This program creates a new file with the union of the words of two input files.

Union

/*
  Copyright (C) 2015  Katerine Muñoz Tello <[email protected]>

  This program is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
  (at your option) any later version.

  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program.  If not, see <http://www.gnu.org/licenses/>.

*** Nombre: Katerine Muñoz Tello.
*** Objetivo: Genere un nuevo archivo que contenga la unión de las palabras de ambos archivos.
*** SO: Ubuntu 15.04
*** IDE: Sublime text 3.
*** Compilador: gcc version 4.9.2 (Ubuntu 4.9.2-10ubuntu13).
*/

#include <iostream>
#include <string>
#include <fstream>
#include <stdlib.h>
using namespace std;
                                  
void agregarTexto(string palabra);
int Iguales(string palabra, ifstream &file);

int main(){
  string palabra, palabra2;

  ifstream texto1;
  ifstream texto2;

  texto1.open("texto3.txt");
  texto2.open("texto4.txt");

  if(texto1.fail() && texto2.fail()){
    cout << "El archivo no existe.";
    system("PAUSE");
    exit (1);
  }
  else{
    while(texto1 >> palabra){
      agregarTexto(palabra);
    }
    while(texto2 >> palabra2){
      agregarTexto(palabra2);
    }
  }

  texto1.close();
  texto2.close();
  return 0;
}

void agregarTexto(string palabra){
  string en_archivo;
  int incluir = 1;

  fstream newfile;
  newfile.open("texto_union.txt", ios::in);

  while(newfile >> en_archivo){
    if(palabra == en_archivo){
      incluir = 0;
    }
  }

  if(incluir == 1){
    newfile.close();
    newfile.open("texto_union.txt", ios::app);
    newfile << palabra << "\t";
  }

  newfile.close();
}

Disclaimer

I know is a really bad habit for me to code in both spanish and english (as you may see, there're variables on those languages) BUT I'm coding in C++ since the begining of the year, so, excuse me please :)