How to use vector acsess element through function - visual-c++

Question

This code is made for the multiplication of two arrays given by the user
typedef vector<vector<int> > arr ;
void multiply (arr &arr1 ,arr &arr2 )
{
arr res ;
unsigned new_row = arr1.size() ;
unsigned new_col = arr2.at(0).size();
for(int i = 0 ; i < new_row ; i++)
{
vector <int> vec ;
res.push_back(vec ) ;
for(int j = 0 ; j<new_col ;j++)
{
int x = 0 ;
res.at(i).push_back(x);
for(unsigned k =0 ; k <arr2.size();k++)
{
res.at(i).at(j) += arr1.at(i).at(k)*arr2.at(k).at(j);
}
cout<< res.at(i).at(j) ;
}
}
}
int main()
{
unsigned rows_number1 = 0 , columns_number1 = 0 ;
arr arr1 ;
cout<<"MATRIX A "<<endl<<endl ;
cout << "The Rows : " ;
cin >> rows_number1 ;
cout << "The Columns :" ;
cin>> columns_number1 ;
for(int i = 0 ; i<rows_number1;i++)
{
vector<int> newr ;
arr1.push_back(newr);
for(int j = 0; j<columns_number1 ;j++)
{
int x ;
cout<<"The Member ("<<i+1<<","<<j+1 <<") :" ;
cin>>x ;
arr1.at(i).push_back(x);
}
}
unsigned rows_number2 = 0 , columns_number2 = 0 ;
arr arr2 ;
cout<<"MATRIX B "<<endl<<endl ;
cout << "The Rows : " ;
cin >> rows_number2 ;
cout << "The Columns :" ;
cin>> columns_number2 ;
for(int i = 0 ; i<rows_number2;i++)
{
vector<int> newr ;
arr1.push_back(newr);
for(int j = 0; j<columns_number2 ;j++)
{
int x ;
cout<<"The Member ("<<i+1<<","<<j+1 <<") :" ;
cin>>x ;
arr1.at(i).push_back(x);
}
}
system("cls");
if(columns_number1!=rows_number2)
{
cout<<"Error Multiplication Dimensions" <<endl ;
}
else
{
cout << "A * B ="<<endl;
multiply(arr1,arr2);
}
}
why there is an error and what is the other way ??
how i can improve the code to multiplicate two arrays
Edited : i tried with two 2*2 arrays using console input and output and this is my full code
The problem is out_of_range exception but i don't know why

Answer 1

When you mulitply to matrices, there are strict constraints on the input.
The number of columns of the first matrix must be the same as the number of rows of the second matrix.
When your matrix is represented by a std::vector<std::vector<int>>, you'll have to make sure that ALL the nested std::vectors are of the same size.
Also, don't assume that arr2 is non-empty. When it is empty, arr2.at(0) will thrown an exception.
Here's a more robust version of your function.
void multiply (arr &arr1 ,arr &arr2 )
{
arr res;
unsigned num_rows1 = arr1.size();
if ( num_rows1 == 0 )
{
// Can't do much.
// Return.
return res;
}
unsigned num_cols1 = arr1[0].size();
if ( num_cols1 == 0 )
{
// Can't do much.
// Return.
return res;
}
// Inner vector size check of arr1.
for(unsigned int i = 1 ; i < num_rows1 ; i++)
{
if ( num_cols1 != arr1[i].size() )
{
throw std::runtime_error("Bad input");
}
}
// Make sure the number of columns in arr1 is the same as
// number of rows in arr2.
unsigned num_rows2 = arr2.size();
if ( num_cols1 != num_rows2 )
{
throw std::runtime_error("Bad input");
}
unsigned num_cols2 = arr2[0].size();
if ( num_cols2 == 0 )
{
// Can't do much.
// Return.
return res;
}
// Inner vector size check of arr2.
for(unsigned int i = 1 ; i < num_rows2 ; i++)
{
if ( num_cols2 != arr2[i].size() )
{
throw std::runtime_error("Bad input");
}
}
// All inputs appear to be valid.
// Now, do the multiplication.
unsigned new_row = num_rows1;
unsigned new_col = num_cols2;
res.resize(num_rows1);
for(unsigned i = 0 ; i < new_row ; i++)
{
for(unsigned j = 0 ; j<new_col ;j++)
{
int x = 0 ;
res.at(i).push_back(x);
for(unsigned k =0 ; k < num_cols1; k++)
{
res.at(i).at(j) += arr1.at(i).at(k)*arr2.at(k).at(j);
}
cout<< res.at(i).at(j) ;
}
}
}