Matrix multiplication using 2d vector - c++

Question

Following is my code:
void matrix(int rowsInA, int columnsInA, int columnsInB){
std::vector< vector<int> > a;
std::vector< vector<int> > b;
for (int i = 0; i < rowsInA; i++) {
vector<int> myRow(1);
a.push_back(myRow);
for (int j = 0; j < columnsInA-1; j++) {
int x = rand() % 100;;
myRow.push_back(x);
}
}
for (int i = 0; i < rowsInA; i++) {
vector<int> myRow(1);
b.push_back(myRow);
for (int j = 0; j < columnsInB-1; j++) {
int x = rand() % 100;;
myRow.push_back(x);
}
}
std::vector< vector<int> > c = multiply(a, b);
for (int i = 0; i < rowsInA; i++) {
for (int j = 0; j < columnsInB; j++) {
cout << c[i][j];
}
}
}
std::vector< vector<int> > multiply(std::vector< vector<int> > a , std::vector< vector<int> > b) {
int rowsInA = 9;
int columnsInA = 9; // same as rows in B
int columnsInB = 9;
std::vector< vector<int> > d;
for (int i = 0; i < rowsInA; i++) {
for (int j = 0; j < columnsInB; j++) {
vector<int> myRow;
myRow.push_back(a[i][0]);
d.push_back(myRow);
for (int k = 0; k < columnsInA; k++) {
myRow.push_back(myRow[i]+ a[i][k] * b[k][j]);//error here
}
}
}
return d;
}
first function matrix() creates two vector within a vector and assigns random values to it and then call multiply in which vectors are multiplied.
It is giving vector subscript out of range error

Answer 1

Your code:
for (int i = 0; i < rowsInA; i++) {
vector<int> myRow(1);
a.push_back(myRow);
for (int j = 0; j < columnsInA-1; j++) {
int x = rand() % 100;;
myRow.push_back(x);
}
}
Constructs a single element vector, pushes it on to a, appends some
values to your temporary, and then throws it away. C++ is not Java or C#. The vector at the back of a is not changed by changes to myRow.
You also seem to be putting a fixed value as the first element of myRow, and
then appending randomness to it. Is that what you meant?
What you need is:
for (int i = 0; i < rowsInA; i++) {
vector<int> myRow;
myRow.reserve(columnsInA);
myRow.push_back(0); // First element fixed.
for (int j = 1; j < columnsInA; j++) {
const int x = rand() % 100;;
myRow.push_back(x);
}
a.push_back(myRow);
}
Alternatively, making all the values random, and working directly with the vector:
a.reserve(rowsInA);
for (int i = 0; i < rowsInA; i++) {
a.push_back( {} );
a.back().reserve(columnsInA);
for (int j = 0; j < columnsInA; j++) {
const int x = rand() % 100;;
a.back().push_back(x);
}
}
You have a similar problem with initializing B, and your multiply loop needs to accumulate a[i][k] * b[k][j] into a temporary sum, then push that onto myRow, and finally push myRow onto d.
Finally, when you have got this working, look up how to write a Matrix class that stores all the data in a vector with rows*cols elements and then uses an indexing function to refer to it. Your cache will thank you.