My code works just fine when I use map< array<int,FIXEDSIZE>, int> but not when I use unordered_map< array<int,FIXEDSIZE>, int>.
It creates this massive list of errors so I don't really know what's wrong. Things like "value" is not a member, or "no match for operator[]", etc.
This is how I am using my map (which I name cache):
if (cache.find(key) != cache.end()) return cache[key];
and
cache[key] = valueToMemoize;
This is basically what boost::hash_combine boils down to:
void hash_combine(std::size_t& seed, std::size_t value) {
seed ^= value + 0x9e3779b9 + (seed<<6) + (seed>>2);
}
A simple hasher for containers - hash all of their elements using std::hash, and combine them.
struct container_hasher {
template<class T>
std::size_t operator()(const T& c) const {
std::size_t seed = 0;
for(const auto& elem : c) {
hash_combine(seed, std::hash<typename T::value_type>()(elem));
}
return seed;
}
};
Use:
std::unordered_map<std::array<int, 10>, int, container_hasher> my_map;
For cheaper lookup, do
auto r = cache.find(key);
if(r != cache.end()) return r->second;
For std::map, you might want to use lower_bound instead, to help with the later insertion:
auto lb = cache.lower_bound(key);
if(lb != cache.end() && lb->first == key) return lb->second;
cache.emplace_hint(lb, key, valueToMemoize);
You need to define your custom hash object like below:
template<typename T, std::size_t N>
class arrayHash {
public:
std::size_t operator()(std::array<T, N> const &arr) const {
std::size_t sum(0);
for(auto &&i : arr) sum += std::hash<T>()(i);
return sum;
}
};
And then define your unordered_map as:
std::unordered_map<std::array<int, FIXEDSIZE>, int, arrayHash<int, FIXEDSIZE>> umap;
Live Demo
Related
I want a get_value template function.
please see the following code:
template<typename T>
(something i want) get_value(const T& m, int key) {
auto it = m.upper_bound(key);
return it == m.begin() ? (*it).second : (*--it).second; // please notice here,
// if my instance is map<int, map<string, int>> the return type should be m.second's type
// that's map<string, int>
}
int main() {
std::map<int, std::map<std::string, int>> m;
auto& it = get_value(m, 10);
}
as you can see, the template function should have a return type, which is depend on instance's second type, is there any method i can get this type to make the code runnable?
The "second type" in a std::map<K,V> is called std::map<K,V>::mapped_type. However, you can use auto to let the compiler deduce that for you:
template<typename T>
auto get_value(const T& m, int key) {
auto it = m.upper_bound(key);
return it == m.begin() ? (*it).second : (*--it).second; // please notice here,
}
or with explicit type:
template<typename T>
typename T::mapped_type get_value(const T& m, int key) {
auto it = m.upper_bound(key);
return it == m.begin() ? (*it).second : (*--it).second; // please notice here,
}
If you can use C++14 standard or above, the safest way to go would be to use decltype(auto) as the return type:
template<typename T>
decltype(auto) get_value(const T& m, int key);
The difference to a plain auto is that decltype(auto) preserves cv-qualifiers, and in your case you most likely want to forward exactly the same type that std::map gives you.
For example, since the actual code uses std::map<int, std::map<std::string, int>>, you might want to avoid copying the return value every time, and decltype(auto) will achieve that.
There is a good talk by Jason Turner and Ben Deane from C++Now 2017 called "Constexpr all the things" which also gives a constexpr vector implementation. I was dabbling with the idea myself, for educational purposes. My constexpr vector was pure in the sense that pushing back to it would return a new vector with added element.
During the talk, I saw a push_back implementation tat looks like more or less following:
constexpr void push_back(T const& e) {
if(size_ >= Size)
throw std::range_error("can't use more than Size");
else {
storage_[size_++] = e;
}
}
They were taking the element by value and moving it but, I don't think this is the source of my problems. The thing I want to know is, how this function could be used in a constexpr context? This is not a const member function, it modifies the state. I don think it is possible to do something like
constexpr cv::vector<int> v1;
v1.push_back(42);
And if this is not possible, how could we use this thing in constexpr context and achieve the goal of the task using this vector, namely compile-time JSON parsing?
Here is my version, so that you can see both my new vector returning version and the version from the talk. (Note that performance, perfect forwarding etc. concerns are omitted)
#include <cstdint>
#include <array>
#include <type_traits>
namespace cx {
template <typename T, std::size_t Size = 10>
struct vector {
using iterator = typename std::array<T, Size>::iterator;
using const_iterator = typename std::array<T, Size>::const_iterator;
constexpr vector(std::initializer_list<T> const& l) {
for(auto& t : l) {
if(size_++ < Size)
storage_[size_] = std::move(t);
else
break;
}
}
constexpr vector(vector const& o, T const& t) {
storage_ = o.storage_;
size_ = o.size_;
storage_[size_++] = t;
}
constexpr auto begin() const { return storage_.begin(); }
constexpr auto end() const { return storage_.begin() + size_; }
constexpr auto size() const { return size_; }
constexpr void push_back(T const& e) {
if(size_ >= Size)
throw std::range_error("can't use more than Size");
else {
storage_[size_++] = e;
}
}
std::array<T, Size> storage_{};
std::size_t size_{};
};
}
template <typename T>
constexpr auto make_vector(std::initializer_list<T> const& l) {
return cx::vector<int>{l};
}
template <typename T>
constexpr auto push_back(cx::vector<T> const& o, T const& t) {
return cx::vector<int>{o, t};
}
int main() {
constexpr auto v1 = make_vector({1, 2, 3});
static_assert(v1.size() == 3);
constexpr auto v2 = push_back(v1, 4);
static_assert(v2.size() == 4);
static_assert(std::is_same_v<decltype(v1), decltype(v2)>);
// v1.push_back(4); fails on a constexpr context
}
So, this thing made me realize there is probably something deep that I don' know about constexpr. So, recapping the question; how such a constexpr vector could offer a mutating push_back like that in a constexpr context? Seems like it is not working in a constexpr context right now. If push_back in a constexpr context is not intended to begin with, how can you call it a constexpr vector and use it for compile-time JSON parsing?
Your definition of vector is correct, but you can't modify constexpr objects. They are well and truly constant. Instead, do compile-time calculations inside constexpr functions (the output of which can then be assigned to constexpr objects).
For example, we can write a function range, which produces a vector of numbers from 0 to n. It uses push_back, and we can assign the result to a constexpr vector in main.
constexpr vector<int> range(int n) {
vector<int> v{};
for(int i = 0; i < n; i++) {
v.push_back(i);
}
return v;
}
int main() {
constexpr vector<int> v = range(10);
}
Your return cx::vector<int>{o, t}; will produce a compilation error when o and t are of types cx::vector<T> and T respectively, because those are different types, while all elements of std::initializer_list<T> should be of same type (o is not expanded into a list of its elements).
If you're merely after your 'pure' implementation of push_back, then you can make do with standard arrays:
#include <array>
template <typename T, std::size_t N>
constexpr auto push_back(std::array<T, N> const& oldArr, T const& el) {
std::array<T, N+1> newArr{};
std::copy(begin(oldArr), end(oldArr), begin(newArr));
newArr[N] = el;
return newArr;
}
int main() {
constexpr auto a1 = std::to_array({1, 2, 3});
static_assert(a1.size() == 3);
constexpr auto a2 = push_back(a1, 4);
static_assert(a2.size() == 4);
// This assert will still fail though, because push_back's implementation
// above not only returns new array, but also a new type.
// For example, std::array<int, 3> is not the same type as std::array<int, 4>
//static_assert(std::is_same_v<decltype(a1), decltype(a2)>);
}
I am looking for a hash function for std::vector, which would be independent from vector's item's ordering.
In other words I am looking for a hash implementation,
that would give me same result for
std::vector<int> v1(1,2,3);
std::vector<int> v2(2,3,1);
std::vector<int> v3(1,3,2);
Any ideas on how I might accomplish this?
template<template<class...>class element_hash=std::hash>
struct symmetric_range_hash {
template<class T>
std::size_t operator()( T const& t ) const {
std::size_t r = element_hash<int>{}(0); // seed with the hash of 0.
for (auto&& x:t) {
using element_type = std::decay_t<decltype(x)>;
auto next = element_hash<element_type>{}(x);
r = r + next;
}
return r;
}
};
That should do it. We gather the hashes via + which is symmetric.
+ is better than ^ because it takes longer to get a cycle. With ^, {1,1} and {2,2} would hash the same (and in general even numbers of anything "disappear"). With + they instead get multiplied.
So the end result is the sum, for each distinct value in the array, of the hash of that value times its count, mod "max(size_t)+1".
Note that an unordered_map requires both a hash and an equality. If you want collision, you'll need to also write an ==.
struct unordered_equal {
template<class C>
bool operator()(C const& lhs, C const& rhs)const {
using std::begin;
using K = std::decay_t< *decltype(begin(lhs)) > >;
std::unordered_map< K, std::size_t > counts;
for (auto&& k : lhs) {
counts[k]++;
}
for (auto&& k : rhs) {
counts[k]--;
}
for (auto&& kv : counts)
if (kv.second != 0) return false;
return true;
}
};
using c++ std's unordered_map i want to map an integer triplet to a single integer, i usually don't use hash tables(didn't know they were so cool), but i don't know the right approach in this case, using the default hashing function should i map the triplets directly (something like < < int,int >,int >->int)
std::unordered_map <std::make_pair <make_pair <int,int>,int>,int> hash;
or maybe use a function to map the triplet to a single value and the use that value with the default function?
int mapping(int a, int b, int c){
}
std::unordered_map <int,int> hash;
both approaches work but i'd like to know wich one is the most efficient one. thank you
First off, you would use std::tuple<int, int, int> as the key type.
Next, you need a way to hash a tuple given that you can hash each element. There is a function called hash_combine in Boost that does that, but for reasons unclear to me, that one was not included in the standard. Anyway, here it goes:
#include <tuple>
#include <utility>
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);
}
template <class Tuple, std::size_t Index = std::tuple_size<Tuple>::value - 1>
struct tuple_hash_impl
{
static inline void apply(std::size_t & seed, Tuple const & tuple)
{
tuple_hash_impl<Tuple, Index - 1>::apply(seed, tuple);
hash_combine(seed, std::get<Index>(tuple));
}
};
template <class Tuple>
struct tuple_hash_impl<Tuple, 0>
{
static inline void apply(std::size_t & seed, Tuple const & tuple)
{
hash_combine(seed, std::get<0>(tuple));
}
};
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;
}
};
template<typename ...Args> struct hash<tuple<Args...>>
{
inline size_t operator()(const tuple<Args...> & v) const
{
size_t seed = 0;
tuple_hash_impl<tuple<Args...>>::apply(seed, v);
return seed;
}
};
}
"Most efficient" seems like something depending on your compiler, but I would say that the make_pair solution looks like a mess. Better use your own hash function... just make sure you make up a decent one :)
Your solution with a pair of pairs should be pretty efficient. It will be difficult to map three integers to something simpler as far as hashing is concerned.
I have trouble describing my problem so I'll give an example:
I have a class description that has a couple of variables in it, for example:
class A{
float a, b, c, d;
}
Now, I maintain a vector<A> that contains many of these classes. What I need to do very very often is to find the object inside this vector that satisfies that one of it's parameters is maximal w.r.t to the others. i.e code looks something like:
int maxi=-1;
float maxa=-1000;
for(int i=0;i<vec.size();i++){
res= vec[i].a;
if(res > maxa) {
maxa= res;
maxi=i;
}
}
return vec[maxi];
However, sometimes I need to find class with maximal a, sometimes with maximal b, sometimes the class with maximal 0.8*a + 0.2*b, sometimes I want a maximal a*VAR + b, where VAR is some variable that is assigned in front, etc. In other words, I need to evaluate an expression for every class, and take the max. I find myself copy-pasting this everywhere, and only changing the single line that defines res.
Is there some nice way to avoid this insanity in C++? What's the neatest way to handle this?
Thank you!
I know this thread is old, but i find it quite useful to implement a powerful argmax function in C++.
However, as far as i can see, all the given examples above rely on std::max_element, which does comparison between the elements (either using a functor or by calling the operator<). this can be slow, if the calculation for each element is expensive. It works well for sorting numbers and handling simple classes, but what if the functor is much more complex? Maybe calculating a heuristic value of a chess position or something else that generate a huge tree etc.
A real argmax, as the thread starter mentioned, would only calculate its arg once, then save it to be compared with the others.
EDIT: Ok i got annoyed and had too much free time, so i created one < C++11 and one C++11 version with r-value references, first the C++11 version:
#include <iostream>
#include <algorithm>
#include <iterator>
#include <vector>
template<typename IteratorT, typename HeuristicFunctorT>
IteratorT argmax(IteratorT && it, const IteratorT & end, const HeuristicFunctorT & functor) {
IteratorT best(it++);
typename HeuristicFunctorT::result_type best_value(functor(*best));
for(; it != end; ++it) {
typename HeuristicFunctorT::result_type value(functor(*it));
if (value > best_value) {
best_value = value;
best = it;
}
}
return best;
}
template<typename IteratorT, typename HeuristicFunctorT>
inline IteratorT argmax(const IteratorT & begin, const IteratorT & end, const HeuristicFunctorT & functor) {
return argmax(IteratorT(begin), end, functor);
}
class IntPairFunctor : public std::unary_function< std::pair<int, int>, int > {
public:
int operator() (const std::pair<int, int> & v) const {
return v.first + v.second;
}
};
std::pair<int, int> rand_pair() {
return std::make_pair(rand(), rand());
}
int main(int argc, const char **argv) {
srand(time(NULL));
std::vector< std::pair<int, int> > ints;
std::generate_n(std::back_insert_iterator< std::vector< std::pair<int, int> > >(ints), 1000, rand_pair);
std::vector< std::pair<int, int> >::iterator m (argmax(ints.begin(), ints.end(), IntPairFunctor()));
std::cout << std::endl << "argmax: " << *m << std::endl;
}
The non C++11 version is much simpler, only the template:
template<typename IteratorT, typename HeuristicFunctorT>
IteratorT argmax(IteratorT it, const IteratorT & end, const HeuristicFunctorT & functor) {
IteratorT best(it++);
typename HeuristicFunctorT::result_type best_value(functor(*best));
for(; it != end; ++it) {
typename HeuristicFunctorT::result_type value(functor(*it));
if (value > best_value) {
best_value = value;
best = it;
}
}
return best;
}
Note that neither version requires any template arguments, the only requirement is that the heuristic implements the unary_function class
template <typename F>
struct CompareBy
{
bool operator()(const typename F::argument_type& x,
const typename F::argument_type& y)
{ return f(x) < f(y); }
CompareBy(const F& f) : f(f) {}
private:
F f;
};
template <typename T, typename U>
struct Member : std::unary_function<U, T>
{
Member(T U::*ptr) : ptr(ptr) {}
const T& operator()(const U& x) { return x.*ptr; }
private:
T U::*ptr;
};
template <typename F>
CompareBy<F> by(const F& f) { return CompareBy<F>(f); }
template <typename T, typename U>
Member<T, U> mem_ptr(T U::*ptr) { return Member<T, U>(ptr); }
You need to include <functional> for this to work. Now use, from header <algorithm>
std::max_element(v.begin(), v.end(), by(mem_ptr(&A::a)));
or
double combination(A x) { return 0.2 * x.a + 0.8 * x.b; }
and
std::max_element(v.begin(), v.end(), by(std::fun_ptr(combination)));
or even
struct combination : std::unary_function<A, double>
{
combination(double x, double y) : x(x), y(y) {}
double operator()(const A& u) { return x * u.a + y * u.b; }
private:
double x, y;
};
with
std::max_element(v.begin(), v.end(), by(combination(0.2, 0.8)));
to compare by a member or by linear combinations of a and b members. I split the comparer in two because the mem_ptr thing is damn useful and worth being reused. The return value of std::max_element is an iterator to the maximum value. You can dereference it to get the max element, or you can use std::distance(v.begin(), i) to find the corresponding index (include <iterator> first).
See http://codepad.org/XQTx0vql for the complete code.
This is what functors and STL are made for:
// A class whose objects perform custom comparisons
class my_comparator
{
public:
explicit my_comparator(float c1, float c2) : c1(c1), c2(c2) {}
// std::max_element calls this on pairs of elements
bool operator() (const A &x, const A &y) const
{
return (x.a*c1 + x.b*c2) < (y.a*c1 + y.b*c2);
}
private:
const float c1, c2;
};
// Returns the "max" element in vec
*std::max_element(vec.begin(), vec.end(), my_comparator(0.8,0.2));
Is the expression always linear? You could pass in an array of four coefficients. If you need to support arbitrary expressions, you'll need a functor, but if it's just an affine combination of the four fields then there's no need for all that complexity.
You can use the std::max_element algorithm with a custom comparator.
It's easy to write the comparator if your compiler supports lambda expressions.
If it doesn't, you can write a custom comparator functor. For the simple case of just comparing a single member, you can write a generic "member comparator" function object, which would look something like this:
template <typename MemberPointer>
struct member_comparator
{
MemberPointer p_;
member_comparator(MemberPointer p) : p_(p) { }
template <typename T>
bool operator()(const T& lhs, const T& rhs) const
{
return lhs.*p_ < rhs.*p_;
}
};
template <typename MemberPointer>
member_comparator<MemberPointer> make_member_comparator(MemberPointer p)
{
return member_comparator<MemberPointer>(p);
}
used as:
// returns an iterator to the element that has the maximum 'd' member:
std::max_element(v.begin(), v.end(), make_member_comparator(&A::d));
You could use the std::max_element STL algorithm providing a custom comparison predicate each time.
With C++0x you can even use a lambda function for it for maximum conciseness:
auto maxElement=*std::max_element(vector.begin(), vector.end(), [](const A& Left, const A& Right) {
return (0.8*Left.a + 0.2*Left.b)<(0.8*Right.a + 0.2*Right.b);
});
Sample of using max_element/min_element with custom functor
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
struct A{
float a, b, c, d;
};
struct CompareA {
bool operator()(A const & Left, A const & Right) const {
return Left.a < Right.a;
}
};
int main() {
vector<A> vec;
vec.resize(3);
vec[0].a = 1;
vec[1].a = 2;
vec[2].a = 1.5;
vector<A>::iterator it = std::max_element(vec.begin(), vec.end(), CompareA());
cout << "Largest A: " << it->a << endl;
it = std::min_element(vec.begin(), vec.end(), CompareA());
cout << "Smallest A: " << it->a << endl;
}