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Reading or writing a double into a bin file

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Join Date: Jan 2007

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fesago90

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Reading or writing a double into a bin file

Jan 24th, 2007

Aright, now all of you might be tired of my sometimes info-lacking posts about binary files.
... but I have another quick question:
How do I write and read a double in a binary file?
Code as simple as the following does not work (which is the usual code for writing/reading an integer, string, etc. to a binary file)

 Help with Code Tags 
 c++ Syntax (Toggle Plain Text) 
#include <iostream>
#include <fstream>
 
using namespace std;
 
int main(int argc, char *argv[])
{
    double pi = 3.14;
    double read;
    ofstream binf1("bin.txt");
    binf1.close();
    fstream binf("bin.txt", ios::binary|ios::in|ios::out);
    binf.write(reinterpret_cast<char*>(&binf), sizeof(double));
    binf.read(reinterpret_cast<char*>(&read), sizeof(double));
    cout<< "\n" << read << "\n";
    system("PAUSE");
    return EXIT_SUCCESS;
}
#include <iostream>
#include <fstream>

using namespace std;

int main(int argc, char *argv[])
{
    double pi = 3.14;
    double read;
    ofstream binf1("bin.txt");
    binf1.close();
    fstream binf("bin.txt", ios::binary|ios::in|ios::out);
    binf.write(reinterpret_cast<char*>(&binf), sizeof(double));
    binf.read(reinterpret_cast<char*>(&read), sizeof(double));
    cout<< "\n" << read << "\n";
    system("PAUSE");
    return EXIT_SUCCESS;
}

Thanks

Last edited by fesago90; Jan 24th, 2007 at 10:11 pm.

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John A

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Re: Reading or writing a double into a bin file

Jan 24th, 2007

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
    binf.write(reinterpret_cast<char*>(&binf), sizeof(double));
    binf.write(reinterpret_cast<char*>(&binf), sizeof(double));

Why are you writing to binf the address of itself? I don't know how much good that's going to do you...

Which is probably where your problem lies.

Also, you don't have to write double as a string to your binary file. If you wanted to, you could directly write the value to the file.

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Ravalon

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Posting Whiz in Training

Re: Reading or writing a double into a bin file

Jan 25th, 2007

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Originally Posted by fesago90

Aright, now all of you might be tired of my sometimes info-lacking posts about binary files.
... but I have another quick question:
How do I write and read a double in a binary file?
Code as simple as the following does not work (which is the usual code for writing/reading an integer, string, etc. to a binary file)
Help with Code Tags

c++ Syntax (Toggle Plain Text)
#include <iostream>
#include <fstream>
 
using namespace std;
 
int main(int argc, char *argv[])
{
    double pi = 3.14;
    double read;
    ofstream binf1("bin.txt");
    binf1.close();
    fstream binf("bin.txt", ios::binary|ios::in|ios::out);
    binf.write(reinterpret_cast<char*>(&binf), sizeof(double));
    binf.read(reinterpret_cast<char*>(&read), sizeof(double));
    cout<< "\n" << read << "\n";
    system("PAUSE");
    return EXIT_SUCCESS;
}
#include <iostream> #include <fstream> using namespace std; int main(int argc, char *argv[]) { double pi = 3.14; double read; ofstream binf1("bin.txt"); binf1.close(); fstream binf("bin.txt", ios::binary|ios::in|ios::out); binf.write(reinterpret_cast<char*>(&binf), sizeof(double)); binf.read(reinterpret_cast<char*>(&read), sizeof(double)); cout<< "\n" << read << "\n"; system("PAUSE"); return EXIT_SUCCESS; }
Thanks

You have two problems. First, when trying to write a double to a file, it's a good idea to use the value of the double and not the value of the stream.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
binf.write(reinterpret_cast<char*>(&pi), sizeof(double));
binf.write(reinterpret_cast<char*>(&pi), sizeof(double));

Second, two-way streams are tricky to use. It's better to just use two one-way streams so you don't have to worry about seeking at the right time or flushing or any of that stuff. Your code should work like this.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
#include <fstream>
 
using namespace std;
 
int main(int argc, char *argv[])
{
    double pi = 3.14;
    double read;
    fstream binf("atest.txt", ios::binary|ios::in|ios::out);
    binf.write(reinterpret_cast<char*>(&pi), sizeof(double));
    binf.seekg( ios::beg );
    binf.read(reinterpret_cast<char*>(&read), sizeof(double));
    cout<< "\n" << read << "\n";
    return 0;
}
#include <iostream>
#include <fstream>
 
using namespace std;
 
int main(int argc, char *argv[])
{
    double pi = 3.14;
    double read;
    fstream binf("atest.txt", ios::binary|ios::in|ios::out);
    binf.write(reinterpret_cast<char*>(&pi), sizeof(double));
    binf.seekg( ios::beg );
    binf.read(reinterpret_cast<char*>(&read), sizeof(double));
    cout<< "\n" << read << "\n";
    return 0;
}

But I would do it like this.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
#include <fstream>
 
 
int main()
{
  using namespace std;
 
  ofstream ofs( "atest.txt", ios::binary );
 
  if ( ofs ) {
    double pi = 3.14;
 
    ofs.write( reinterpret_cast<char*>( &pi ), sizeof pi );
    // Close the file to unlock it
    ofs.close();
 
    // Use a new object so we don't have to worry
    // about error states in the old object
    ifstream ifs( "atest.txt", ios::binary );
    double read;
 
    if ( ifs ) {
      ifs.read( reinterpret_cast<char*>( &read ), sizeof read );
      cout << read << '\n';
    }
  }
 
  return 0;
}
#include <iostream>
#include <fstream>


int main()
{
  using namespace std;

  ofstream ofs( "atest.txt", ios::binary );

  if ( ofs ) {
    double pi = 3.14;

    ofs.write( reinterpret_cast<char*>( &pi ), sizeof pi );
    // Close the file to unlock it
    ofs.close();

    // Use a new object so we don't have to worry
    // about error states in the old object
    ifstream ifs( "atest.txt", ios::binary );
    double read;

    if ( ifs ) {
      ifs.read( reinterpret_cast<char*>( &read ), sizeof read );
      cout << read << '\n';
    }
  }

  return 0;
}

It's also good to mention that the file you produce is only guaranteed to be readable by a program written using the same version of the same compiler that the output program was written with. A lot of people don't like that portability issue, and will read and write their binary values manually instead of using read() and write().

Did I mention that text is probably a better choice than binary?

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