C++: How to use a 'for' loop to multiply 2D arrays?

Question

I'm writing a program to populate two multidimensional arrays and multiply their elements using for loops only. The code's pretty simple yet I'm stuck with a host of errors from the multiplication loop. I can't wrap my head around this one.

Is there a problem with my loop syntax? I'll attach the code below. Lines 48 and 49 are the loop code for multiplication. Thanks in advance.

#include <iostream>
#include <iostream>
using namespace std;

int main()
{

cout << "Creating array: " << endl;

cout << " " << endl;

cout << "Please enter the number of rows: "; 
int numRows2 = 0;
cin >> numRows2;

cout << " " << endl;

cout << "Please enter the number of columns: ";
int numCols2 = 0;
cin >> numCols2;

double myNums2[numRows2][numCols2];

for (int counter = 0; counter < numRows2; counter++)
    {
        for (int counter_a = 0; counter_a < numCols2; counter_a++)
           {
            cout << "Enter Element [" << counter << "][" << counter_a << "]: ";
            cin >> myNums2[counter][counter_a];
            }
    }

cout << " " << endl;

cout << "Displaying contents of the array: " << endl;

for (int counter = 0; counter < numRows2; ++counter)
    {
        for (int counter_a = 0; counter_a < numCols2; ++counter_a)

            cout << "Element [" << counter <<"][" << counter_a << "]: " << myNums2[counter][counter_a] << endl;
    }


cout << " " << endl;

cout << "Multiplying each element in myNums1 by each element in myNums2: " << endl;

for (int indexRow1 = 0, int indexCol1 = 0; indexRow1 < numRows, indexCol1 < numCols; ++indexRow1, ++indexCol1)
    for (int indexRow2 = 0, int indexCol2; indexRow2 < numRows2, indexCol2 < numCols2; ++indexRow2, ++indexCol2)
        {
            cout << myNums1[indexRow1][indexRow2] << " x " << myNums2[indexRow2][indexCol2] << " = " << myNums1[indexRow1][indexCol1] * myNums2[indexRow2][indexCol2] << endl;
        }

return 0;
}

`cin >> numCols2; double myNums2[numRows2][numCols2];` is a VLA and is not valid C++. Use `std::vector` instead of arrays. — cigien, Jan 29 '21 at 05:09
That syntax works perfectly when I comment out the multiplication code. This specific project requires the use of arrays. — c.dasa, Jan 29 '21 at 05:21
I'm sorry to hear that the project *requires* you to use invalid code. Are you sure that `vector` is not allowed? If so, maybe you're supposed to dynamically allocate the arrays, or maybe you can create arrays with a constant size that will not be exceeded. — cigien, Jan 29 '21 at 05:39
I'll have to double-check the project requirements... I may have misread something. Using std::vector and dynamically allocating the arrays is the better solution. — c.dasa, Jan 29 '21 at 06:20
In the last 'cout': myNums1[R1][R2] << " x " << myNums2[R2][C2] = myNums1[R1][C1] * myNums2[R2][C2]. The indexes is knid of mixed. Is that you intention? — ytlu, Jan 29 '21 at 08:05

score 1 · Accepted Answer · answered Jan 29 '21 at 06:02

You can not declare two variables in the for loop as you are doing it. You can check this thread for ways to achieve it.

On the snippet you share, certain variables are not declared, numRows, numCols, myNums1.

indexCols2 is not initialized, even though the for loop is not correct as it is.

As @cigien mentioned in the comment, to use an array you will have to allocate memory dynamically if you want to read the size from the user.

score 1 · Answer 2 · answered Jan 29 '21 at 06:24

There are two errors in your code at the for loops:

for (int indexRow1 = 0, int indexCol1 = 0; indexRow1 < numRows, indexCol1 < numCols; ++indexRow1, ++indexCol1)
    for (int indexRow2 = 0, int indexCol2; indexRow2 < numRows2, indexCol2 < numCols2; ++indexRow2, ++indexCol2)

First: This declaration is wrong

for (int indexRow1 = 0, int indexCol1 = 0, ...)

The correct one is:

for (int indexRow1 = 0, indexCol1 = 0, ...)

Second: You didn't declare numRows and numCols before the for loop. You may need to check your code and declare them.

C++: How to use a 'for' loop to multiply 2D arrays?

2 Answers2