ifstream in loop stuck on the same value after returning to start of file

Question

I'm trying to write a function that reads values from a file and puts them in a matrix. The matrix (of two columns) is made by scanning the number of rows in the file and using that number as the number of rows in the matrix. To read the values, the ifstream object reader is brought back to the start of the file. However, after doing so, reader is stuck on an integer (I think it's a garbage value) for the entire loop. The function that dynamically allocates the matrix works fine.

I included the MCVE below.

#include <fstream>
#include <iostream>
#include <string>

using namespace std;

int main(){

    string fileChoice;
    cout << "Choose a file to open: ";
    cin >> fileChoice;

    ifstream reader;
    reader.open(fileChoice);
    if (reader.fail()){
        cerr << fileChoice << " could not be opened" << endl;
        system("pause");
        exit(1);
    }

    // https://stackoverflow.com/questions/26903919/c-allocate-dynamic-array-inside-a-function
    int** Matrix = new int*[4];  
    for (int i = 0; i < 4; i++)  {
        Matrix[i] = new int[2];
    }

    reader.seekg(0, ios::beg);
    for (int i = 0; i < 4; i++){
        for (int j = 0; j < 2; j++){
            reader >> Matrix[i][j];
            cout << Matrix[i][j] << " ";
        }
    }

    system("pause");
    exit(0);
}

Here's the data in the sample file I used:

This is what I expected as output for cout:

1 10 2 10 11 20 23 30

But this is what I got instead:

-842150451 -842150451 -842150451 -842150451 -842150451 -842150451 -842150451 -842150451

Additionally, when changing

    reader.seekg(0, ios::beg);
    for (int i = 0; i < 4; i++){
        for (int j = 0; j < 2; j++){
            reader >> Matrix[i][j];
            cout << Matrix[i][j] << " ";
        }
    }

to

    int beg;
    reader.seekg(0, ios::beg);
    for (int i = 0; i < 4; i++){
        for (int j = 0; j < 2; j++){
            reader >> beg;
            cout << beg << " ";
        }
    }

I get the following output:

-858993460 -858993460 -858993460 -858993460 -858993460 -858993460 -858993460 -858993460

You never check `reader`'s state? Post a [mcve] reproducing your problem as required here please. — πάντα ῥεῖ, Mar 23 '19 at 17:54
`-842150451` is 0xcdcdcdcd = uninitailzed heap memory https://stackoverflow.com/questions/127386/in-visual-studio-c-what-are-the-memory-allocation-representations — drescherjm, Mar 23 '19 at 18:08
Do you have a call to `reader.clear()` before the call to `seekg()`? If not, start by adding that. — Jerry Coffin, Mar 23 '19 at 18:08
-842150451 sound like unitilialized. Are you sure that your array has the correct size? Can you show us how you create it? (Or better, as πάντα ῥεῖ already said, do a MVCE) — Aziuth, Mar 23 '19 at 18:47
@Aziuth I added the code for making the matrix in the description — DangIt, Mar 23 '19 at 20:56

score 0 · Accepted Answer · answered Mar 24 '19 at 00:50

When I take the code you now have in the question, and add reader.clear(); to get this:

#include <fstream>
#include <iostream>
#include <string>

using namespace std;

int main(){

    string fileChoice;
    cout << "Choose a file to open: ";
    cin >> fileChoice;

    ifstream reader;
    reader.open(fileChoice);
    if (reader.fail()){
        cerr << fileChoice << " could not be opened" << endl;
        system("pause");
        exit(1);
    }

    // https://stackoverflow.com/questions/26903919/c-allocate-dynamic-array-inside-a-function
    int** Matrix = new int*[4];  
    for (int i = 0; i < 4; i++)  {
        Matrix[i] = new int[2];
    }

    reader.clear();
    reader.seekg(0, ios::beg);
    for (int i = 0; i < 4; i++){
        for (int j = 0; j < 2; j++){
            reader >> Matrix[i][j];
            cout << Matrix[i][j] << " ";
        }
    }
}

...and run it on a file containing the data you gave in the question, the output I get is as follows:

1 10 2 10 11 20 23 30

I don't understand how `reader.clear()` fixes the problem. What flags would the `ifstream` object have raised? — DangIt, Mar 24 '19 at 09:27

ifstream in loop stuck on the same value after returning to start of file

1 Answers1