A problem while reading data from file to vector.

nanchuangyeyu · #1 06-May-2009, 03:30

Hi,
I am back again...

I have written a 2*3 matrix[1,2,3;4,5,6] to file. And then I tried to read the data back to initialize a 2D C++ vector.
The code is as following:

CPP / C++ / C Code:

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

using namespace std;

int main()
{
	ofstream out_file;
	ifstream in_file;
    
	char *out_name = "test.bin";
	out_file.open(out_name,ios::binary | ios::app);

	short matrix[2][3] = {1,2,3,4,5,6};
	// write matrix to a disk file.
	for (int y=0; y<2; y++)
	{
		for (int x=0; x<3; x++)
		{
			out_file.write(reinterpret_cast<char*>(&matrix[y][x]),sizeof(short));
		}
	}
	out_file.close();
	cout<< "Matrix has been written to test.bin."<<endl;

	vector< vector<short> >vec_2d;
	vector<short>row;

 	in_file.open(out_name,ios::binary);
	short m;
	while(!in_file.eof())
	{
		for (int y=1; y<4; y++)
		{
			in_file.read(reinterpret_cast<char*>(&m),sizeof(short));
			row.push_back(m);
			if (y == 3)
			{
				vec_2d.push_back(row);
				row.clear();
			}
		}
	}
        for (int i=0; i< vec_2d.size(); i++)
        {
	      for (int j=0; j< vec_2d[i].size(); j++)
	      {
		     cout<<" "<<vec_2d[i][j];
	      }
	      cout<<endl;
        }
        in_file.close();
        return 0;
}

However, the vector is printed out as [1,2,3;4,5,6;6,6,6]. I tried to fix it but in vain. Anybody can point out the mistake for me? Thank u in advance.
nanchuangyeyu

Kimmo · #2 06-May-2009, 08:08

The eof flag is set only AFTER you have tried to read past the end of file, not WHEN you read the last of something in the file.

After going through the while loop twice, your program hasn't yet encountered the end of file, since it hasn't tried to read anything that would be past the end. Thus, your loop will run a third time. This time, however, in the first run through the for loop, your program tries to read past the end of file and the eof flag is set. in_file is not in good() state anymore and it cannot read anything, so the value of m is what was last read in; 6. Thus the third row.

nanchuangyeyu · #3 06-May-2009, 08:50

Thank you for your help Kimmo. So what should I do in order to obtain the desired result? I just want to stop exactly after all data in the file is read.

Kimmo · #4 06-May-2009, 11:20

Quote:

Originally Posted by nanchuangyeyu

Thank you for your help Kimmo. So what should I do in order to obtain the desired result? I just want to stop exactly after all data in the file is read.

When reading a matrix from a file, you probably know already how many rows and how many columns there are in the matrix? In other words, you know how much data you need to read. So instead of reading all the data until the end of the file is reached, you could read the data as far as you need to and only check if the end of the file is reached before all the expected data was read.

As for your problem here, you could just change

CPP / C++ / C Code:

in_file.read(reinterpret_cast<char*>(&m),sizeof(short));
row.push_back(m);

to

CPP / C++ / C Code:

in_file.read(reinterpret_cast<char*>(&m),sizeof(short));
if (in_file.eof()) // if at end of file, break loop
    break;
row.push_back(m);

Or if you're not a fan of break

CPP / C++ / C Code:

if (!in_file.eof())
    row.push_back(m);

I must admit I'm so rusty that I don't know how you would go about checking if what you just read was the last bytes in the file. Whatever the way, I feel it might be more complicated than the way I've showed here.

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A problem while reading data from file to vector.

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