How to access to subsequence of a valarray considering it as a 2D matrix in C++

I'm learning C++, so please be patient with me.
I have a std::valarray in which there are double elements and I consider it as a 2D matrix.

class Matrix {

    valarray<double> elems;

    int r, c;

public:

/* type? operator(int r) { return ? } */

//...

}

I want to overload the operator, so that I can get a row of the matrix, and after that, I want have the m[r][c] access operator.

Is there any way to get a row, as a sequence of double using std::slice in the valarray, so that if I change a value, it is changed also in the matrix?

I've read this definition in valarray:

std::slice_array<T> operator( std::slice slicearr );

My operator must have std::slice_array<double>& as returned type?

Thanks.

edited Nov 19 '18 at 15:51

Swordfish

9,34811336

asked Nov 19 '18 at 15:36

Sam

357

What is proxy class in C++

– Swordfish
Nov 19 '18 at 15:56

I asked a similar question back in 2016! :D

– Paula_plus_plus
Nov 19 '18 at 16:45

add a comment |

I'm learning C++, so please be patient with me.
I have a std::valarray in which there are double elements and I consider it as a 2D matrix.

class Matrix {

    valarray<double> elems;

    int r, c;

public:

/* type? operator(int r) { return ? } */

//...

}

I want to overload the operator, so that I can get a row of the matrix, and after that, I want have the m[r][c] access operator.

Is there any way to get a row, as a sequence of double using std::slice in the valarray, so that if I change a value, it is changed also in the matrix?

I've read this definition in valarray:

std::slice_array<T> operator( std::slice slicearr );

My operator must have std::slice_array<double>& as returned type?

Thanks.

edited Nov 19 '18 at 15:51

Swordfish

9,34811336

asked Nov 19 '18 at 15:36

Sam

357

What is proxy class in C++

– Swordfish
Nov 19 '18 at 15:56

I asked a similar question back in 2016! :D

– Paula_plus_plus
Nov 19 '18 at 16:45

add a comment |

I'm learning C++, so please be patient with me.
I have a std::valarray in which there are double elements and I consider it as a 2D matrix.

class Matrix {

    valarray<double> elems;

    int r, c;

public:

/* type? operator(int r) { return ? } */

//...

}

I want to overload the operator, so that I can get a row of the matrix, and after that, I want have the m[r][c] access operator.

Is there any way to get a row, as a sequence of double using std::slice in the valarray, so that if I change a value, it is changed also in the matrix?

I've read this definition in valarray:

std::slice_array<T> operator( std::slice slicearr );

My operator must have std::slice_array<double>& as returned type?

Thanks.

edited Nov 19 '18 at 15:51

Swordfish

9,34811336

asked Nov 19 '18 at 15:36

Sam

357

I'm learning C++, so please be patient with me.
I have a std::valarray in which there are double elements and I consider it as a 2D matrix.

class Matrix {

    valarray<double> elems;

    int r, c;

public:

/* type? operator(int r) { return ? } */

//...

}

I want to overload the operator, so that I can get a row of the matrix, and after that, I want have the m[r][c] access operator.

Is there any way to get a row, as a sequence of double using std::slice in the valarray, so that if I change a value, it is changed also in the matrix?

I've read this definition in valarray:

std::slice_array<T> operator( std::slice slicearr );

My operator must have std::slice_array<double>& as returned type?

Thanks.

c++

edited Nov 19 '18 at 15:51

Swordfish

9,34811336

asked Nov 19 '18 at 15:36

Sam

357

edited Nov 19 '18 at 15:51

Swordfish

9,34811336

asked Nov 19 '18 at 15:36

Sam

357

edited Nov 19 '18 at 15:51

Swordfish

9,34811336

edited Nov 19 '18 at 15:51

Swordfish

9,34811336

edited Nov 19 '18 at 15:51

Swordfish

9,34811336

asked Nov 19 '18 at 15:36

Sam

357

asked Nov 19 '18 at 15:36

Sam

357

asked Nov 19 '18 at 15:36

Sam

357

What is proxy class in C++

– Swordfish
Nov 19 '18 at 15:56

I asked a similar question back in 2016! :D

– Paula_plus_plus
Nov 19 '18 at 16:45

add a comment |

What is proxy class in C++

– Swordfish
Nov 19 '18 at 15:56

I asked a similar question back in 2016! :D

– Paula_plus_plus
Nov 19 '18 at 16:45

What is proxy class in C++

– Swordfish
Nov 19 '18 at 15:56

I asked a similar question back in 2016! :D

– Paula_plus_plus
Nov 19 '18 at 16:45

add a comment |

1 Answer
1

active

oldest

votes

I don't think std::slice and std::slice_array is what you're looking for, especially the latter is nothing but a helper type with a very limited public interface. You can instead return an proxy object. Here's a possible example of how to implement that.

class Matrix {

    /* ... */



    class RowProxy {

       public:

          RowProxy(std::valarray<double>& elems, int c, int row) :

              elems(elems), c(c), row(row) {}



          double& operator(int j)

          {

             return elems[row*c + j];

          }



       private:

          std::valarray<double>& elems;

          int row;

          int c;

    };



    RowProxy operator(int i)

    {

       return RowProxy(elems, c, i);

    }

};

This way, you can access the data with two operator.

Matrix m(2, 4); // Assuming the ctor initializes elemens with row*column



m[0][0] = 1.234;

m[1][0] = 2.234;

m[1][3] = -52.023;

Note that both Matrix and RowProxy are missing overloads and proper handling for const-ness, and variable names are poor. Also, you might want to think about an out-of-bounds error handling strategy. But it may serve as a starting point for your implementation.

answered Nov 19 '18 at 16:03

lubgr

10.6k21845

ok, thanks! In this way I can access the element with m[r][c]. I would ask you, if I want to return an object like a Matrix_row() that contain the "reference" to elements in Matrix object, with the syntax m, how can I do? Is it possible?

– Sam
Nov 19 '18 at 17:08

1

@Sam This is exactly the proxy object. You can use it like this: auto row = m[0]. Then, index the row as row[0]. This is exactly the same as m[0][0].

– lubgr
Nov 19 '18 at 19:13

oooh now I got it, thanks!! Just a question. You speak about error handling strategy, It's a good practise to use the valarray exception in constructor without checking size? I mean, in the constructor I pass int r, int c and I create a valarray with r * c elements, but if r * c is < 0 valarray throw an exception. It's a good practise to use that exception or it is better that I check size before create valarray?

– Sam
Nov 20 '18 at 10:44

1

@Sam I think it's fine to do that. Should ideally be documented somewhere. Note, that there is another error that might occur: accessing the array with an out-of-bounds index. This is quite common, you can have a look how std::vector handles that - there are two functions, std::vector::at, which throws on out-of-bounds, and std::vector::operator which result in UB when called with an out-of-bounds index.

– lubgr
Nov 21 '18 at 19:46

Ok thanks!! I will do it.

– Sam
Nov 21 '18 at 20:40

add a comment |

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1 Answer
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class Matrix {

    /* ... */



    class RowProxy {

       public:

          RowProxy(std::valarray<double>& elems, int c, int row) :

              elems(elems), c(c), row(row) {}



          double& operator(int j)

          {

             return elems[row*c + j];

          }



       private:

          std::valarray<double>& elems;

          int row;

          int c;

    };



    RowProxy operator(int i)

    {

       return RowProxy(elems, c, i);

    }

};

This way, you can access the data with two operator.

Matrix m(2, 4); // Assuming the ctor initializes elemens with row*column



m[0][0] = 1.234;

m[1][0] = 2.234;

m[1][3] = -52.023;

answered Nov 19 '18 at 16:03

lubgr

10.6k21845

ok, thanks! In this way I can access the element with m[r][c]. I would ask you, if I want to return an object like a Matrix_row() that contain the "reference" to elements in Matrix object, with the syntax m, how can I do? Is it possible?

– Sam
Nov 19 '18 at 17:08

1

@Sam This is exactly the proxy object. You can use it like this: auto row = m[0]. Then, index the row as row[0]. This is exactly the same as m[0][0].

– lubgr
Nov 19 '18 at 19:13

oooh now I got it, thanks!! Just a question. You speak about error handling strategy, It's a good practise to use the valarray exception in constructor without checking size? I mean, in the constructor I pass int r, int c and I create a valarray with r * c elements, but if r * c is < 0 valarray throw an exception. It's a good practise to use that exception or it is better that I check size before create valarray?

– Sam
Nov 20 '18 at 10:44

1

@Sam I think it's fine to do that. Should ideally be documented somewhere. Note, that there is another error that might occur: accessing the array with an out-of-bounds index. This is quite common, you can have a look how std::vector handles that - there are two functions, std::vector::at, which throws on out-of-bounds, and std::vector::operator which result in UB when called with an out-of-bounds index.

– lubgr
Nov 21 '18 at 19:46

Ok thanks!! I will do it.

– Sam
Nov 21 '18 at 20:40

add a comment |

class Matrix {

    /* ... */



    class RowProxy {

       public:

          RowProxy(std::valarray<double>& elems, int c, int row) :

              elems(elems), c(c), row(row) {}



          double& operator(int j)

          {

             return elems[row*c + j];

          }



       private:

          std::valarray<double>& elems;

          int row;

          int c;

    };



    RowProxy operator(int i)

    {

       return RowProxy(elems, c, i);

    }

};

This way, you can access the data with two operator.

Matrix m(2, 4); // Assuming the ctor initializes elemens with row*column



m[0][0] = 1.234;

m[1][0] = 2.234;

m[1][3] = -52.023;

answered Nov 19 '18 at 16:03

lubgr

10.6k21845

ok, thanks! In this way I can access the element with m[r][c]. I would ask you, if I want to return an object like a Matrix_row() that contain the "reference" to elements in Matrix object, with the syntax m, how can I do? Is it possible?

– Sam
Nov 19 '18 at 17:08

1

@Sam This is exactly the proxy object. You can use it like this: auto row = m[0]. Then, index the row as row[0]. This is exactly the same as m[0][0].

– lubgr
Nov 19 '18 at 19:13

oooh now I got it, thanks!! Just a question. You speak about error handling strategy, It's a good practise to use the valarray exception in constructor without checking size? I mean, in the constructor I pass int r, int c and I create a valarray with r * c elements, but if r * c is < 0 valarray throw an exception. It's a good practise to use that exception or it is better that I check size before create valarray?

– Sam
Nov 20 '18 at 10:44

1

@Sam I think it's fine to do that. Should ideally be documented somewhere. Note, that there is another error that might occur: accessing the array with an out-of-bounds index. This is quite common, you can have a look how std::vector handles that - there are two functions, std::vector::at, which throws on out-of-bounds, and std::vector::operator which result in UB when called with an out-of-bounds index.

– lubgr
Nov 21 '18 at 19:46

Ok thanks!! I will do it.

– Sam
Nov 21 '18 at 20:40

add a comment |

class Matrix {

    /* ... */



    class RowProxy {

       public:

          RowProxy(std::valarray<double>& elems, int c, int row) :

              elems(elems), c(c), row(row) {}



          double& operator(int j)

          {

             return elems[row*c + j];

          }



       private:

          std::valarray<double>& elems;

          int row;

          int c;

    };



    RowProxy operator(int i)

    {

       return RowProxy(elems, c, i);

    }

};

This way, you can access the data with two operator.

Matrix m(2, 4); // Assuming the ctor initializes elemens with row*column



m[0][0] = 1.234;

m[1][0] = 2.234;

m[1][3] = -52.023;

answered Nov 19 '18 at 16:03

lubgr

10.6k21845

class Matrix {

    /* ... */



    class RowProxy {

       public:

          RowProxy(std::valarray<double>& elems, int c, int row) :

              elems(elems), c(c), row(row) {}



          double& operator(int j)

          {

             return elems[row*c + j];

          }



       private:

          std::valarray<double>& elems;

          int row;

          int c;

    };



    RowProxy operator(int i)

    {

       return RowProxy(elems, c, i);

    }

};

This way, you can access the data with two operator.

Matrix m(2, 4); // Assuming the ctor initializes elemens with row*column



m[0][0] = 1.234;

m[1][0] = 2.234;

m[1][3] = -52.023;

answered Nov 19 '18 at 16:03

lubgr

10.6k21845

answered Nov 19 '18 at 16:03

lubgr

10.6k21845

answered Nov 19 '18 at 16:03

lubgr

10.6k21845

answered Nov 19 '18 at 16:03

lubgr

10.6k21845

ok, thanks! In this way I can access the element with m[r][c]. I would ask you, if I want to return an object like a Matrix_row() that contain the "reference" to elements in Matrix object, with the syntax m, how can I do? Is it possible?

– Sam
Nov 19 '18 at 17:08

1

@Sam This is exactly the proxy object. You can use it like this: auto row = m[0]. Then, index the row as row[0]. This is exactly the same as m[0][0].

– lubgr
Nov 19 '18 at 19:13

oooh now I got it, thanks!! Just a question. You speak about error handling strategy, It's a good practise to use the valarray exception in constructor without checking size? I mean, in the constructor I pass int r, int c and I create a valarray with r * c elements, but if r * c is < 0 valarray throw an exception. It's a good practise to use that exception or it is better that I check size before create valarray?

– Sam
Nov 20 '18 at 10:44

1

@Sam I think it's fine to do that. Should ideally be documented somewhere. Note, that there is another error that might occur: accessing the array with an out-of-bounds index. This is quite common, you can have a look how std::vector handles that - there are two functions, std::vector::at, which throws on out-of-bounds, and std::vector::operator which result in UB when called with an out-of-bounds index.

– lubgr
Nov 21 '18 at 19:46

Ok thanks!! I will do it.

– Sam
Nov 21 '18 at 20:40

add a comment |

ok, thanks! In this way I can access the element with m[r][c]. I would ask you, if I want to return an object like a Matrix_row() that contain the "reference" to elements in Matrix object, with the syntax m, how can I do? Is it possible?

– Sam
Nov 19 '18 at 17:08

1

@Sam This is exactly the proxy object. You can use it like this: auto row = m[0]. Then, index the row as row[0]. This is exactly the same as m[0][0].

– lubgr
Nov 19 '18 at 19:13

oooh now I got it, thanks!! Just a question. You speak about error handling strategy, It's a good practise to use the valarray exception in constructor without checking size? I mean, in the constructor I pass int r, int c and I create a valarray with r * c elements, but if r * c is < 0 valarray throw an exception. It's a good practise to use that exception or it is better that I check size before create valarray?

– Sam
Nov 20 '18 at 10:44

1

@Sam I think it's fine to do that. Should ideally be documented somewhere. Note, that there is another error that might occur: accessing the array with an out-of-bounds index. This is quite common, you can have a look how std::vector handles that - there are two functions, std::vector::at, which throws on out-of-bounds, and std::vector::operator which result in UB when called with an out-of-bounds index.

– lubgr
Nov 21 '18 at 19:46

Ok thanks!! I will do it.

– Sam
Nov 21 '18 at 20:40

ok, thanks! In this way I can access the element with m[r][c]. I would ask you, if I want to return an object like a Matrix_row() that contain the "reference" to elements in Matrix object, with the syntax m, how can I do? Is it possible?

– Sam
Nov 19 '18 at 17:08

@Sam This is exactly the proxy object. You can use it like this: auto row = m[0]. Then, index the row as row[0]. This is exactly the same as m[0][0].

– lubgr
Nov 19 '18 at 19:13

oooh now I got it, thanks!! Just a question. You speak about error handling strategy, It's a good practise to use the valarray exception in constructor without checking size? I mean, in the constructor I pass int r, int c and I create a valarray with r * c elements, but if r * c is < 0 valarray throw an exception. It's a good practise to use that exception or it is better that I check size before create valarray?

– Sam
Nov 20 '18 at 10:44

@Sam I think it's fine to do that. Should ideally be documented somewhere. Note, that there is another error that might occur: accessing the array with an out-of-bounds index. This is quite common, you can have a look how std::vector handles that - there are two functions, std::vector::at, which throws on out-of-bounds, and std::vector::operator which result in UB when called with an out-of-bounds index.

– lubgr
Nov 21 '18 at 19:46

Ok thanks!! I will do it.

– Sam
Nov 21 '18 at 20:40

add a comment |

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