEquals definition if you expect the comparison to be done differently.
If you are fine with using Boost, a cleaner solution is to rely on Boost's implementation of the hash function for pairs (which in fact does exactly what kerrek-sb explains in his answer). Therefore all you have to do is:
#include <unordered_map>
#include <boost/functional/hash.hpp>
using namespace std;
using namespace boost;
unordered_map<pair<int, int>, int, hash<pair<int, int>>> table;