Reading Input From File and Storing into 2D Array

Question

I need to read in a text file that contains only integers and each one is separated by new line. example would be:

0
1
2
...
64
repeating 0 to 64 64 times

Essentially the file is 64*64 lines long, containing an integer for each line.

I need to store each integer (line) in ldisk, my 2D array, but am having serious problems doing so. I understand my code has an error because I am trying to store a string in a char, but I am not sure how to get around this. By the way, ldisk must be a 2-D array of chars. I would love some advice/feedback on my current code posted below, or an alternative solution. NOTE: I am a beginner at C++ PS: I know similar topics exist, but mine is more to the problem of getting around the type conversion or just converting it properly so I can store more than a single digit integer into my 2D array, because I have it working where I can store only the first digit where I want in my 2D array, but run into problems if there is more than 1 digit.

 int main(){
      char **ldisk;
      ldisk = new char*[64];
      for (int i = 0; i<64; i++)
        {
          ldisk[i]= new char[64];
        }
      int counter = 0;
      string line;
      ifstream inFile("example2.txt");
      while ( getline(inFile, line))
        {
          int first, second;
          first = counter/64;
          second = counter%64;
          cout << line;
          ldisk[first][second]= line;
        }
      return 0;
    }

EDIT: My apologies I have no idea how to do a table.

I want ldisk[0][0] to be 0,
 then ldisk[0][1] to be 1,
 then ldisk[0][2] to be 2,
 etc,
 etc,
 then ldisk[0][63] to be 64

Eventually it will fill up such that ldisk[63][63] = 64

Is the blank line between each line containing a single number intentional ? if not I'll remove them. Separate note: there is zero-reason to use hand-rolled dynamic allocation for this. you will be likely shocked how little code is needed if the desired outcome is what I think it is. — WhozCraig
– WhozCraig, Commented Apr 25, 2014 at 18:41
It's accidental. For some reason I was unable to do just one on each line. Sorry, I'm new to stackoverflow — colin
– colin, Commented Apr 25, 2014 at 18:41
Great. Now, given the input you have above (which I assume is the tip of the iceberg) what exactly to you expect your 2D array to look like if properly formed? update the question with such a sample result, in table-form please. Doing so brings much clarity to understanding where your code goes afoul and what it should be doing instead. — WhozCraig
– WhozCraig, Commented Apr 25, 2014 at 20:16
Hi, sorry I don't know how to do tables, but I tried explaining as best as I could — colin
– colin, Commented Apr 25, 2014 at 21:05

R Sahu · Accepted Answer · 2014-04-26 02:56:20Z

2

This is the problem:

ldisk[first][second]= line;

The type of ldisk[first][second] is char. You are trying to assign a std::string to it.

You can make your life a lot simpler by using a std::vector<std::string>.

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

using namespace std;

int main(){

    vector<string> ldisk;

    int counter = 0;
    string line;
    ifstream inFile("example2.txt");
    while ( getline(inFile, line))
    {
       cout << line;
       ldisk.push_back(line);
    }
    return 0;
}

Update

If you must have char** ldisk, you can change main to:

int main()
{
   char **ldisk;
   ldisk = new char*[64];
   for (int i = 0; i<64; i++)
   {
      ldisk[i]= new char[64];
   }
   int counter = 0;
   string line;
   ifstream inFile("example2.txt");
   while ( getline(inFile, line) && counter < 64 )
   {
      cout << line << endl;
      if ( line.size() >= 64 )
      {
         cout << "Line is longer than 63 characters. Copying only 63 characters from it.\n";
         strncpy(ldisk[counter], line.c_str(), 63);
         ldisk[counter][63] = '\0';
      }
      else
      {
         strcpy(ldisk[counter], line.c_str());
      }
      ++counter;
   }

   return 0;
}

edited Apr 26, 2014 at 2:56

answered Apr 25, 2014 at 18:43

R Sahu

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4 Comments

colin Over a year ago

Thanks, but ldisk must be char

colin Over a year ago

This gave me a segmentation fault. When I modified it a bit I was only getting the last integer in the textfile but it was being stored in the entire 2D array.

R Sahu Over a year ago

@user3529578 I am not sure why you are getting segmentation. Perhaps you are reading more than 64 lines of text. I am going to update the code to check for the number of lines read so you don't accidentally read more than 64 lines of text.

M.M Over a year ago

@R Sahu, add a check that strcpy does not overflow buffer.

M.M · Accepted Answer · 2014-04-26 01:15:34Z

0

Change your loop to:

for (counter = 0; counter < 64*64; ++counter)
{
    int item;

    if ( !(inFile >> item) )
    {
         cerr << "File only contained " << counter << "items.\n";
         return 1;
    }

    if ( item < CHAR_MIN || item > CHAR_MAX )
    {
         cerr << "Item " << counter << " invalid value " << item << "\n";
         return 2;
    }

    ldisk[counter/64][counter%64] = item;
}

The missing ingredient is that you were not trying to convert the string in your file into an integer value. You may need #include <climits>.

answered Apr 26, 2014 at 1:15

M.M

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