Filling a vector of pairs

问题

I want to fill a vector with 8 pairs. Each pair represents the moves in x and y coordinates a knight in a game of chess can make. At the moment I'm doing it like this

vector<pair<int,int>> moves;

pair<int,int> aPair;
aPair.first = -2;
aPair.second = -1;
moves.push_back(aPair);
aPair.first = -2;
aPair.second = 1;
moves.push_back(aPair);
aPair.first = -1;
aPair.second = -2;
moves.push_back(aPair);
aPair.first = -1;
aPair.second = 2;
moves.push_back(aPair);
aPair.first = 1;
aPair.second = -2;
moves.push_back(aPair);
aPair.first = 1;
aPair.second = 2;
moves.push_back(aPair);
aPair.first = 2;
aPair.second = -1;
moves[6].push_back(aPair);
aPair.first = 2;
aPair.second = 1;
moves.push_back(aPair);

I'm doing this to learn about the Std library. This seems like a hopelessly inefficient way of solving this problem.

Anyone have a more elegant solution?

回答1:

Loops to the rescue:

for(int k = 0; k < 2; k++)
    for(int i = -1; i < 2; i += 2)
        for(int j = -1; j < 2; j+= 2)
            result.push_back(make_pair(i * (k+1), j * (((k + 1) % 2) + 1)));

Output: http://ideone.com/2B0F9b

回答2:

If you have C++11 (otherwise you can't write >>), you can use the following:

vector<pair<int,int>> moves = {
  {-2, -1},
  {-2,  1},
  {-1, -2},
  {-1,  2},
  { 1, -2},
  { 1,  2},
  { 2, -1},
  { 2,  1}
};

回答3:

In C++98/03:

moves.push_back(std::make_pair(-2, -1));

In C++11:

moves.emplace_back(-2, -1);

Alternatively in C++11:

std::vector<std::pair<int, int>> moves = { { -2, -1}, ... };

回答4:

If you don't have C++11 you can utilize make_pair, pre-allocate the space for the vector without initializing the elements using reserve, and then utilize push_back without new allocations being done.

For example:

vector<pair<int,int> > moves;
moves.reserve(8);
moves.push_back(make_pair(-2, -1));
    // and so on

Even if you have C++11 this technique is useful if you need to compute the elements on the fly rather than hard code them.

回答5:

Try that:

vector<pair<int,int>> moves{{-2, -1}, {2, 1}, {-1, -2}, {-1, 2},
                            {1, -2},  {1, 2}, {2, -1},  {2, 1}};

Initializer list together with Uniform Initialization gives a lot of power in C++11.

回答6:

Here's another method of doing the same thing.

template <class VectorClass>
class CreateVector
{
public:
    typedef typename VectorClass::value_type value_type;
    CreateVector(const value_type& value)
    {
        mVector.push_back(value);
    }

    CreateVector& operator()(const value_type& value)
    {
        mVector.push_back(value);
        return *this;
    }

    inline operator VectorClass() const
    {
        return mVector;
    }
private:
    VectorClass mVector;
};

Usage:

vector<pair<int,int>> moves = CreateVector<vector<pair<int,int> > >
(make_pair(1,2))
(make_pair(2,3))
(make_pair(3,4))
(make_pair(4,5));

EDIT: Provided you're not using C++11, this would be one way. Otherwise, I would suggest to go the way @ipc suggested.

回答7:

If you're using C++11, you might want to consider std::array instead of std::vector. Like a normal array, the std array has a fixed number of elements and makes more conceptual sense if you know in advance how much data you use.

回答8:

Hopefully a more readable version with loops:

vector<pair<int, int>> result;
for(int moveX=1; moveX<=2; moveX++)
{
    for(int signX=-1; signX<=1; signX+=2)
    {
        for(int signY=-1; signY<=1; signY+=2)
        {
            result.push_back(make_pair(moveX*signX, (3-moveX)*signY));
        }
    }
}

Full program produces the following vector:

{-1, -2},
{-1, 2},
{1, -2},
{1, 2},
{-2, -1},
{-2, 1},
{2, -1},
{2, 1},

来源：https://stackoverflow.com/questions/13406790/filling-a-vector-of-pairs

标签

c++

stl