How to read in a matrix from a file in C++?

Question

I am attempting to read in an n × n matrix from a file and then store that matrix in a one dimensional array. I would also like to store the value for n. I have looked into various approaches but can't seem to apply them to what I'm trying to achieve.

This is what I have so far but I'm unsure what to put into the while loop.

/*READ IN THE IMAGE MATRIX FROM THE FILE*/

String lineA;
ifstream imFile;
imFile.open("imageMatrixDefined.txt");
if(imFile.fail()){
    cerr << "File cannot be found or opened" << endl;
    exit(1);
}

if(imFile.is_open(){
    cout << "file opened successfully!" << endl;
    while(!imFile.eof()){    
    }
}

The input file could look like the following:

1    2    3
2    3    1
3    3    2

where a tab separates the elements. Any suggestions would be greatly appreciated as I'm new to C++.

How does your input format look? But `while (!imFile.eof()) { … }` is almost certainly wrong anyway. — 5gon12eder, Feb 28 '16 at 16:51
Please add that information to your question directly by [edit]ing it. — 5gon12eder, Feb 28 '16 at 16:54
See [why it is wrong](http://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong). — n. m. could be an AI, Feb 28 '16 at 16:55
The idea is to try and allow the file to be changed so that it can read in any size so n can change. — Tony Davis, Feb 28 '16 at 16:59
The tutor has given you the skeleton of the code. Put in the line in the while loop — Ed Heal, Feb 28 '16 at 17:00
Search the internet for "c++ read file matrix". There are too many of these similar questions already asked here. — Thomas Matthews, Feb 28 '16 at 17:03
Possible duplicate of [read matrix from a file in C C++](http://stackoverflow.com/questions/1492517/read-matrix-from-a-file-in-c-c) — 5gon12eder, Feb 28 '16 at 17:05
The duplicate question has the same file format but wants to store the data in a vector of vectors instead of a single vector. You should be able to transfer the concept, though. — 5gon12eder, Feb 28 '16 at 17:06

score 0 · Accepted Answer · answered Feb 28 '16 at 17:16

Arrays have a fixed size.
You have to get the value for n first before initializing an array.

It's good to double check if you would like to store that matrix in a one dimensional array or a two dimensional array. If it is a one dimensional array, checking how the matrix is stored in a one dimensional array is good. Some stores it first row, second row, ..., and nth row, and some stores it first column, second column, ..., and nth column.

The matrix is n × n, so the number of columns is equal to the number of rows.
The file stores one row at a time. Getting the value of n is not difficult.

The while loop is critical, but before the while loop, getting the value of n is the first step of solving the problem.

When the value of n is gotten, the array is easy to be initialized. You can read the line one by one from the file in the while loop, get each matrix element by using the delimiter as a tab character, and store the matrix element in the array.

score 0 · Answer 2 · edited Oct 06 '22 at 10:18

You have to bind a custom input stream with your input file. Say your input file is "in.txt", and it looks the same as you have specified.

The following code then reads from the file, stores it in a 1D array and prints the resulting matrix in matrix form:

#include <iostream>
#include <fstream>
using namespace std;

int main () {
  ifstream myfile;
  myfile.open ("in.txt");

  cout << "Reading from a file.\n";
  int arr[10];
  int k = 0;
  for (int i = 0; i < 3; ++i)
  {
    for (int j = 0; j < 3; ++j)
    {
        myfile >> arr[k];           //Read it in a 1D array   
        cout << arr[k] << "  ";
        ++k;
    }
    cout << "\n";
  }

  myfile.close();

  return 0;
}

Output:

Reading from a file.
1  2  3  
2  3  1  
3  3  2

score 0 · Answer 3 · answered Oct 06 '22 at 04:47

Reading a 2D array to a file. Input is assumed to be comma separated, one row per line and x amount of columns per row, each column separated by a tab (\t) character. No EOL (end of line) tab's required, just a newline (\n). Remember to set rows and columns variables correctly. Code is tested to work correctly.

using namespace std;
#include <iostream>
#include <fstream>

int main() {
  int rows=10, columns=10;

  ifstream f("input.txt");
  int input[rows][columns];
  for (int i=0; i<rows; i++){
    for (int j=0; j<columns; j++){
      f >> input[i][j];
    }
  }
  f.close();

  // Here you can add processing of the file, for example, print it:

  cout << "Input:" << endl;
  for (int i=0; i<rows; i++){
    for (int j=0; j<columns; j++){
      cout << input[i][j] << "  ";
    }
    cout << endl;
  }
  return 0;
}

How to read in a matrix from a file in C++?

3 Answers3