動態數組

Question

我做的時刻下面的練習：

的通用矩陣類（15磅）

a) Create a class called Matrix, it should contain storage for M*N numbers of type double. Just like earlier, when choosing how to store your data it is often useful to know what we are going to use the data for later. In matrix operations we are going to access the different elements of the matrix based on their column and/or row, therefore it is useful to order the members of Matrix as an array. Also, we are going to need to change the size of the data stored in the matrix, therefore it should be dynamically allocated.

b) Create constructors for the matrix.

Create the following three constructors: Matrix() • Default constructor, should initialize the matrix into the invalid state.

explicit Matrix(unsigned int N) • Should construct a valid NxN Matrix, initialized as an identity matrix. (The explicit keyword is not in the syllabus, but it should be used here.)

Matrix(unsigned int M, unsigned int N) • Should construct a valid MxN Matrix, initialized as a Zero matrix. (All elements are zero.)

~Matrix() • The destructor of Matrix, should delete any dynamically allocated memory.

我班這麼遠如下：

class Matrix{ 
    private: 
     int rows; 
     int columns; 
     double* matrix; 
    public: 
     Matrix(); 
     explicit Matrix(int N); 
     Matrix(int M, int N); 
     ~Matrix(); 
}; 

和我的代碼的其餘部分：

Matrix::Matrix(){ 
    double * matrix = NULL; 
} 

Matrix::Matrix(int N){ 
    double * matrix = new double[N * N]; 
    this->rows = N; 
    this->columns = N; 

    for(int i = 0; i < N; i++){ 
     for(int j = 0; j < N; j++){ 
      if(i==j) 
       matrix[i * N + j] = 1; 
      else 
       matrix[i * N + j] = 0; 
     } 
    } 
} 

Matrix::Matrix(int M, int N){ 
    double * matrix = new double[M * N]; 
    this->rows = M; 
    this->columns = N; 

    for(int i = 0; i < M; i++){ 
     for(int j = 0; j < N; j++) 
      matrix[i * N + j] = 0; 
    } 
} 

Matrix::~Matrix(){ 
    delete [] matrix; 
} 

我是否正確創建了動態數組和構造函數？ 我後來在練習中使用三種不同的構造函數創建三個不同的數組。我如何做到這一點？如果我嘗試這樣的事情

Matrix::Matrix(); 
Matrix::Matrix(3); 

或

Matrix::Matrix(3,4) 

我得到以下錯誤：

Unhandeled exception at 0x773c15de in Øving_6.exe: 0xC0000005: Access violation reading location 0xccccccc0.

我到底做錯了什麼？

Answer 1

在你的構造函數，你定義一個局部變量

double * matrix = new double[N * N]; 

其陰影同名的成員變量，那麼該成員從來沒有初始化。

所有你應該需要的是將其更改爲

matrix = new double[N * N]; 

而且這非常不-C++使用this->的成員訪問，除非它是絕對必要的消歧（這是幾乎從來沒有）

Answer 2

在你的三個構造函數中，你用本地構造函數掩蓋了實例變量矩陣。試試這個：

Matrix::Matrix(){ 
this->matrix = NULL; 
} 

Matrix::Matrix(int N){ 
this->matrix = new double[N * N]; 
this->rows = N; 
this->columns = N; 

for(int i = 0; i < N; i++){ 
    for(int j = 0; j < N; j++){ 
     if(i==j) 
      matrix[i * N + j] = 1; 
     else 
      matrix[i * N + j] = 0; 
    } 
} 
} 

Matrix::Matrix(int M, int N){ 
this->matrix = new double[M * N]; 
this->rows = M; 
this->columns = N; 

for(int i = 0; i < M; i++){ 
    for(int j = 0; j < N; j++) 
     matrix[i * N + j] = 0; 
} 

}

Answer 3

你會找到更多的「C++」（並且有時只有這樣，才能初始化成員）：

Matrix::Matrix(int M, int N): rows (M), 
           columns (N), 
           matrix (new double[M * N]) 
{ 
    for(int i = 0; i < M; i++) 
     for(int j = 0; j < N; j++) 
      matrix[i * N + j] = 0; 

} 

現在嘗試明白這一點：

Matrix::Matrix(  int N): rows (N), 
           columns (N), 
           matrix (new double[N * N]) 
{ 
    for(int i = 0; i < N; i++) 
     for(int j = 0; j < N; j++) 
      matrix[i * N + j] = (i==j); 

} 

如果你使用：

class Matrix{ 
private: 
    int rows; 
    int columns; 
    std::unique_ptr<double[]> matrix; 

你會發現，你不需要析構函數，以及其他一些inerest事情。另外，請閱讀我的其他答案。