Prefill a std::vector at initialization?

I want to create a vector of vector of a vector of double and want it to already have (32,32,16) elements, without manually开发者_高级运维 pushing all of these back. Is there a way to do it during initialization? (I don't care what value gets pushed).

I want a 3-dimensional array, the first dimension has 32, the second dimension has 32 and the third dimension has 16 elements.

One liner:

std::vector< std::vector< std::vector<double> > > values(32, std::vector< std::vector<double> >(32, std::vector<double>(16, 0.0)));

Or breaking the lines:

typedef std::vector<double> v_type;
typedef std::vector<v_type> vv_type;
typedef std::vector<vv_type> vvv_type;

vvv_type values(32, vv_type(32, v_type(16, 0.0)));

I would remark that this allocate a fair lot of objects (32*32*16).

Would a single vector work ?

std::vector<double> values(32*32*16, 0.0)

That would be 32*32*16-1 less new.

One of the ctors for a vector allows you to specify both the size and a value to be copied into the elements for the vector. I'm not quite sure what you means by "(32,32,16)" elements, but you could do something like:

// create a vector containing 16 elements set to 2
std::vector<int> temp(16, 2);   

// create a vector of 32 vectors, each with 16 elements set to 2
std::vector<std::vector<int> > values(32, temp);

Here you go:

vector<vector<vector<int> > > k(32, vector<vector<int> >(32, vector<int>(16,0)));

I am not even going to ask why you would want such a monstrosity and not just use a single vector with your own indexing scheme.

Since the size is known before hand and you will need to convert the whole array to a 1D array (for OpenGL) why not create a class for it? I have created a template-based implementation of a 3D array which I hope will be useful to you.

Usage:

typedef Array3D<double, 3, 3, 3> DoubleArray333;
DoubleArray333 array;
Double val = 0.0;
for (size_t i = 0; i < DoubleArray333::SizeX; ++i)
{
 for (size_t j = 0; j < DoubleArray333::SizeY; ++j)
 {
  for (size_t k = 0; k < DoubleArray333::SizeZ; ++k)
  {
   array(i, j, k) = val++;
  }
 }
}
for (size_t i = 0; i < DoubleArray333::ArraySize; ++i)
{
 std::cout<<array[i]<<" ";
}

The Array3D header file:

#pragma once

#include <exception>
#include <sstream>
#include <utility>
#include <memory>

/**
 * \brief   A 3D array of variable type and size.
 * \author  Vite Falcon
 * \date    08/06/2010
**/
template <typename T, int x, int y, int z>
class Array3D
{
private:
    T* m_array;

    /**
     * \brief   Validate the index range of different dimensions.
     * \remarks Vite Falcon, 08/06/2010.
     * \author  Vite Falcon
     * \date    08/06/2010
     * \exception   std::out_of_range   Thrown when one of the indices is out-of-range.
     * \param   x   The width index.
     * \param   y   The height index.
     * \param   z   The depth index.
    **/
    inline void validateRange(size_t x, size_t y, size_t z)
    {
        if (x >= SizeX || y >= SizeY || z >= SizeZ)
        {
            std::stringstream ss;
            ss<<"Index out of range when accessing ("<<x<<", "<<y<<", "<<z<<
                ") when total size is ("<<SizeX<<", "<<SizeY<<", "<<SizeZ<<").";
            throw std::out_of_range(ss.str());
        }
    }

    /**
     * \brief   Validates the given array index.
     * \remarks Vite Falcon, 08/06/2010.
     * \author  Vite Falcon
     * \date    08/06/2010
     * \exception   std::out_of_range   Thrown when the index is out-of-range.
     * \param   index   Zero-based index of the array.
    **/
    inline void validateIndex(size_t index)
    {
        if (index >= ArraySize)
        {
            std::stringstream ss;
            ss<<"Index out of range when accessing array by index "<<index<<
                " when total array size is "<<ArraySize<<".";
            throw std::out_of_range(ss.str());
        }
    }
public:
    static const size_t SizeX;
    static const size_t SizeY;
    static const size_t SizeZ;
    static const size_t ArraySize;

    typedef Array3D<T,x,y,z> MyType;

    /**
     * \brief   Default constructor. 
     * \author  Vite Falcon
     * \date    08/06/2010
    **/
    Array3D(void)
        :m_array(new T[ArraySize])
    {
    }

    /**
     * \brief   Copy constructor.
     * \author  Vite Falcon
     * \date    08/06/2010
     * \param   other   The other.
    **/
    Array3D(const MyType& other)
        :m_array(new T[ArraySize])
    {
        memcpy_s(m_array, sizeof(T)*ArraySize, other.m_array, sizeof(T)*ArraySize);
    }

    /**
     * \brief   Destructor.
     * \author  Vite Falcon
     * \date    08/06/2010
    **/
    ~Array3D(void)
    {
        delete[] m_array;
        m_array = 0;
    }

    /**
     * \brief   Gets the value at a particular array index.
     * \author  Vite Falcon
     * \date    08/06/2010
     * \param   array_index Zero-based index of the array.
     * \return  The value at the given index.
    **/
    inline T& at(size_t array_index)
    {
        return (*this)[array_index];
    }

    /**
     * \brief   Gets the value at a particular array index (const version)
     * \author  Vite Falcon
     * \date    08/06/2010
     * \param   array_index Zero-based index of the array.
     * \return  The value at the given index.
    **/
    inline const T& at(size_t array_index) const
    {
        return (*this)[array_index];
    }

    /**
     * \brief   Gets the value in the array from the given 3D indices.
     * \author  Vite Falcon
     * \date    08/06/2010
     * \param   x   The width index.
     * \param   y   The height index.
     * \param   z   The depth index.
     * \return  The value at the given indices.
    **/
    inline T& at(size_t x, size_t y, size_t z)
    {
        return (*this)(x, y, z);
    }

    /**
    * \brief    Gets the value in the array from the given 3D indices (const version).
     * \author  Vite Falcon
     * \date    08/06/2010
     * \param   x   The width index.
     * \param   y   The height index.
     * \param   z   The depth index.
     * \return  The value at the given indices.
    **/
    inline const T& at(size_t x, size_t y, size_t z) const
    {
        return (*this)(x, y, z);
    }

    /**
     * \brief   The '()' operator to access the values as a 3D array.
     * \author  Vite Falcon
     * \date    08/06/2010
     * \return  The value at the given indices.
     *
     * \param   x   The width index.
     * \param   y   The height index.
     * \param   z   The depth index.
    **/
    inline T& operator ()(size_t x, size_t y, size_t z)
    {
        validateRange(x, y, z);
        return m_array[x*SizeY*SizeZ + y*SizeZ + z];
    }

    /**
     * \brief   The '()' operator to access the values as a 3D array (const version).
     * \author  Vite Falcon
     * \date    08/06/2010
     * \return  The value at the given indices.
     *
     * \param   x   The width index.
     * \param   y   The height index.
     * \param   z   The depth index.
    **/
    inline const T& operator()(size_t x, size_t y, size_t z) const
    {
        validateRange(x, y, z);
        return m_array[x*SizeY*SizeZ + y*SizeZ + z];
    }

    /**
     * \brief   The '[]' operator to access the values as a 1D array.
     * \author  Vite Falcon
     * \date    08/06/2010
     * \param   array_index Zero-based index of the array.
     * \return  Value at the given index.
    **/
    inline T& operator[](size_t array_index)
    {
        validateIndex(array_index);
        return m_array[array_index];
    }

    /**
     * \brief   The '[]' operator to access the values as a 1D array.
     * \author  Vite Falcon
     * \date    08/06/2010
     * \param   array_index Zero-based index of the array.
     * \return  Value at the given index.
    **/
    inline const T& operator[](size_t array_index) const
    {
        validateIndex(array_index);
        return m_array[array_index];
    }

    /**
     * \brief   Fills the array with the given value.
     * \author  Vite Falcon
     * \date    08/06/2010
     * \param   val The value to fill the array.
    **/
    void fill(const T& val)
    {
        for (size_t i = 0; i < ArraySize; ++i)
        {
            m_array[i] = val;
        }
    }

    /**
     * \brief   Gets the raw array.
     * \author  Vite Falcon
     * \date    08/06/2010
     * \return  The 1D array.
    **/
    T* getArray()
    {
        return m_array;
    }

    /**
     * \brief   Swaps the current array with the given one.
     * \author  Vite Falcon
     * \date    08/06/2010
     * \param [in,out]  other   The other.
    **/
    void swap(MyType& other)
    {
        std::swap(m_array, other.m_array);
    }

    /**
     * \brief   Copy operator.
     * \author  Vite Falcon
     * \date    08/06/2010
     * \param   other   The other.
     * \return  A shallow copy of this object.
    **/
    MyType& operator = (const MyType& other)
    {
        MyType temp(other);
        swap(temp);
        return *this;
    }
};

template <typename T, int x, int y, int z>
const size_t Array3D<T, x, y, z>::SizeX = x;

template <typename T, int x, int y, int z>
const size_t Array3D<T, x, y, z>::SizeY = y;

template <typename T, int x, int y, int z>
const size_t Array3D<T, x, y, z>::SizeZ = z;

template <typename T, int x, int y, int z>
const size_t Array3D<T, x, y, z>::ArraySize = x*y*z;

You can also get the pointer to the array as a 1D by using the function:
double* double_array = array.getArray();

EDIT: Changed the usage to show it with double rather than int.