Question

I’m very much still a beginner at programming but I have come across the error “lvalue required as left operand of assignment” and I am unsure about how to resolve this issue after looking through various other discussions. The error appears in a class I made for Matrices when I overloaded certain operators. Here is part of the code,

#ifndef MATRIX_H
#define MATRIX_H

#include <iostream>
#include "MVector.h"


class Matrix {
private:
vector<double> columns;
vector<vector<double> > A;
public:
//constructor
explicit Matrix(){};
explicit Matrix(int n,int m):columns(m),A(n,columns){};
explicit Matrix(int n,int m,double x):columns(m,x),A(n,columns){};
//destructor
~Matrix(){};
//equate matrices
Matrix &operator=(const Matrix &rhs) {A=rhs.A;return *this;};
//set all values to a double
Matrix &operator=(double x) 
{
    int rows=this->rows();
    int cols=this->cols();
    for (int i=0;i<rows;i++) 
    {
        for (int j=0;j<cols;j++)
        {
            A[i][j]=x;
        }
    }
}
//access data in matrix (const)
double operator()(int i,int j) const {return A[i][j];};
//access data in matrix
double operator()(int i,int j) {return A[i][j];};
//returns the number of rows
int rows() const {return A.size();};
//returns the number of cols
int cols() const {return columns.size();};
//check if square matrix or not
bool check_if_square() const 
{
    if (rows()==cols()) return true;
    else return false;
}
};

and this is one of the overloaded operators which produces the error

const Matrix operator+(const Matrix &A,const Matrix &B)
{
//addition of matrices
//check dimensions
if (!(A.cols()==B.cols()) || !(A.rows()==B.rows()))
{
    cout << "Error: Dimensions are different \n Ref: Addition of Matrices"; 
    throw;
}
else
{
    int dim_rows = A.rows();
    int dim_cols = B.cols();
    Matrix temp_matrix(dim_rows,dim_cols);
    for (int i=0;i<dim_rows;i++)
    {
        for (int j=0;j<dim_cols;j++)
        {
            temp_matrix(i,j)=A(i,j) + B(i,j);
        }
    }
    return temp_matrix;
}
}

I assume I’ve done something wrong, if anyone can help and explain what I’m doing wrong that would be really appreciated. Thanks for the help!

Answer 1

It means that you cannot assign to the result of an rvalue-expression, in this case the temporary returned by operator()(int,int). You probably want to change your non-const operator()(int,int) in the Matrix class to be:

double& operator()( int x, int y ) { return A[i][j]; }

Additionally (and unrelated to the question) you might want to simplify your matrix class and store only the dimensions and a single one dimensional vector to store all the elements. Then the accessors would perform some basic arithmetic (something like row*columns()+column) to get to the actual value in the one dimensional vector.