How to read a 2d array from a file without knowing its length in C++?

Question

Like the title says I'm trying to read an unknown number of integers from a file and place them in a 2d array.

#include <iostream>
#include <fstream>
using namespace std;
int main()
{

fstream f;int i,j,n,a[20][20];char ch;

i=0;j=0;n=0;
f.open("array.txt", ios::in);
while(!f.eof())
{
    i++;
    n++;
    do
    {
        f>>a[i][j];
        j++;
        f>>ch;
    }
    while(ch!='\n');
}

for(i=1;i<=n;i++)
{
    for(j=1;j<=n;j++)
        cout<<a[i][j]<<endl;
    cout<<endl;
}
return 0;

}

and my "array.txt" file :

1 1 1
2 2 2
3 3 3

After compiling the program, it prints this

Answer 1

Try:

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
using namespace std;
int main() {
  fstream f;
  int i, j, n, a[20][20];
  string buf;

  i = 0;
  j = 0;
  n = 0;
  f.open("array.txt", ios::in);
  while (1) {
    getline(f, buf);
    if (f.eof()) break;
    stringstream buf_stream(buf);
    j = 0;
    do {
      buf_stream >> a[i][j];
      j++;
    } while (!buf_stream.eof());
    i++;
    n++;
  }

  for (i = 0; i < n; i++) {
    for (j = 0; j < n; j++) cout << a[i][j] << " ";
    cout << endl;
  }
  return 0;
}

Also, if you really want to read arbitrarily large arrays, then you should use std::vector or some such other container, not raw arrays.

Answer 2

As your input file is line oriented, you should use getline (C++ equivalent or C fgets) to read a line, then an istringstream to parse the line into integers. And as you do not know a priori the size, you should use vectors, and consistently control that all lines have same size, and that the number of lines is the same as the number of columns.

Last but not least, you should test eof immediately after a read and not on beginning of loop.

Code becomes:

#include <iostream>
#include <fstream>
#include <vector>
#include <string>
#include <sstream>
using namespace std;
int main()
{

    fstream f;
    int i=0, j=0, n=0;
    string line;
    vector<vector<int>> a;
    f.open("array.txt", ios::in);
    for(;;)
    {
        std::getline(f, line);
        if (! f) break; // test eof after read
        a.push_back(vector<int>());
        std::istringstream fline(line);
        j = 0;
        for(;;) {
            int val;
            fline >> val;
            if (!fline) break;
            a[i].push_back(val);
            j++;
        }
        i++;
        if (n == 0) n = j;
        else if (n != j) {
            cerr << "Error line " << i << " - " << j << " values instead of " << n << endl;
        }
    }
    if (i != n) {
        cerr << "Error " << i << " lines instead of " << n << endl;
    }

    for(vector<vector<int>>::const_iterator it = a.begin(); it != a.end(); it++) {
        for (vector<int>::const_iterator jt = it->begin(); jt != it->end(); jt++) {
            cout << " " << *jt;
        }
        cout << endl;
    }
    return 0;
}

Answer 3

You may want to look into using a vector so you can have a dynamic array.