Assertion failure multiplying Eigen matrices

Question

I am writing a c++ program for least square leaner regression problem in interpolation. I use Eigen for matrix operations. The problem I am getting is when I run the program it shows an error displaying an assertion error. Here is my code:

#include <iostream>
#include <Eigen/Dense>
using Eigen::MatrixXd;
using namespace std;
int main()
{
    int i;
    int nmbrOfPoints;
    cout<<" Enter the number of data points : ";
    cin>>nmbrOfPoints;

    MatrixXd matY(nmbrOfPoints,1);       //initialize matrix Y
    MatrixXd matX(nmbrOfPoints,2);       //initialize matrix X
    MatrixXd matXdup(nmbrOfPoints,2);      //initialize matrix X duplicate
    MatrixXd matAns(2,1);


    for(i=0;i<nmbrOfPoints;i++)
    {
        matX(i,0)=1;                    // storing the 1 st column of the matrix x, all 1s.
        matXdup(i,0)=1;
    }

    cout<<"Enter all sample points (x and y values ): "<<endl;

    for(i=0;i<nmbrOfPoints;i++)
    {
        cin>>matX(i,1)>>matY(i,0); // read both (x,f(x)) ,, store x values to matrix x and y values to matrix y
    }

    for(i=0;i<nmbrOfPoints;i++)
    {
        matXdup(i,1)=matX(i,1);    //copying matrix x to its duplicate
    }

    cout<<"\n \n";
    cout << matX << endl;
    cout<<"\n \n";
    cout << matY << endl;
    cout<<"\n \n";
    cout << matXdup << endl;

    // find the transpose of matrix x

    cout << "\nHere is the transposed matrix x duplicate:\n" << endl;
    matXdup.transposeInPlace();


    cout << matXdup << endl;
    cout<<"\n \n";
    cout << matX << endl;

    //find the multiplication of x and transpose of x

    matX = matX* matXdup;   // now the matrix x holds the multiplication of transpose of x and x

    cout << "\nmultiplication of x and xdup:\n" << endl;
    cout << matX << endl;
    cout<<"\n \n";

    //find the inverse of x

    double q,a,b,c,d;

    a=matX(0,0);
    b=matX(0,1);
    c=matX(1,0);
    d=matX(1,1);

    q=1/((a*d)-(b*c));

    matX(0,0) = d*q;
    matX(0,1) = b*-1*q;             //now matrix x holds the inverse of x
    matX(1,0) = c*-1*q;
    matX(1,1) = a*q;

    cout<<"\n \n";
    cout << "\n inverse of x:\n" << endl;
    cout << matX << endl;

    //find the multiplication of transpose of x(x duplicate matrix) and y

     matY = matXdup* matY;   // now the matrix x duplicate holds the multiplication of y and x transpose

    //find the multiplication of x(inverse of xt*x) and matXdup (xt*y)

    // matAns = matY* matX;

     cout << "\nfinal answers :\n" << endl;
     cout << "\n *********************:\n" << endl;

     cout << matY << endl;
     cout<<"\n \n";
     cout << matX << endl;

     cout << "\nfinal answer FINAL :\n" << endl;
     cout << "\n *********************:\n" << endl;
     matAns = matY* matX;
     cout << matAns << endl;

     /*cout<<"\n matx dup = \n";
     cout << matXdup << endl;
     cout<<"\n maty =  \n";
     cout << matY << endl;
     cout<<"\n \n";*/

     return 0;


}

I am getting the error from the final multiplication part which is matAns = matY* matX:

Assertion failed: a_lhs.cols() == a_rhs.rows() && "invalid matrix product" && "if you wanted a coeff-wise or a dot product use the respective explicit functions"

When I remove that statement code works. Up to that point the code works fine. Can someone explain me what is assertion problem and how to fix it here?

What is the error? Eigen assertions usually have useful information in them. — David Brown
Please compile with all warnings and debugging info (e.g. g++ -Wall -g) and learn how to use the debugger (e.g. gdb on Linux). — Basile Starynkevitch
In general an assertion error indicates that you violated the 'contract' of the function you called in some way. You haven't provided much detail but since you are doing matrix multiplication I would guess that you've picked two matrices whose size means they cannot be multiplied. — john
error says Assertion failed: a_lhs.cols() == a_rhs.rows() && "invalid matrix product" && "i f you wanted a coeff-wise or a dot product use the respective explicit functions — buddhi weerasinghe

ggael ggael · Accepted Answer · 2013-11-23T10:37:33

matY is a 2x1 vector and matX is a NxN matrix, so the product matY * matX is invalid. Are you sure you don't want to compute matX as:

matX = matXdup * matX;

and matAns as:

matAns = matX * matY;

?

BTW, no need to explicitly transpose matXdup with transposeInPlace, you can directly do:

matX = matXdup.transpose() * matX;

Moreover, when a dimension is known at compiletime and that this dimension is very small, better specify it. For instance, matY should rather be a VectorXd. The result of matXdup.transpose() * matX should rather be stored in a Matrix2d object. Then call inverse() instead of writing your own inverse routine (you need to include <Eigen/LU>:

Matrix2d XX = matXdup.transpose() * matX; 
Vector2d Y = matXdup * matY;
Vector2d ans = XX.inverse() * Y;

Assertion failure multiplying Eigen matrices

1 Answers