Reading/writing array from file c++

Question

I am a beginner in c++ and i'm trying to figure how the in/output in a file works. This program is supposed to write an array of integers into a file,then read it from file, sort it,get the number of square integers and then write the results back into the file.However,when i try to compile it , it tells me that " "<<" operand is illegal for class". Could anyone please tell me what is the matter? I'm trying to avoid using .get and .put.

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

void sorting(int *p);
int squares(int *p);
int main() {
   int i, arr[10], *p = &arr[0], sq;
   char name[50];
   cout << "Give the file's name:";
   cin >> name;
   ofstream myfile(name, ios::out);
   cout << "Give the elements:";
   for (i = 0; i < 10; i++)
      myfile << *(p + i);
   myfile.close();

   ifstream myfile(name, ios::in);
   for (i = 0; i < 10; i++)
      myfile >> *(p + i);
   sq = squares(arr);
   sorting(arr);
   myfile.close();
   ofstream myfile(name, ios::app);
   for (i = 0; i < 10; i++) {
      myfile << "The sorted array is:";
      myfile << *(p + i);
   }
   myfile << "The number of square numbers is: " << sq;
   myfile.close();
   return 0;
}
void sorting(int *p) {
   int temp;
   for (int i = 0; i < 9; i++)
      for (int j = i + 1; j < 10; j++)
         if (*(p + i)>*(p + j)) {
            temp = *(p + i);
            *(p + i) = *(p + j);
            *(p + j) = temp;
         }
}
int squares(int *p) {
   int count = 0;
   for (int i = 0; i < 10; i++)
      if ((*(p + i) % 2) == 0)
         count++;

   return count;
}

Added the C++ tag for you. Do something about the indentation. — Ed Heal
– Ed Heal, Commented Jan 18, 2014 at 8:12
Well, first of all, you redefine myfile several times. Instead, every time you use ifstream myfile(name, ios::in); append a number. So ifstream myfile1(name, ios::in);, ifstream myfile2(name, ios::in);, and ifstream myfile3(name, ios::in);. — zero298
– zero298, Commented Jan 18, 2014 at 8:19

zero298 · Accepted Answer · 2014-01-18 08:21:39Z

1

You are redefining myfile several times. Change main() so that each declaration is unique. This compiles on GCC

int main() {
   int i, arr[10], *p = &arr[0], sq;
   char name[50];
   cout << "Give the file's name:";
   cin >> name;
   ofstream myfile1(name, ios::out);
   cout << "Give the elements:";
   for (i = 0; i < 10; i++)
      myfile1 << *(p + i);
   myfile1.close();

   ifstream myfile2(name, ios::in);
   for (i = 0; i < 10; i++)
      myfile2 >> *(p + i);
   sq = squares(arr);
   sorting(arr);
   myfile2.close();
   ofstream myfile3(name, ios::app);
   for (i = 0; i < 10; i++) {
      myfile3 << "The sorted array is:";
      myfile3 << *(p + i);
   }
   myfile3 << "The number of square numbers is: " << sq;
   myfile3.close();
   return 0;
}

answered Jan 18, 2014 at 8:21

zero298

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4 Comments

Georgiana Over a year ago

But now it doesn't let me give the numbers from console

zero298 Over a year ago

I think you need more << cin then. You only ask the user for one variable, the file name.

Georgiana Over a year ago

The problem was solved.The only question left would be: if for example i use fstream stream_name(filename, ios::in | ios::out),would i be able to read from and write to the file using only that one stream?

David G Over a year ago

@Georgiana std::basic_fstream<...> is a bidirectional file stream, so yes, you have both read and write functionality. But note that the explicit specification of the ::out and ::in open modes is unnecessary as that is how it is opened by default. You can simply open it using std::fstream f(filename) if no other open modes are needed.

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