Multiplication error in matrices if I do it in a loop

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This is my function

C++

void clear_up(MatrixXd A, int dim)  //to set all initial values of the matrix to 0
{
    for(int i=0;i<dim;i++)
    {   for(int j=0;j<dim;j++)
        {   A(i,j)=0;   }   }
}

int sumrow(MatrixXd A,int row, int dim)   //adding up elements of a particular row
{
    int sum=0;
    for(int i=0;i<dim;i++)
    {
        sum=sum+A(row,i);
    }
    return sum;
}

MatrixXd generate_B(MatrixXd A, int dim)  //A is a symmetric matrix with diagonal elements 0 and other elements either 0 or 1
{
    cout<<"A \n"<<A<<"\n";
    MatrixXd B(dim,dim), inter1(dim,dim),   inter2(dim,dim),    D(dim,dim);
    clear_up(B,dim);    clear_up(inter1,dim);    clear_up(inter2,dim);  clear_up(D,dim);
    for(int i=0;i<dim;i++)
    {
        for(int j=0;j<dim;j++)
        {
            if(i==j)
            {
                D(i,j)= 1/sqrt(sumrow(A,i,dim));     //D is thus a diagonal matrix
            }
            else    {   D(i,j) = 0;     }
        }
    }
    cout<<"D \n"<<D<<"\n";
    for(int a=0;a<dim;a++)
    {
        for(int b=0;b<dim;b++)
        {
            for(int c=0;c<dim;c++)
            {
                inter1(a,b)+= A(a,c)*D(c,b);    //inter1 = A*D
            }
        }
    }
    cout<<"inter1 \n"<<inter1<<"\n";
    for(int x=0;x<dim;x++)
    {
        for(int y=0;y<dim;y++)
        {
            for(int z=0;z<dim;z++)
            {
                inter2(x,y)+= D(x,z)*inter1(z,y);   //inter2 = D*A*D
            }
        }
    }
    cout<<"inter2 \n"<<inter2<<"\n";
    for(int X=0; X<dim; X++)
    {   for(int Y=0; Y<dim; Y++)
        {   if(X==Y)    {   B(X,Y) = 1 - inter2(X,Y);   }
            else    {   B(X,Y) = 0 - inter2(X,Y);   }
        }       //B = I - D*A*D (I = identity matrix)
    }
    cout<<"B \n"<<B<<"\n";
    clear_up(inter1,dim);   clear_up(inter2,dim);   clear_up(D,dim);
    return B;
}

A is a symmetric matrix with diagonal elements as 0 and non diagonal elements as 0 or 1. Mathematically, B should be a symmetric matrix with diagonal elements 1, which it is for 1 iteration. However, when I run it in a loop, some matrices are symmetric whereas some are not. What could be the reason?

What I have tried:

Tried splitting it into different functions. Also tried clearing up junk values.

Posted 7-Jun-17 7:32am

Member 13246365

Updated 7-Jun-17 8:16am

Patrice T

v3

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ZurdoDev 7-Jun-17 13:55pm

Debug your code and find out what is wrong.

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OriginalGriff · Answer 1 · 2017-06-07T08:03:00

Quote:
Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:
Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16
Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

C#
private int Double(int value)
   {
   return value * value;
   }
Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on your line:

C++
cout<<"A \n"<<A<<"\n";
and run your app. Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?

This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!

Yes, I could probably tell you what "the problem" is - but it's not difficult to do this yourself, and you will learn something really worthwhile at the same time!

Patrice T · Answer 2 · 2017-06-07T08:16:00

Quote:
However, when I run it in a loop, some matrices are symmetric whereas some are not. What could be the reason?

Serious ? you did not even dared to give an example of the problem.
The minimum is an example with calling sequence to reproduce the problem.

There is a tool that allow you to see what your code is doing, its name is debugger. Mastering the debugger is not optional, it is mandatory for any programmer, no exception.
It is also a great learning tool because it show you reality and you can see which expectation match reality.
When you don't understand what your code is doing or why it does what it does, the answer is debugger.
Use the debugger to see what your code is doing. Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute, it is an incredible learning tool.

Debugger - Wikipedia, the free encyclopedia[^]
Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't find bugs, it just help you to. When the code don't do what is expected, you are close to a bug.

Multiplication error in matrices if I do it in a loop

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