Reading 2-dimensional table to array

Question

I have a simple program that tries to read 2-dimensional data into a heap allocated array of floating-points. The program is as follows.

#include <iostream>
#include <fstream>


void read_file(std::ifstream &inFile,  float **arr, unsigned m, unsigned n)
{

    for(unsigned i = 0; i < m; ++i) {
        for(unsigned j = 0; j < n; ++j) {
            inFile >> arr[i][j];
            std::cout << arr[i][j];
        }
        std::cout << std::endl;
    }

}


int main() {

    const unsigned m = 20;
    const unsigned n = 1660;

    float **A = new float *[m]; // alloc pointers to rows 1,...,m
    for(unsigned row = 0; row < m; row++) A[row] = new float [n]; // alloc columns for each row pointer

    std::ifstream inFile("data.dat");
    read_file(inFile, A, m, n);



    // DEALLOC MEMORY
    for(unsigned row = 0; row < m; row++) delete [] A[row]; // dealloc columns for each row pointer
    delete [] A; // dealloc row pointers


    return EXIT_SUCCESS;
}

The data is a table of 0-1 entries (see here: data) which is nicely row orientated and has 20 rows and 1660 columns. I added the printing into the read_file function to see what goes wrong and it prints only zeros, but the correct amount at least (20*1660 zeros).

The data seems to be tab delimitered; is that the problem or is my approach completely invalid?

I started with `vector` but then I tried to get to know the `new` keyword. — ELEC, Dec 30 '16 at 10:04
Here is a nice list of [C++ books](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). I suggest you take a look at it. For a *2D array* use vector of vectors. — , Dec 30 '16 at 10:05

score 1 · Accepted Answer · answered Dec 30 '16 at 10:10

It's exactly what may happen if the file doesn't exist.

You should check that the file exists after creating the inFile object, for instance like this:

std::ifstream inFile("data.dat");
if (!inFile)
{
   std::cerr << "cannot open input" << std::endl;
   return 1;
}

If file doesn't exist, cin doesn't put data in your array, and there are chances that you get 0 all the way (I had 0 + other strange stuff), so to sum it up undefined behaviour

Note that if the file exists, your program works as expected. It would be even better to check the file after having read a value to be sure that the file has as many values as expected:

for(unsigned j = 0; j < n; ++j) {
    inFile >> arr[i][j];
    if (!inFile)
    {
        std::cerr << "file truncated " << i << " " << j << std::endl;
        return;
    }
    std::cout << arr[i][j];
}

Thank you Jean. You were correct, I needed to specify the working directory to my IDE, now it works! — ELEC, Dec 30 '16 at 10:17

Reading 2-dimensional table to array

1 Answers1