C++ Dynamic 2D-Array with pointers(Matrix)

Question

everyone:

Im creating a program that is able to create Matrices and and perform various operations on them for a course at school. They require us to overload the operators with with the appropriate Matrix Operations.

I am having a hard time with the following function:

typedef double matrixType;


using namespace std;


class Matrix{
protected:
    int m,n; // m:row size n:column size
    matrixType **a; //Allows us to acces the a(ij) i,j position of the matrix


//==================================================
// (==Operator)Verifies if two given Matrices are equal
//==================================================

bool Matrix::operator==(const Matrix &B){

bool flag=false;


if(B.m ==m && B.n ==n){

    for (int row=0; row<m; row++) {
        for (int col=0; col<n; col++) {
            if (B[row][col] != a[row][col]) {
                flag=false;
            }
        }
    }
    flag= true;
}

else{
    flag=false;

}

return flag;


}

Xcode warns me that at the following line:

 if (B[row][col] != a[row][col])

type 'const Matrix' doesn't provide a subscript operator

Note: Function Headers,constructors and other functions have been omitted from this code portion.

Any help would be greatly appreciated. Thank you.

Lior Kogan · Accepted Answer · 2011-10-19 21:06:58Z

5

Given your implementation, it should be if (B.a[row][col] != a[row][col])

BTW: You should read this page if you plan to implement your own matrix class.

edited Oct 19, 2011 at 21:06

answered Oct 19, 2011 at 21:01

Lior Kogan

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1 Comment

Mooing Duck Over a year ago

He should probably also give it a subscript operator.

cli_hlt · Accepted Answer · 2011-10-19 21:02:33Z

1

What you meant to do was

if (B.a[row][col] != a[row][col]) {
                flag=false;
}

And if you give it a moment of thought, couldn't you simply do

if (B.a[row][col] != a[row][col]) {
                return false;
}

and skip the flag variable altogeher? If yes, why, if not, why not (awards you bonus points ;).

If you want to really do B[][] you would have to implement operator[] for your class:

matrixType* operator[](size_t index) 
{
  return a[index];
}

answered Oct 19, 2011 at 21:02

cli_hlt

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2 Comments

edu222 Over a year ago

The reason I was using a flag was because I was worried by only having return statements inside an IF structure. I did try to do it without a flag and got a Warning from my IDE: 'control may reach end of non-void function'

cli_hlt Over a year ago

@edu222 Well yes, you should put another return statement at the end. So you need 3: 1. return false if B.m!=m&&B.n!=n 2. return false if the if fails (thats also called early-out, imagine a 10^10x10^10 matrix and all the unnecessary comparisions when they differ at index 0,0 ;) 3. return true at the end of the method.

totowtwo · Accepted Answer · 2011-10-19 21:11:59Z

1

Take a cue from boost::ublas::matrix, boost::gil::view_type, OpenCV::Mat, etc and use operator(int,int) for indexes instead of the subscript operator. It is worlds easier to implement, maintain, and debug.

answered Oct 19, 2011 at 21:11

totowtwo

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1 Comment

edu222 Over a year ago

HI, Thanks for the suggestion, I will check it as soon as I have some time. For now Ill finish my homework with the current implementation since I have it almost ready.:)

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