Am I assigning values to the array incorrectly?

Question

I have a code that is supposed to take a file with n lines, each with p doubles, and add each double to a 2d array. For some reason when I try to print the array it shows every other row being zeroed out. What am I doing wrong?

The text file would look like this

3.0 5.0 9.0 1.0
7.0 10.0 2.0 6.0
4.0 8.0 11.0 12.0
20.0 19.0 15.0 13.0
29.0 24.0 17.0 21.0

Here is my code

fileInStream.open(input);

cellArr = new double[rows * cols];

int i, j = 0;

while (getline(fileInStream, line)){
  stringstream ss(line);
  while (ss >> cell){
    *(cellArr + i * cols + j) = stod(cell);
    j++;
  }      
  i++;
}

for (int i = 0; i < rows; i++) {
  for (int j = 0; j < cols; j++) {
    cout << *(cellArr + i * cols + j) << " ";
  }
  cout << endl;
}

My output looks like this

Tip: In C++ use `std::vector` and stay far, far away from `new[]`. — tadman, Oct 07 '22 at 05:56
`*(cellArr + i * cols + j) = stod(cell)`? Please don't. If you're using an array, just use it normally, like `cellArr[i * cols + j]`. It makes it *vastly* easier to follow what you're doing. — tadman, Oct 07 '22 at 05:56
Step 1: print the values when you read them to verify that the input is what you assume. — molbdnilo, Oct 07 '22 at 06:00
@Coder99 It only initializaes `j`. Every variable need its own initializer. A segfault when you modify something trivial is a very solid indication of undefined behaviour somewhere. — molbdnilo, Oct 07 '22 at 06:01
@Coder99 Use `std::vector` instead of using `new[]` and `delete[]`. — Jason, Oct 07 '22 at 06:09
`int i=0, j=0;` And you never reset `j` on each row advance either. — WhozCraig, Oct 07 '22 at 06:10
@Coder99 please learn how to use a debugger: https://stackoverflow.com/questions/25385173/what-is-a-debugger-and-how-can-it-help-me-diagnose-problems — πάντα ῥεῖ, Oct 07 '22 at 06:15
Do not edit your question to correct your code after answer's suggestions. Rollbacked. — YSC, Oct 07 '22 at 08:03

Chris · Answer 1 · 2022-10-07T06:24:21.120

You've made a few basic logic mistakes. i is unitialized, yielding undefined behavior. Also, j is never reset. It should reset to 0 at the beginning of each row.

As it is, if you run this for a simple 2x2 array, assuming i is 0 to start, we have:

i = 0, j = 0 -> cellArr[i * cols + j] -> cellArr[0]
i = 0, j = 1 -> cellArr[i * cols + j] -> cellArr[1]
i = 1, j = 2 -> cellArr[i * cols + j] -> cellArr[4]
i = 1, j = 3 -> cellArr[i * cols + j] -> cellArr[5]

Code I used to test, modified from your code:

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

int main() {
    int rows = 2;
    int cols = 2;

    auto cellArr = new double[rows * cols];

    int i = 0;
    string line;

    while (getline(cin, line)) {
       double cell;
       int j = 0;
       stringstream ss(line);

       while (ss >> cell) {
          cellArr[i * cols + j] = cell;
          j++;
       }

       i++;
    }

    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < cols; j++) {
            cout << cellArr[i * cols + j] << " ";
        }

        cout << endl;
    }

    return 0;
}

We can use for loops to clean this up a bit.

#include <iostream>
#include <string>
#include <sstream>
#include <memory>

using namespace std;

int main() {
    int rows = 2;
    int cols = 2;

    auto cellArr = make_unique<double[]>(rows * cols);

    int i = 0;

    string line;

    for (int i = 0; i < rows && getline(cin, line); ++i) {
       stringstream ss(line);
       for (int j = 0; j < cols && ss >> cellArr[i * rows + j]; ++j);
    }

    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < cols; j++) {
            cout << cellArr[i * cols + j] << " ";
        }
        cout << endl;
    }

    return 0;
}

Am I assigning values to the array incorrectly?

1 Answers1