C++ How to insert array to unordered_map as its key?

Question

Hi I used to have a unordered_set to hold my 16 int array, now I need to store one more int as its bucket. I wonder if I can insert the array into my unordered_set, or can I use the same template I used to use?

#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);
}

Can I just change it to this?

std::unordered_map<std::array<int, 16>,int > 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,24);
    }

And I couldn't understand the template, I wonder what is "h = h * 31 + hasher(a[i]);"?

Thank you!!!

"I wonder what is h = h * 31 + hasher(a[i]);" - In this line you just calculate a hash for your array. What exactly do not you understand? — awesoon
– awesoon, Commented Jun 14, 2013 at 15:48
@soon what is 31? I asked about this and some nice guy gave me this template... — weeo
– weeo, Commented Jun 14, 2013 at 15:56
31 is just a constant. It depends on your restrictions for elements in an array. — awesoon
– awesoon, Commented Jun 14, 2013 at 16:09

awesoon · Accepted Answer · 2013-06-14 15:47:08Z

1

Can I just change it to this?

Firstly, your array initialization is wrong:

std::array<int, 16> sn = {{1,2,3,4,5,6,0,8,9,10,11,12,13,14,7,15}};
//                        ^                                     ^

Since std::array has no constructor with std::initializer_list as argument. So, first level for initializing an object, second for initializing an array in the object.

Secondly, from reference:

std::pair<iterator,bool> insert( const value_type& value );

template <class P> 
std::pair<iterator,bool> insert( P&& value );

So, you should pass std::pair(or something, convertible to std::pair), for example:

closelist.insert({sn,24});

Or, simpler:

closelist[sn] = 24;

answered Jun 14, 2013 at 15:47

awesoon

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Comments

Andrey Tuganov · Accepted Answer · 2013-06-14 17:05:17Z

1

How to use any object as a key:

Serialize the object into a byte array (for an array of ints just use the binary data as it is)
Compute the cryptographic hash (MD5 or SHA)
Convert the cryptographic hash into a fingerprint value (for example cast its first 64 bits into uint64_t)
Use this fingerprint as a map key

The disadvantage is that you might need to resolve collisions somehow.

answered Jun 14, 2013 at 17:05

Andrey Tuganov

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1 Comment

Martin Nyolt Over a year ago

You usually do not use any cryptographic hash unless you really need it. Cryptographic hashes are computationally more intensive. Additionally, finding a perfect hash function (w/o collisions for your application) is always a challenge. In most cases, the standard implementation works very well.

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