从矩阵构建子矩阵

Question

我正在尝试将给定的矩阵拆分为 4，但出现了一些错误，而且我无法弄清楚原因。我对这门语言有点陌生。有人知道我做错了什么吗？

void split_tl(T **matrice, unsigned int dim){
    if(dim == 1){
        return;
    }

    T **tl = new T*[dim/4];
    for(unsigned int i = 0; i<dim/4;++i){
        tl[i] = new T[dim/4];
    }

    for(unsigned int i=0; i<dim;++i){
        for(unsigned int j=0; j<dim;j++){
            if((i<dim/2) && (j<dim/2)){
                tl[i][j] = matrice[i][j];
            } else{
                std::cout << "no ";
            }
        }
        std::cout << std::endl;
    }
}

在这个函数中，我试图获取矩阵的左上角。

int **matrice = new int*[2];

for(unsigned int i = 0; i<2;++i){
    matrice[i] = new int[2];
}

for(unsigned int i = 0; i<2;++i){
    for(unsigned int j = 0; j<2;++j){
        matrice[i][j] = i+j;
    }
}

这是我发送的矩阵。它是一个 2x2 矩阵，仅用于测试目的。

这些是来自 Valgrind 的错误：

==133== Invalid read of size 8
==133== Invalid write of size 4
==133== Process terminating with default action of signal 11 (SIGSEGV)
==133== Access not within mapped region at address 0x0

Answer 1

如果dim是矩阵的边，分配给四分之一矩阵应该是dim/2。

下面是您使用的代码：

if((i<dim/2) && (j<dim/2)){
  tl[i][j] = matrice[i][j];
}

此处tl可能超出分配

Answer 2

你做错了什么？您正在分配内存并传递指针。构建一个合适的矩阵 class，例如（非常简化的版本）：

template <typename T, unsigned Rows, unsigned Cols>
struct generic_matrix {
    
    using datatype = T;
    
    static constexpr unsigned rows = Rows;
    static constexpr unsigned cols = Cols;
    
    datatype data[rows][cols];

    constexpr datatype& operator()(unsigned row, unsigned col) noexcept
    { return data[row][col]; }
    
    constexpr const datatype& operator()(unsigned row, unsigned col) const noexcept
    { return data[row][col]; }
    
};

子矩阵:

/* Returns a submatrix of the matrix m, 
 * by deleting the row r and the column c.*/
template <typename M>
auto submatrix(const M& m, unsigned r, unsigned c) noexcept
{
    generic_matrix<typename M::datatype, M::rows-1, M::cols-1> res;
    
    for (unsigned row = 0, i = 0; row < M::rows; ++row) {
        if (row == r) continue; //this row we do not want
        for (unsigned col = 0, j = 0; col < M::cols; ++col) {
            if (col == c) continue; //this col we do not want
            res(i,j) = m(row,col);
            ++j;
        }
        ++i;
    }

    return res;
}

打印:

template <typename M>
void printmatrix(const M& m) noexcept
{
    for (unsigned r=0; r < M::rows; ++r) {
        for (unsigned c=0; c < M::cols; ++c) {
            std::cout << m(r,c) << ' ';
        }
        std::cout << '\n';
    }
}

测试:

int main()
{
    int n=0;
    generic_matrix<int, 3, 3> m;
    for (int r = 0; r < 3; ++r)
        for (int c = 0; c < 3; ++c)
            m(r,c) = ++n;

    printmatrix(m);
    std::cout << '\n';

    printmatrix(submatrix(m, 0, 0));
    std::cout << '\n';

    printmatrix(submatrix(m, 1, 1));
    std::cout << '\n';

    printmatrix(submatrix(m, 2, 2));

    return 0;
}

注意: 这只是一个提示。如您所见，不需要分配或转换。

从矩阵构建子矩阵

Building a submatrix from a matrix

c++

matrix

submatrix