C -Output multiplication of two matrices using 2D matrix-CodePudding

I am new in programming and was doing a question about multipliaction of two matrix which are inputted from the user. I think I wrote the correct code for it. However, the output is a null matrix and I cannot pin point the mistake.

#include<iostream>
using namespace std;
int main(){
    int row1,col1,row2,col2,val;
    cin>>row1>>col1>>col2;
    int arr1[row1][col1];
    int arr2[row2][col2];
    int arr3[row1][col2];
    row2=col1;
    for(int i=1;i<=row1;i  ){
        for(int j=1;j<=col1;j  ){
            cin>>val;
            arr1[row1][col1]=val;
        }
        cout<<endl;
    }
    for(int i=1;i<=row2;i  ){
        for(int j=1;j<=col2;j  ){
            cin>>val;
            arr1[row2][col2]=val;
        }
        cout<<endl;
    }
    for(int i=1;i<=row1;i  ){
       for(int k=1;k<=col2;k  ){
          for(int j=1;j<=col1;j  ){
                arr3[i][k] =arr1[i][j]*arr2[j][k];
                cout<<arr3[i][k]<<" ";
            }
            cout<<endl;
        }
     }

}

CodePudding user response：

When you are setting up your matrices (arr1 and arr2) you are using an incorrect index.

for(int j=1;j<=col1;j  ){
    arr1[row1][col1]

should be

for(int j=1;j<=col1;j  ){
    arr1[i][j]

Also, as mentioned, indices in C and C go from 0 to N-1, so you are accessing one value out-of-bounds. You should use:

for (int j = 0; j < col1; j  ) {
    arr1[i][j]

Also, you are setting up arr1 twice, but forgetting to set up arr2.

CodePudding user response：

On top of the 0-based index issue already mentioned, the size of an array must be known at compile-time in C . This is undefined behavior:

int row1,col1;
cin>>col1>>col2;
int arr1[row1][col1]; // row1 and col1 are only known at runtime

Try this instead

int rows, cols;
cin >> rows >> cols;
std::vector<std::vector<int>> mat(rows, std::vector<int>(cols, 0));