Reading data from file and performing matrix multiplication in c++ - c++

Question

I am new to C++, and am trying to perform a matrix multiplication taking data from a file. But I am unable to get the multiplication part. Someone please help me to fix this.
While am trying to do the multiplication of 2 matrices but am unable to get the output.
#include <iostream>
#include <fstream>
using namespace std;
int main()
{
string line;
int d[3][3],e[3][3],f[3][3];
int i=0;
int x=0;
int j=0;
string a[20];
string b[20];
string c[20];
ifstream myfile;
myfile.open("numeric.txt");
while(getline (myfile,line))
{
if(line!=" ")
{
a[x]=line;
b[x]=line;
cout<<a[x]<<endl;
x++;
}
}
cout<<"first Matrix"<<endl;
for(i=0;i<3;i++)
{
for(j=0;j<=4;j++)
{
cout<<a[i][j]<<"";
}
cout<<endl;
}
cout<<"Second Matrix"<<endl;
for(i=1;i<4;i++)
{
for(j=0;j<6;j++)
{
cout<<b[i][j]<<"";
}
cout<<endl;
}
cout<<"Multiplication"<<endl;
for(i=0;i<3;i++)
{
for(j=0;j<=3;j++)
{
c[i][j]=0;
for(int k=0;k<3;k++)
{
c[i][j]+=a[i][k]*b[k][j];
}
}
}
cout<<"Multiplication Result"<<endl;
for(i=0;i<3;i++)
{
for(j=0;j<3;j++)
{
cout<<c[i][j]<<" ";
}
cout<<endl;
}
return 0;
}

Answer 1

Here is a complete example on how you write (fixed size) matrices to a file and how you can build them reading from that file:
#include <iostream>
#include <fstream>
#include <cmath>
using namespace std;
int main() {
const size_t matrixSize = 3;
//
ofstream matrixOutput("matrix.txt");
for (size_t i = 0; i < matrixSize; i++) {
for (size_t j = 0; j < matrixSize; j++) {
matrixOutput << j*i << ' ';
}
matrixOutput << '\n';
}
for (size_t i = 0; i < matrixSize; i++) {
for (size_t j = 0; j < matrixSize; j++) {
matrixOutput << j*i * 2 << ' ';
}
matrixOutput << '\n';
}
matrixOutput.close();
//
ifstream matrixData("matrix.txt");
size_t matrixInput[matrixSize][matrixSize];
size_t matrixInput2[matrixSize][matrixSize];
size_t position = 0;
size_t number = 0;
while (matrixData >> number) {
const size_t matrixNumber = size_t(floor(position / (matrixSize*matrixSize)));
const size_t row = size_t(floor(position / matrixSize)) % matrixSize;
switch (matrixNumber) {
case 0:
matrixInput[row][position % 3] = number; break;
case 1:
matrixInput2[row][position % 3] = number; break;
}
position++;
}
matrixData.close();
cout << "Matrices: " << endl;
for (size_t i = 0; i < matrixSize; i++) {
for (size_t j = 0; j < matrixSize; j++) {
cout << matrixInput[i][j] << ' ';
}
cout << endl;
}
cout << endl;
for (size_t i = 0; i < matrixSize; i++) {
for (size_t j = 0; j < matrixSize; j++) {
cout << matrixInput2[i][j] << ' ';
}
cout << endl;
}
cout << endl;
cout << "Matrices multiplication: " << endl;
for (size_t i = 0; i < matrixSize; i++) {
for (size_t j = 0; j < matrixSize; j++) {
cout << matrixInput[i][j] * matrixInput2[i][j] << ' ';
}
cout << endl;
}
// ...
}
Note: if you want you can write the matrix size to the file too, so you can retrieve it later and build custom size matrices.