Reading arrays from binary file C++

Question

#include <iostream>
#include <cstdlib>
#include <ctime>
#include <fstream>

using namespace std;

int main()
{
    srand(time(0));
    const int array_size=10,n=10;
    int i,j,k;
    fstream io("io.dat", ios_base::binary);
    float a[array_size];
    for (i=0;i<n;i++) {
        cout<<"Inputting "<<i+1<<" array:\n";
        for (j=0;j<array_size;j++) {
            a[j]=(float)rand()/(RAND_MAX+1)*200-100;
            cout<<i+1<<"["<<j<<"]="<<a[j]<<endl;
        }
        io.write((char *)&a,sizeof(a));
    }
    io.close();
    io.open("io.dat", ios_base::binary);
    j=1;
    while (!io.eof()) {
        cout<<"Reading "<<j<<" array:"<<endl;
        io.read((char *)&a,sizeof(a));
        for (i=0,k=0;i<array_size;i++) {
            cout<<j<<"["<<i<<"]="<<a[i]<<endl;
            if (a[i]<0) k++;
        }
        cout<<"Number of negative elements in "<<j++<<" array="<<k<<endl;
    }
    return 0;
}

I am stuck in reading arrays from binary file. The problem is that condition of breaking reading cycle doesn't even works. Program reads the same array again and again.

eof as loop continuation condition is almost always wrong. instead use streams state after read operation. — Cheers and hth. - Alf, Mar 15 '15 at 09:08
Don't use e.g. `!in.eof()` as loop condition, the `eofbit` flag will not be set until *after* you try to read from beyond the end of the file. That will cause the loop to iterate once to many. Instead if you read e..g [this `read` reference](http://en.cppreference.com/w/cpp/io/basic_istream/read) you will see that it returns the stream, and you can use the stream as a boolean condition, like `while (in.read(...))` — Some programmer dude, Mar 15 '15 at 09:10
@JoachimPileborg is right. [Read this!](http://stackoverflow.com/questions/5431941/while-feof-file-is-always-wrong) — notadam, Mar 15 '15 at 10:05

score 0 · Answer 1 · answered Mar 15 '15 at 09:58

This sort of a problem is caused because you are using !in.eof(). One of the easiest way to solve this is to read inside the while loop like in this code

io.open("io.dat", ios_base::binary);
j=1;
while ( io.read( (char *) &a, sizeof(a) ) )    //  changed the !in.eof
 {
    cout<<"Reading "<<j<<" array:"<<endl;  
    // Removed the read form here
    for (i=0,k=0;i<array_size;i++) 
     {
        cout<<j<<"["<<i<<"]="<<a[i]<<endl;
        if (a[i]<0) k++;
     }
    cout<<"Number of negative elements in "<<j++<<" array="<<k<<endl;
 }

Well, that should solve the problem with your output.

Reading arrays from binary file C++

1 Answers1