loading into 2d array from file in c++

Question

I am having trouble reading numbers from a file into a 2d array in c++. It reads the first row just fine but the rest of the rows are populated with 0's. I have no idea what I'm doing wrong.

#include <iostream>
#include <fstream>

using namespace std;

int main()
{
    int myarray[20][20];

    int totRow = 20, totCol = 20, number, product, topProduct = 0, row, col, count;
    char element[4];

    ifstream file;

    file.open( "c:\\2020.txt" );

    if( !file )
    {
        cout << "problem";
        cin.clear();
        cin.ignore(255, '\n');
        cin.get();

        return 0;
    }

    while( file.good())
    {
        for( row = 0; row < totRow; row++ )
        {
            for( col = 0; col < totCol; col++ )
            {
                file.get( element, 4 );
                number = atoi( element );
                myarray[row][col] = number;
                cout << myarray[row][col] << " ";
            }
            cout << endl;

        }
        file.close();
    }

You could probably replace your entire while loop with std::copy_n( std::istream_iterator<int>( file ), 20*20, &myarray[0][0] );. — Robᵩ
– Robᵩ, Commented Oct 25, 2012 at 19:59
@trumpetlicks - No, that will work even if he has newlines. - ideone.com/u3Ww8P — Robᵩ
– Robᵩ, Commented Oct 25, 2012 at 20:01

Olaf Dietsche · Accepted Answer · 2012-10-25 20:06:11Z

3

If there are only numbers in your file, you can just read them with the >> operator. Change your inner loop to:

for( col = 0; col < totCol; col++ )
{
    file >> myarray[row][col];
    cout << myarray[row][col] << " ";
}

The problem with file.get() is, it doesn't read beyond newline \n. See: std::basic_istream::get

edited Oct 25, 2012 at 20:06

answered Oct 25, 2012 at 19:53

Olaf Dietsche

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2 Comments

Olaf Dietsche Over a year ago

@trumpetlicks No, this doesn't matter. The >> operator skips whitespace before reading the integer.

Olaf Dietsche Over a year ago

@trumpetlicks Look at std::basic_istream::operator>>

jrok · Accepted Answer · 2012-10-25 20:16:30Z

2

You're closing the file inside the while loop:

while( file.good())
    {
        for( row = 0; row < totRow; row++ )
        {
            for( col = 0; col < totCol; col++ )
            {
                file.get( element, 4 );
                number = atoi( element );
                myarray[row][col] = number;
                cout << myarray[row][col] << " ";
            }
            cout << endl;

        }
        file.close();   // <------ HERE
    } // end of while loop is here

You obviously can't read from a closed stream. Now, because you're trying to read all the data in the first iteration of the while loop, this doesn't seem to be your immediate problem. Note however, that the stream can still be good() even after you've read all the meaningful data (for example if there's a traling new-line character) and in that case, you'll enter the loop for the second time. That's a bug.

edited Oct 25, 2012 at 20:16

answered Oct 25, 2012 at 19:55

jrok

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3 Comments

trumpetlicks Over a year ago

While not good form, Im not sure this is his ultimate problem.

jrok Over a year ago

@trumpetlicks My best guess, unless OP posts a sample input.

Olaf Dietsche Over a year ago

@jrok But outside the reading loop ;-) When the file is closed, he's already gone over the whole myarray.

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