C++ How to scan 2D array from a text file and sum the rows?

Question

File info:

6 10

0 1 0 1 0 1 1 1 1 1

0 1 1 1 1 1 1 1 1 0

0 0 1 1 1 1 1 1 1 0

0 1 1 1 1 0 1 0 1 1

0 1 1 1 1 1 0 1 1 1

0 1 1 1 0 1 1 0 1 1

The script:

#include <iostream>
#include <fstream>

using namespace std;

int main()
{
    int x; int y;   
    int masyvas[x][y];  // masyvas is 2D array

    int sum = 0;

    ifstream D ("Duomenys.txt"); // the files name

    D >> x >> y; // x and y is 6 and 10
    // The scanning/reading 
    for (int i = 1; i < x + 1; i++)
    {
        for (int j = 1; j < y + 1; j++)
        {           
            D >> masyvas[i][j]; 
        }       
    }   
    // the printing
    for (int i = 1; i < x + 1; i++){
        for (int j = 1 ; j < y + 1; j++){

            cout << masyvas[x][j] << "  ";
            sum = sum + masyvas[i][j];                              
        }
        cout << sum << endl;            
        sum = 0;
    }   
    D.close();  

    return 0;
}

Now when I test this file ( compile it ) getting no errors but I get this.

Result:

0 1 0 1 0 1 1 1 1 1 7

0 1 0 1 0 1 1 1 1 1 7

0 1 0 1 0 1 1 1 1 1 7

0 1 0 1 0 1 1 1 1 1 7

0 1 0 1 0 1 1 1 1 1 7

0 1 0 1 0 1 1 1 1 1 7

Now, why does it repeat? Where am I doing it wrong? And did I do right with the rows? ( trying to sum them up)

The wanted result:

0 1 0 1 0 1 1 1 1 1 7

0 1 1 1 1 1 1 1 1 0 8

0 0 1 1 1 1 1 1 1 0 7

0 1 1 1 1 0 1 0 1 1 7

0 1 1 1 1 1 0 1 1 1 8

0 1 1 1 0 1 1 0 1 1 7

How can I achieve that?

Answer 1

There is a few problems with your code. First, x and y are not initialized before the line:

int masyvas[x][y];  // masyvas is 2D array

Then, when you read into the array:

D >> masyvas[i][j];

the indices you use (i and j) range from 1 to x and from 1 to y. But arrays in C/C++ are 0-based, thus i and j should range from 0 to x-1 and 0 to y-1.

I would suggest that instead of loops and arrays, you use the C++ way: containers (vector) and iterators.

For instance, instead of calling "D >> masyvas[i][j]" y times, you can simply use

copy_n(istream_iterator<int>(input_data),
       cols,
       back_inserter(row));

This simply says that "cols" integers are read from input_data and then appended to the back of the vector called "row".

For output you can use a similar technique: the following line says that the entire "row" vector is copied to the output "cout", such that between each two elements there is a space:

copy(row.begin(), row.end(),
     ostream_iterator<int>(cout," "));

Finally, to compute the total of a row, we can use the function "accumulate" which does just that:

cout << accumulate(row.begin(), row.end(), 0) << "\n";

The complete code is below:

#include <iostream>
#include <fstream>
#include <iterator>
#include <algorithm>
#include <numeric>
#include <vector>
int main(int argc, char *argv[])
{
    using namespace std;
    int rows, cols;
    ifstream input_data("Duomenys.txt");
    input_data >> rows >> cols;
    for(int i = 0; i < rows; i++)
        {
            vector<int> row;
            copy_n(istream_iterator<int>(input_data),
                         cols,
                         back_inserter(row));
            copy(row.begin(), row.end(),
                     ostream_iterator<int>(cout," "));
            cout << accumulate(row.begin(), row.end(), 0) << "\n";
        }
    return 0;
}

Answer 2

A few things to note:

1. In your 'printing' for loops, you're cout'ing masyvas[x][j] with an x. Shouldn't that be masyvas[i][j] with an i?

2. Arrays in C++ start with 0 and go up to arraySize - 1 but your for loops start at index 1 and go to arraySize + 1. Suppose I write a for loop like this:

   int array[10] = {1, 2, 3, 5, 8, 13, 21, 34, 55, 89};
   for (int x = 1; x < 10 + 1; x++) {
       cout << array[10] << " " << endl;
   }
   

   We get the first index in an array using 0, not 1. So instead of printing out the sequence of numbers in the array, it's going to start at the 1st index (2) and go until the 11th index (out of range) and crash my program.

3. How is your array creation not giving you compiler errors? It does on mine. When you create an int, C++ isn't going to initalize it to 0 so you don't know what could be in it. As an example:

   int x;
   int y;
   cout << x << ", " << y << endl;
   

   This won't compile for me. It tells me that x contains -858993460. Imagine trying to create an array with that as one of the sizes. The two most basic solutions would be vector and dynamic arrays using the new operator.

Hope this helps.

Answer 3

I think your main problem here is your declaration of the 2d array:

int masyvas[x][y];

x and y are not known at compile time, you need to either use dynamic memory allocation, use something like std::vector, or set masyvass size to something larger than you'll need:

int masyvas[100][100];

Try that instead.