Сортировка в файле. Структура данных

#include <iostream>
#include <cstdlib>
#include <cstring>
#include <cstdio>
#include <fstream>
#define N 1000
using namespace std;
 
struct animal
{
    char name[30];
    char continent[30];
    float weight;
    int speed;        
    }; 
    
//----------------------
void vvod(int n, animal mas[]) 
{int i;
for(i=0; i<n; i++){
    cout<<"\nDate of "<<i+1<<". animal: "<<"\nName: ";
    if (i==0)
    {gets(mas[i].name); gets(mas[i].name);}
    else {gets(mas[i].name); gets(mas[i].name);} 
    cout<<"Continent: ";
    gets(mas[i].continent); 
    cout<<"Weight: ";
    cin>>mas[i].weight;
    cout<<"Speed: ";
    cin>>mas[i].speed;
    
    }
}
//-----------------------------
void vivod (int v, animal mas[])
{int i;
for(i=0; i<v; i++){
cout<<"\nAnimal:";
cout<<"\nName: "<<mas[i].name;
cout<<"\nContinent: "<<mas[i].continent;
cout<<"\nWeight: "<<mas[i].weight;
cout<<"\nSpeed: "<<mas[i].speed;
cout<<endl;
    }
}
//--------------------------TXT-------------------------
void write_file (int n, animal mas[])
{ int i;
ofstream f1;
f1.open("Animals.txt", ios::app);
for(i=0; i<n; i++){
f1<<mas[i].name<<"\n";
f1<<mas[i].continent<<"\n";
f1<<mas[i].weight<<"\n";
f1<<mas[i].speed<<"\n";
   }
f1.close(); 
}
 
void read_file (int v, animal mas[])
{ int i=0;
ifstream f1;
f1.open("Animals.txt");
if (!f1) {cout<<"\nFile open error"; 
    system("Pause>>nul"); exit(1);}
do{
f1>>mas[i].name;
f1>>mas[i].continent;
f1>>mas[i].weight;
f1>>mas[i].speed;
i++;
}
while(!f1.eof());
//--------------------------------Search-------------------------------------
char buf[N];
int n_record=i, n_animal=0;      
while(true)
{          cout<<"\nType animal`s name or continent (to be end type - "end")  "; 
           cin>>buf;
           if (strcmp(buf,"end")==0) break;
           bool not_found=true;
           for(i=0;i<n_record;i++)
           {                      if (strstr(mas[i].name, buf) || strstr(mas[i].continent, buf))
                                  {cout<<"Animal: "<<mas[i].name<<"\n"<<"Continent: "<<mas[i].continent<<"\n"<<"Weight: "<<mas[i].weight<<"\n"<<"Speed: "<<mas[i].speed<<"\n";
                                  cout<<endl; 
                                  n_animal++;                                 
                                  not_found=false;  
               }         
           }
if (not_found) cout<<"No this animal!\n";
}
f1.close();
}
 
void sort_file (int v, animal mas[])
{ int i=0;
ifstream f1;
f1.open("Animals.txt");
if (!f1) {cout<<"\nFile open error"; 
    system("Pause>>nul"); exit(1);}
    do{
f1>>mas[i].name;
f1>>mas[i].continent;
f1>>mas[i].weight;
f1>>mas[i].speed;
i++;
}
while(!f1.eof());
//----------------------------------SORTING-----------------------------------(ГДЕ ТУТ ПРОБЛЕМА?)-------------------
int n_record=i;
char x;
cout<<"Sort by speed or by weight: ";
cin>>x;
 
for (i=0;i<n_record-1;i++)
{
int min=i; 
for (int j=i+1; j<n_record; j++)
if(x=='speed')
{
if (mas[j].speed<mas[min].speed) min=j; 
animal a=mas[i];
mas[i]=mas[min];
mas[min]=a;
}
else cout<<"Wrong!";
 
if(x=='weight')
{
if (mas[j].weight<mas[min].weight) min=j; 
animal a=mas[i];
mas[i]=mas[min];
mas[min]=a;
}
else cout<<"Wrong!";
 
}
 
ofstream f2("Sort baze.txt");
for(i=1;i<n_record;i++)
{
f2<<"Animal: "<<mas[i].name<<"\n";
f2<<"Continent: "<<mas[i].continent<<"\n";
f2<<"Weight: "<<mas[i].weight<<"\n";
f2<<"Speed: "<<mas[i].speed<<"\n"; 
f2<<endl;
}
f2.close();
cout<<"Sort be successful!!\n\n";
f1.close();
}
 
int main()
{animal mas[N], mastxt[N], masbin[N];
int n,v;
cout<<"Animal add n= ";
cin>>n;
cout<<"How much animals open to read v= ";
cin>>v;
vvod(n, mas);
vivod(n, mas);
//------- TXT file-----------
write_file(n, mas);
read_file(v, mastxt);
cout<<"\nDates of txt file: ";
vivod(v, mastxt);
sort_file (v, mastxt);
system("Pause>>nul");
return 0;
}

int n_record=i;
 
for (i=0;i<n_record-1;i++)
{
int min=i; 
for (int j=i+1; j<n_record; j++)
if (mas[j].weight<mas[min].weight) min=j; 
animal a=mas[i];
mas[i]=mas[min];
mas[min]=a;
}