Can't read integer from file to matrix - c++

Question

I have to read from a file an array of numbers with an unknown size and save it as a matrix. The code must be as compact as possible, which is why I don't want to read file as string and then convert it to an int.
int main()
{
ifstream infile("array.txt");
int n, counter = 0, **p;
while (!infile.eof()) {
counter++;
}
counter = sqrt(counter);
cout << "counter is " << counter << endl;
p = new int*[counter];
for (int i = 0; i < counter; i++)
p[i] = new int[counter];
while (!infile.eof()) {
for (int i = 0; i < counter; i++) {
for (int j = 0; j < counter; j++)
p[i][j] = n;
}
}
for (int i = 0; i < counter; i++) {
for (int j = 0; j < counter; j++) {
cout << p[i][j] << " ";
}
cout << endl;
}
_getch();
return 0;
}
Here is my code, it was made for a square matrix. The problem is, I can't read the file at second time to save the numbers to the matrix.

Answer 1

You have a lot of problems in your code. A big one is that you have several infinite loops and aren't even reading from a file. An even bigger problem is that you're not using C++ constructs. I've written a small program that does what you're trying to do using more C++ concepts. In this case, you should use a std::vector - they will handle all the dynamic sizing for you.
test.cc
#include <iostream>
#include <vector>
#include <sstream>
#include <fstream>
#include <string>
// Nobody wants to write `std::vector<std::vector<int>>` more than once
using int_matrix = std::vector<std::vector<int>>;
void populate_matrix(int_matrix& mat, const std::string& line) {
int num;
std::stringstream ss(line);
std::vector<int> row;
// Push ints parsed from `line` while they still exist
while(ss >> num) {
row.push_back(num);
}
// Push the row into the matrix
mat.push_back(row);
}
// This is self-explanatory, I hope
void print_matrix(const int_matrix& mat) {
size_t n = mat.at(0).size();
for(size_t i = 0; i < n; ++i) {
for(size_t j = 0; j < n; ++j) {
std::cout << mat.at(i).at(j) << " ";
}
std::cout << std::endl;
}
}
int main(int argc, char** argv) {
int_matrix mat;
// Pass the file as a command-line arg. Then you don't need to worry about the path as much.
if(argc != 2) {
std::cout << "Number of arguments is wrong\n";
return EXIT_FAILURE;
}
// Open file with RAII
std::ifstream fin(argv[1]);
std::string line;
// Handle each line while we can still read them
while(std::getline(fin, line)) {
populate_matrix(mat, line);
}
print_matrix(mat);
return EXIT_SUCCESS;
}
This code assumes the text file looks something like this:
numbers.txt
1 2 3
4 5 6
7 8 9
i.e., n lines with n numbers per line separated by whitespace.
To compile and run this code, you can follow these steps:
13:37 $ g++ test.cc -std=c++14
13:37 $ ./a.out /path/to/numbers.txt

Answer 2

As far as I can see the program runs once through the file and later you run another while loop to read from the file. When you read from a file, then its like your "cursor" moves forward. So basicly, if you hit the end, you have to reset the cursor back to the start of the file.
You can set your cursor back with seekg(0).(http://www.cplusplus.com/reference/istream/istream/seekg/)