Construct an empty matrix

Question

I have a class

class A
{
    int *const e;
    const int row, column;
public:
    A(int r, int c); // create a empty matrix with r rows and c columns passed in method
}

int main(int argc, char* argv[])
{
    A test(2,2); 
    return 0;
}

Question is how can I write a constructor that create a matrix which I can use ?

if you want any prebuilt matrix functionality look into libraries like Eigen — chris, Dec 04 '14 at 01:48
`memset` is your friend, assuming you really just want an "empty" (which, BTW, has no real meaning in software development) matrix. Also, since the word `empty` appears in your question title, but not in the body, I have no idea what you're really asking for. — 3Dave, Dec 04 '14 at 02:08
Also, stop using parameter names like `r` and `c`. I would either a) not hire you fresh out of school, or b) fire your ass after a few successive code reviews revealing that variable naming was not your forte'. — 3Dave, Dec 04 '14 at 02:12

Cory Kramer · Answer 1 · 2014-12-04T02:02:25.187

1

Your constructor would just be

A::A(int r, int c) : row{r}, column{c}
{
    e = new int[r * c];
}

Then your destructor should be

A::~A()
{
    delete[] e;
}

And you can access the elements as

for (int i = 0; i < row; ++i)
{
    for (int j = 0; j < column; ++j)
    {
        std::cout << e[i * row + j] << " ";  // Using array arithmetic
    }
    std::cout << std::endl;
}

edited Dec 04 '14 at 02:02

answered Dec 04 '14 at 01:56

Cory Kramer

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Error 1 error C2758: 'A::e' : a member of reference type must be initialized So should I first initialized variable first ? – mskuratowski Dec 04 '14 at 02:49

score 0 · Accepted Answer · answered Dec 04 '14 at 02:02

Your question sound like: How to save parameters into the constant row/column properties.

A::A(int r, int c): row(r), column(c), e(new int[r*c])
{
    for (int i=0; i<r*c; i++) e[i]=0;
}

Also do not forgot the destructor:

virtual ~A(){ delete[] e; };

In the parameter-list of the constructor, parameters are initialized in declaration order, so you cannot use row/column (they are not already initialized).

score 0 · Answer 3 · edited May 23 '17 at 12:13

0

You need a pointer to a pointer to create a [normal] 2-dimensional array so

int ** const e;

Then the constructor can work like:

e = (int**) malloc(sizeof(int**) * a);
for(int i=0;i<a;++i) {
  e[i] = (int*) malloc(sizeof(int*) * b);
}

Your next problem is "how to initialize the constants". For this, you can refer to the answer here: how to initialize const member variable in a class C++

to implement this, put the inittializer code in a function:

initConstArray(const int** const a, int r, int  c) {
    e = (int**) malloc(sizeof(int**) * a);
    for(int i=0;i<r;++i) {
      e[i] = (int*) malloc(sizeof(int*) * b);
      for(int j = 0;j < c; ++j) {
        e[i][j] = a[i][j];
    }

and call this function from the constructor initializer list:

A(int **a, int r, int c) : initConstArray(a, r, c) {
}

edited May 23 '17 at 12:13

Community

1
1

answered Dec 04 '14 at 02:06

gk_2000

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3

malloc? I may consider to use hard-coded pointer instead of smart pointers, but that is too deprecated for me. – Adrian Maire Dec 04 '14 at 02:08
Excuse my code. It is just here for the conceptual explanation. - Thanks – gk_2000 Dec 04 '14 at 02:10
Just joking :-) but yes, there is much better. – Adrian Maire Dec 04 '14 at 02:11

score 0 · Answer 4 · answered Dec 04 '14 at 02:49

Here is another way to implement the matrix with templates:

#include <array>
#include <iostream>

template <std::size_t ROW, std::size_t COLUMN>
class A
{
    std::array<int, ROW*COLUMN> a;
public:
    A():a(){};
};


int main(int argc, char* argv[])
{
    A<3,3> test; 
    //test.printSize();
    //std::cout << test;
    return 0;
}

Much shorter and cleaner.

To make commented lines to work, you must add to the class the following 2 functions:

void printSize(){ std::cout << ROW << "x" << COLUMN << std::endl; };

friend std::ostream& operator<<( std::ostream& os, A<R,C> mat){
    int col_count = 0;
    for (auto id=mat.a.begin(); id!=mat.a.end(); id++){
        os << *id << (++col_count%C==0?"\n":" ");
    }

    return os;
};

Construct an empty matrix

4 Answers4