How to process a binary file in c++?

Question

C++ / C#

Hello! The condition of the problem is as follows: given a binary file of integers, write all prime numbers to another file, swap the 2 smallest numbers in the second file, without using arrays. In fact, the problem is in a small piece of code that puts the cursor in the file in the right place:

fseek (f, sizeof(int)+min11*sizeof (int), SEEK_SET);
  fwrite (&min2, sizeof(int), 1, f);
  fseek(f, sizeof(int)+min22*sizeof (int), SEEK_SET);
  fwrite (&min1, sizeof (int), 1, f);

I've been fighting for several days, I can not understand what the problem is. Just in case, here's the full code:

#include <iostream>
#include <fstream>
#include <cstdio>

using namespace std;
int create_file ()
{
  FILE *f; int n, i, *a;
  cout<<"n="; cin>>n;
  a=new int [n];
  for (i=0; i<n; i++) {
   cout<<"a["<<i<<"]=";  cin>>a[i]; cout<<"\n"; }
  f=fopen("abc.dat", "wb");
  fwrite(&n, sizeof(int), 1, f);
  fwrite(a, sizeof(int), n, f);
  fclose(f);
  cout<<"\nCOMPLETED";
  return 0;
}
bool Prostoe (int n)
{
    int k; bool Pr;
     for (k=2, Pr=true; k <= n/2; k++)
      if (n%k==0) 
      {Pr=false; break;}
     return Pr;
}

int pr_file()
{
  FILE *f;
  int i, n, *a;
  f=fopen ("abc.dat", "rb");
  fread(&n, sizeof (int), 1, f);
  cout<<n<<endl;
  a=new int [n];
  fread (a, sizeof (int), n, f);
  fclose(f);
  f=fopen ("abc1.dat", "wb");
  for (i=0; i<n; i++) 
        if (Prostoe(a[i]))  {
              fwrite(&a[i], sizeof (int), 1, f); 
              cout<<"a["<<i<<"]="<<a[i]<<endl;}
    fclose (f);
    cout<<"\nCOMPLETED\n";
  return 0;
}

int delete_file ()
{
  FILE *f; int a,min2,min1, pr=0,min11, min22;
  f=fopen("abc1.dat", "rb");
  for(pr=0;!feof(f); pr++) {
  fread (&a, sizeof(int), 1, f);
      if (pr==0) {min1=a;min11=pr;} 
      else if (pr==1){ if (a<min1) {min2=min1;min22=min11;min1=a; min11=pr;}
       else {min2=a; min22=pr;}}
      else if (a<min1) {min2=min1;min22=min11; min1=a; min11=pr;}
      else if (a<min2) {min2=a;min22=pr;}}
      fclose(f);
    f=fopen("abc1.dat", "ab+");
   cout<<"min1="<<min1<<endl;
   cout<<"min2="<<min2<<endl;
   cout<<"min11="<<min11<<endl;
   cout<<"min22="<<min22<<endl; 
  fseek (f, sizeof(int)+min11*sizeof (int), SEEK_SET);
  fwrite (&min2, sizeof(int), 1, f);
  fseek(f, sizeof(int)+min22*sizeof (int), SEEK_SET);
  fwrite (&min1, sizeof (int), 1, f);
  fclose (f);
  cout<<"\nCOMPLETED\n";
  return 0;
}

int read_file()
{
  FILE *f;
  int  a;
  f=fopen("abc1.dat", "rb");
  while (!feof(f)) {
  fread (&a, sizeof(int), 1, f);
  cout<<"a="<<a<<"\n"; }
  fclose(f);
   cout<<"\nCOMPLETED\n";
  return 0;
}

int main(int argc, char **argv)
{
  int i;
  cout<<"1. Create file\n2. Search simple numbers\n3. Delete 2 min numbers\n4. Read file\n0. Esc\n";
  cin>>i;
  for (;i!=0; cin>>i) {
  if (i==1)
   create_file();
    if (i==2)
     pr_file();
    if (i==3)
     delete_file();
    if (i==4)
     read_file(); }
  return 0;
}

Answer 1

why in fseek sizeof(int)+ at all?
and if the minimum at offset 0 will you still write at offset 4?

Answer 2

The problem is that in the "a +" mode, writing always occurs at the end of the file.
Quote from man fopen:
Related question on SO:
stackoverflow.com/questions/3645123/opening-a-file...
The standard C language does not provide the ability to write to an arbitrary location in a file. You need to change the algorithm, or use some more low-level and platform-specific tools.

Answer 3

Start by saying that checking for a prime number is enough to drive the cycle to (the root of n) +1 :-) otherwise the teacher will not understand you or where you need to

Answer 4

@encyclopedist , the standard C language allows you to do this.
fsee() function.
You just need to choose the right mode.