Sum up values in multidimensional array in c++ and store it in another multidimensional array

Question

I try to sum up the values in an array, and would like to store it in another array.

#include <cstdlib>
#include <iostream>

using namespace std;


int main() {
    int rev[2][12] = {{10,20,30,40,50,60,70,80,90,100,110,120},
                        {100,200,300,400,500,600,700,800,900,1000,1100,1200}};
    
    int temp = 0;
    for (int j = 0; j<2;j++){
        for(int i = 0; i<12; i++){
            temp += rev[j][i];
        }
        cout << "rev in year " << j+1 << ": " << temp << "\n";
        temp = 0;
    }
    
    int revYear[2][1];
    for (int j = 0; j<2;j++){
        for(int i = 0; i<12; i++){
            revYear[j][0] += rev[j][i];
        }
    }
    cout << "rev in year 1: " << revYear[0][0] << "\n";
    cout << "rev in year 2: " << revYear[1][0] << "\n";
    return 0;
}

The first two for loops give me the desired output I'd like to store in revYear, which I tried in the second step. But it returns:

rev in year 18363800
rev in year 278010

Can anyone help me with this? Is it a compiler issue?

I´m using Mac and Xcode but I also ran the code with MS VS on Windows. Same problem, different output.

Please note: in the upper part, i just wanted to show that I found a way to get the desired output.

Answer 1

The first two for loops give me the desired output

Let's see why

// This variable is declared and contextually assigned a meaningful value: zero.
int temp = 0;

for (int j = 0; j<2;j++){
    for(int i = 0; i<12; i++){
        // Here it's updated, we want it to hold the sum. 
        temp += rev[j][i];
    }
    // Now it has reached its final value and can be shown.
    cout << "rev in year " << j+1 << ": " << temp << "\n";

    // Here it is RESETTED, so that it's ready for the next iteration.
    // It would be an error to start the sum from any value other than zero.
    temp = 0;
}

This snippet produces the correct result, but it's not quite idiomatic. You could rewrite it like the following:

for (int j = 0; j<2; j++)
{
    int temp = 0;  // <--- Initialize inside the loop, at the beginning.
    for(int i = 0; i<12; i++)
    {
        temp += rev[j][i];
    }
    std::cout << "rev in year " << j+1 << ": " << temp << "\n";
}

Even better, you could use one of the algorithms of the Standard Library, std::accumulate:

for (size_t i{}; i < std::size(rev); ++i)
{
    std::cout << "revenue in year " << i + 1 << ": "
              << std::accumulate( std::begin(rev[i]), std::end(rev[i])
                                , 0 )  // <--- Initialization of the sum
              << '\n';
}

Now it should be clear why the second nested loop in the question's posted code fails.

// This variable is declared, but NOT initialized. Its elements have
// UNDETERMINED values, they could be equal to zero only by accident.
int revYear[2][1];

for (int j = 0; j<2;j++){
    for(int i = 0; i<12; i++){
        // While the code accumulates the values correctly, we don't know
        // the initial value, so the result will be likely wrong. 
        revYear[j][0] += rev[j][i];
    }
}

To fix it, we can properly initialize the array (it's still unclear to me why they want a 2D one, but that seems part of assignment).

int revYear[2][1]{};

Which IMHO is preferable to set the correct value at the beginning of the loop.

int revYear[2][1];   
for (int j = 0; j<2;j++)
{
    revYear[j][0] = 0;  // <---   
    // ...
    std::cout << "revenue in year " << j + 1 << ": " << revYear[j][0] << "\n";
}