Comparing numbers to a list of numbers held in a text file

I don't think I need to go into detail about the program I'm trying to make, but just ask if you need to know more about.

-In a certain part of my program, the user is asked to enter a three-digit number.
-The number that that user enters is then stored in a file called "used_codes.log".
-The three-digit number that the user enters is given to them in advance and corresponds to an algorithm with another three-digit number.
-I created another program to generate these two corresponding numbers so I don't have to do the math every time.
-The generator saves the numbers to a file called "issued_codes.log"
-What I want to be able to do, is make the generator, before printing a number, to check "issued_codes.log" and never give the same number twice.
-I also want my program to compare the number submitted by the user to the codes in "used_codes.log", and not accept any codes that have been used before

I was thinking of a code somewhere along the lines of this:

Code:

---

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

int maint()

{
  promptA:
  int usrCode;
  string usedCodes;
  cout << "Please enter your code: " << endl;
  cin >> usrCode;
  ifstream used_codes ("used_codes.log");
  while (! used_codes.eof())
  {
      getline (used_codes, usedCodes);
  }
  used_codes.close();
  if (usrCode  !=  "???")  // This is the problem, I don't know how to tell it that I don't want it to equal anything in the file.
                      //Also, I don't know how to make C++ recognize the individual numbers;
                      //I've only ever retrieved information from a file as a single string variable.
                      //I want to be able to retrieve the individual numbers as int */ )
  {
      ...
  }
  else
  {
      cerr << "You have entered an incorrect code. Please try again." << endl;
      goto promptA;
  }
  ............................
  ............................
  ............................
  system("PAUSE");
  return 0;
}

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BTW, I've tried to convert string data types to int, float, double, and long data types before, but my program shows me a ☺, instead of what I had intended.

Since used_codes is an std::string, you only need to convert usrCode to a string and use the find() string member function...

The conversion happens with string streams:

Code:

---

#include <sstream>

stringstream str_usrCode;
str_usrCode << usrCode;

---

Next you simply see if the code exists in the buffer...

Code:

---

if( used_codes.find(str_usrCode.str(), 0) != std::string::npos ) {
  // found!
} else {
  // not found.
}

---

However, the whole convertion business is not needed if you simply declare usrCode as a string. After all, you are not using it as an int. Then it's just a matter of:

Code:

---

if( used_codes.find(usrCode, 0) != std::string::npos ) {
 // same as before...

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Thanks alot! But now I have another question: how would I go about retrieving data from a file as an int or a double variable? Is it similar to converting int to string?

There are other ways, but I find stringstreams to be the most flexible... and easier to type.

Imagine you have a file "test.txt" with the following contents:
"jmajeremy 34 6754 7.3 user"

You want the contents to map to "string int long double string".

Code:

---

#include <sstream>
#include <fstream>
#include <string>
#include <iostream>

int main()
{

    std::ifstream ifile("test.txt");
    if (!ifile) {
        std::cerr << "Couldn't open file!";
    } else {
        std::string buff;
        getline(ifile, buff); // read first line an place it in string
        ifile.close();  // close file

        //create storage
        std::string name;
        int age;
        long money;
        double value;
        std::string type;

        std::istringstream iss(buff); // create input sstream from string

        iss >> name >> age >> money >> value >> type; // store into variables

        // and to prove it...
        std::cout << name << "\n";
        std::cout << age << "\n";
        std::cout << money << "\n";
        std::cout << value << "\n";
        std::cout << type;
    }

    return 0;
}

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