How to fetch information randomly from files in C++?

Question

I am creating a quiz game in C++. So, the main requirement is that questions should be fetched randomly from file each time program runs. So, How can I do this in C++? Consider following two simple programs.

Write.cpp

#include <iostream>
#include <fstream>
#include <string>
using std::cout;
using std::ofstream;
using std::string;
int main()
{
    ofstream fout("test.txt");
    if(!fout)
        cout<<"Error in opening file";
    string s[3];
    s[0]="C++ provides object oriented string handling";
    s[1]="C++ provides object oriented exception handling";
    s[2]="C++ provides highest flexibility to the programmer";
    for(int i=0;i<3;i++)
        fout<<s[i]<<'\n';
    fout.close();
}

Read.cpp

#include <iostream>
#include <fstream>
#include <string>
using std::cout;
using std::string;
using std::ifstream;
int main()
{
    ifstream fin("test.txt");
    if(!fin)
        cout<<"Error in opening file";
    string s[3];
    for(int i=0;i<3;i++)
        getline(fin,s[i]);
    for(int i=0;i<3;i++)
        cout<<s[i]<<'\n';
    fin.close();
}

What should I do so that when I compile Read.cpp & run Read.exe file the 3 strings should be fetched randomly from file & gets displayed?

Your help is highly appreciated.

Answer 1

If you want to maintain the array of strings in the order you read them from the file

You can achieve that by creating another integer array that contains the numbers [1, 2 .. n-1] (where n is the number of strings) and then shuffling it to obtain a randomized sequences of indices, which you can use to print the strings.

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <algorithm>
#include <random>
#include <chrono>

using std::cout;
using std::string;
using std::ifstream;

int main()
{
    ifstream fin("test.txt");
    if(!fin)
        cout<<"Error in opening file";

    std::vector<string> lines;
    string line;
    while (getline(fin, line))
          lines.push_back(line);

    // Create a std::vector containing {1, 2 .. n-1} 
    // and shuffle it to obtain a random ordering.
    std::vector<int> order;
    for (int i = 0; i < lines.size(); ++i)
    {
       order.push_back(i);
    }

    // C++11
    unsigned seed = std::chrono::system_clock::now().time_since_epoch().count(); // Seed from current time.
    auto engine = std::default_random_engine{seed}; // You may also want to look into uniform_int_distribution.
    std::shuffle(std::begin(order), std::end(order), engine);

    // C++98
    // std::random_shuffle(order.begin(), order.end());

    // Prints strings in random order.
    for (int number : order)
         cout << lines[number] <<'\n';

    fin.close();
}

If you can modify (shuffle) the string array

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <algorithm>
#include <random>
#include <chrono>

using std::cout;
using std::string;
using std::ifstream;

int main()
{
    ifstream fin("test.txt");
    if(!fin)
        cout<<"Error in opening file";

    std::vector<string> lines;
    string line;
    while (getline(fin, line))
          lines.push_back(line);

    // C++11
    unsigned seed = std::chrono::system_clock::now().time_since_epoch().count(); // Seed from current time.
    auto engine = std::default_random_engine{seed}; // You may also want to look into uniform_int_distribution.
    std::shuffle(std::begin(lines), std::end(lines), engine);

    // C++98
    // std::random_shuffle(lines.begin(), lines.end());

    // Prints strings in random order.
    for (string& line : lines)
         cout << line <<'\n';

    fin.close();
}

Try it online!