C++ how to insert array into hash set? - c++

Question

I need to insert a 1D array into the hashset.
But I got error while compiling.
#include <stdio.h>
#include <stdlib.h>
#include <hash_set.h>
using namespace std;
int hash_comp(const int* state1,const int* state2) {
int result = 0;
for (i = 0; i < 16; i++)
{
if (state1[i] != state2[i]) {
result = -1;
}
}
return result;
}
struct eqArray
{
bool operator()(const int* a1,const int* a2) const
{
return hash_comp(a1,a2) == 0;
}
};
hash_set<int*,hash<int*>,eqArray> closelist;
int main(int argc, char** argv)
{
const int sn[16] = {1,2,3,4,5,6,0,8,9,10,11,12,13,14,7,15};
closelist.insert(sn);
return 0;
}
/usr/include/c++/4.2.1/ext/hashtable.h: In member function 'size_t __gnu_cxx::hashtable<_Val, _Key, _HashFcn, _ExtractKey, _EqualKey, _Alloc>::_M_bkt_num_key(const _Key&, size_t) const [with _Val = int*, _Key = int*, _HashFcn = __gnu_cxx::hash<int*>, _ExtractKey = std::_Identity<int*>, _EqualKey = std::equal_to<int*>, _Alloc = std::allocator<int*>]':
/usr/include/c++/4.2.1/ext/hashtable.h:599: instantiated from 'size_t __gnu_cxx::hashtable<_Val, _Key, _HashFcn, _ExtractKey, _EqualKey, _Alloc>::_M_bkt_num(const _Val&, size_t) const [with _Val = int*, _Key = int*, _HashFcn = __gnu_cxx::hash<int*>, _ExtractKey = std::_Identity<int*>, _EqualKey = std::equal_to<int*>, _Alloc = std::allocator<int*>]'
/usr/include/c++/4.2.1/ext/hashtable.h:1006: instantiated from 'void __gnu_cxx::hashtable<_Val, _Key, _HashFcn, _ExtractKey, _EqualKey, _Alloc>::resize(size_t) [with _Val = int*, _Key = int*, _HashFcn = __gnu_cxx::hash<int*>, _ExtractKey = std::_Identity<int*>, _EqualKey = std::equal_to<int*>, _Alloc = std::allocator<int*>]'
/usr/include/c++/4.2.1/ext/hashtable.h:437: instantiated from 'std::pair<__gnu_cxx::_Hashtable_iterator<_Val, _Key, _HashFcn, _ExtractKey, _EqualKey, _Alloc>, bool> __gnu_cxx::hashtable<_Val, _Key, _HashFcn, _ExtractKey, _EqualKey, _Alloc>::insert_unique(const _Val&) [with _Val = int*, _Key = int*, _HashFcn = __gnu_cxx::hash<int*>, _ExtractKey = std::_Identity<int*>, _EqualKey = std::equal_to<int*>, _Alloc = std::allocator<int*>]'
/usr/include/c++/4.2.1/ext/hash_set:197: instantiated from 'std::pair<typename __gnu_cxx::hashtable<_Value, _Value, _HashFcn, std::_Identity<_Tp>, _EqualKey, _Alloc>::const_iterator, bool> __gnu_cxx::hash_set<_Value, _HashFcn, _EqualKey, _Alloc>::insert(const typename __gnu_cxx::hashtable<_Value, _Value, _HashFcn, std::_Identity<_Tp>, _EqualKey, _Alloc>::value_type&) [with _Value = int*, _HashFcn = __gnu_cxx::hash<int*>, _EqualKey = std::equal_to<int*>, _Alloc = std::allocator<int*>]'
src/ods2.cpp:677: instantiated from here

Answer 1

If you use a std::array<int, 16> instead of int*, all your problems will go away. If you have no C++11 compiler, you can use boost::array instead. Also, replace hash_set with unordered_set.
It appears there is no specialization of std::hash for std::arrays, so I wrote my own:
#include <unordered_set>
#include <array>
namespace std
{
template<typename T, size_t N>
struct hash<array<T, N> >
{
typedef array<T, N> argument_type;
typedef size_t result_type;
result_type operator()(const argument_type& a) const
{
hash<T> hasher;
result_type h = 0;
for (result_type i = 0; i < N; ++i)
{
h = h * 31 + hasher(a[i]);
}
return h;
}
};
}
std::unordered_set<std::array<int, 16> > closelist;
int main()
{
std::array<int, 16> sn = {1,2,3,4,5,6,0,8,9,10,11,12,13,14,7,15};
closelist.insert(sn);
}

Answer 2

You didn’t post the actual error message, only the trace. That said, it’s probably because your set uses non-const int* while your data is const.
That said, I’d suggest using unordered_set instead of hash_set(either from std::tr1 if your compiler supports that, or from std if your compiler supports C++11 or from Boost), and heed Fred’s advice of using an {std,boost}::array instead of a raw pointer.

Answer 3

I don't think that there exists a specialized hash<int*>. I added this:
namespace __gnu_cxx{ //I'm not sure what compiler version you used,
//mine wanted this
template<>
struct hash<const int*>{
size_t operator()(const int*a) const{
size_t r = 0;
for (int i=0;i<16;i++)
r = (r<<1) ^ a[i]; //not sure if it makes sense as a hash.
return r;
}
};
}
I also put in some consts and it compiles.