how to open and read a binary file in C++ by a given bin file?

Question

Is there anyone could help me to check where I did wrong? Or explain why? I am a beginner and I tried my best to open the binary file. But it just runs out "file is open" "0". Nothing came out.

The objective: The Count3s program opens a binary file containing 32-bit integers (ints). Your program will count the number of occurrences of the value 3 in this file of numbers. Your objective is to learn about opening and accessing files and apply your knowledge of control structures. The name of the file containing data used by the program is "threesData.bin".

my code as below, please help me if you know it. Thank you in advance!

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

int main()
{
int count=0 ;
ifstream myfile;
myfile.open( "threesData.bin", ios::in | ios :: binary | ios::ate);

if (myfile)
{
    cout << "file is open " << endl;
    cout << count << endl;    }

else 
    cout << "cannot open it" << endl;


return 0;    
}

You only have code to open the file. You don't have any lines of code to read the data. — R Sahu, Jul 15 '16 at 20:10
You might want to read e.g. [this `openmode` reference](http://en.cppreference.com/w/cpp/io/ios_base/openmode) which says that `ate` "seek to the end of stream immediately after open". — Some programmer dude, Jul 15 '16 at 20:11

score 1 · Accepted Answer · edited May 23 '17 at 10:28

First of all you should read from file opened in binary mode with

  myfile.read (buffer,length);

where buffer should be defined as

  int data;

and used as

  myfile.read (&data,sizeof(int));

The second important point is reading from file for more than one number - you need loop that is controled by condition that check stream. For example:

  while (myfile.good() && !myfile.eof())
  {
       // read data
       // then check and count value
  }

And the last thing, you should close file, that was successfully oppened, after you finished reading:

  myfile.open( "threesData.bin", ios::binary); 
  if (myfile)
  {
       while (myfile.good() && !myfile.eof())
       {
           // read data
           // then check and count value
       }
       myfile.close();
       // output results
   }

And some additinal tips:

1) int is not always 32-bit type, so consider using int32_t from <cstdint>; and if your data has more than 1 byte, may be byte order is important, but it was not mentioned in the task description

2) read allows reading more than one data object per one call, but in that case you should read to array instead of one variable

3) read and try examples from references and other available resources like this.

Or better, use `unsigned char buffer[bufsiz];` and decode the octets as appropriate. — Martin Bonner supports Monica, Jul 15 '16 at 20:28
As always, there are infinite number of ways to solve this not so difficult problem. Nydia should try from simple and then go to advanced really C++ programming. — VolAnd, Jul 15 '16 at 20:33
[Why is iostream::eof inside a loop condition considered wrong?](http://stackoverflow.com/q/5605125/14065) You should use `while(myfile.read(&data,sizeof(int))){ /* Read worked */}` — Martin York, Jul 17 '16 at 19:10
Closing a stream manually is not standard either [Do I need to manually close an ifstream?](http://stackoverflow.com/q/748014/14065) — Martin York, Jul 17 '16 at 19:14

how to open and read a binary file in C++ by a given bin file?

1 Answers1