How to search line by line in a text file - c++

Question

123 Michael
456 Calimlim
898 Mykfyy
999 Kyxy
657 mykfyy
898 Help
I'm creating a student attendance keeper system. One of the features of my system is that the student need to register(his/her id and name) first to access the system (login with his/her id)
The problem is that i don't know and i don't want my student to have a similar ID number Like(e.g 898 Mykfyy and 898 Help)
I'm using fstream in my system. I've been thinking that if I want to avoid the duplication i need to Read(ifstream) the .txt file before register(oustream). But i dont know how to read line by line and check if the ID(898) is already use/existed

Answer 1

In C++ one wouldn't deal with lines, but with objects:
#include <limits>
#include <cstdlib>
#include <vector>
#include <string>
#include <fstream>
#include <iostream>
#include <iterator>
#include <algorithm>
struct student_t
{
unsigned id;
std::string name;
};
bool operator==(student_t const &lhs, student_t const &rhs)
{
return lhs.id == rhs.id;
}
std::ostream& operator<<(std::ostream &os, student_t const &student)
{
return os << student.id << ' ' << student.name;
}
std::istream& operator>>(std::istream &is, student_t &student)
{
unsigned id;
if (!(is >> id))
return is;
std::string name;
if (!std::getline(is, name)) {
return is;
}
student = student_t{ id, name };
return is;
}
int main()
{
char const *filename{ "test.txt" };
std::ifstream input{ filename };
if (!input.is_open()) {
std::cerr << "Couldn't open \"" << filename << "\" for reading :(\n\n";
return EXIT_FAILURE;
}
std::vector<student_t> students{ std::istream_iterator<student_t>{ input }, std::istream_iterator<student_t>{} };
input.close();
std::copy(students.begin(), students.end(), std::ostream_iterator<student_t>{ std::cout, "\n" });
student_t new_student;
while (std::cout << "New Student?\n", !(std::cin >> new_student)) {
std::cerr << "Input error :(\n\n";
std::cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
auto it{ std::find(students.begin(), students.end(), new_student) };
if (it != students.end()) {
std::cerr << "Sorry, but a student with id " << new_student.id << " already exists :(\n\n";
return EXIT_FAILURE;
}
std::ofstream output{ filename, std::ios::app };
if (!output.is_open()) {
std::cerr << "Couldn't open \"" << filename << "\" for writing :(\n\n";
return EXIT_FAILURE;
}
output << new_student << '\n';
std::cout << "New student [" << new_student << "] added :)\n\n";
}

Answer 2

The easiest way to do it is probably using std::getline to get the current line as a string:
using namespace std;
ifstream in(fileName);
string line;
while(getline(in, line))
{
// --do something with the line--
}
You will then need to parse each line to get the correct ID
Edit: updated to remove eof()

Answer 3

Depends on how you implemented it.
I suppose the number of people isn't too large so I would just check the Id's before a new ID is added.
Something like
If ID exists, not add.