(C++) Making a random sequence of a set of numbers 1-3

Question

I need my program to make a random sequence from 1-3 each time, but I don't understand how I'd use rand() to make the sequence of numbers 1 to 3 in a different order each program. It can't be the same number again, so I don't know what I'd do to prevent that. An example run would be

123 the first, 231 the second, 321 and so fourth

What would you use to make a sequence that doesn't repeat numbers

You can visit [here](https://www.geeksforgeeks.org/stdnext_permutation-prev_permutation-c/) — Yunus Temurlenk, Jan 20 '20 at 05:24
So you want to randomly select numbers from a sequence without replacement? You can just shuffle it randomly once, and then pop from the back until exhausted. — Jan Christoph Terasa, Jan 20 '20 at 05:39
Wait a second: Do you not want to have repetitions *only within* these 3-digit sequences or don't you want to have repetitions *globally*? Would a sequence 123, 321 be legal? Would you want to produce *arbitrary* random sequences from these three-digit sequences? Would 123, 123, 231, 321, 123 be legal? — Aconcagua, Jan 20 '20 at 07:12

score 1 · Answer 1 · answered Jan 20 '20 at 08:11

The simplest way to generate your sequence would be to use std::shuffle to re-order a vector containing your desired values:

#include <vector>
#include <algorithm>
#include <random>
#include <iostream>

int main()
{
    std::random_device rd;
    std::mt19937 g(rd());
    std::vector<int> elements = { 1, 2, 3 };
    std::shuffle(elements.begin(), elements.end(), g);
    for (int i : elements)
    {
        std::cout << i << "\n";
    }
}

If you really must use rand() (its not generally a very good random number generator) you can just about squeeze it into shuffle too:

#include <vector>
#include <algorithm>
#include <ctime>
#include <iostream>

struct RandDevice
{
    using result_type = uint32_t;
    static result_type max() { return static_cast<result_type>(RAND_MAX); };
    static result_type min() { return 0; };

    result_type operator()() {
        return static_cast<result_type>(rand());
    }
};

int main()
{
    std::vector<int> elements = { 1, 2, 3 };
    srand(time(0));
    std::shuffle(elements.begin(), elements.end(), RandDevice());
    for (int i : elements)
    {
        std::cout << i << "\n";
    }
}

score 0 · Answer 2 · answered Jan 20 '20 at 05:13

0

You can use std::next_permutation

Example here : https://en.cppreference.com/w/cpp/algorithm/next_permutation

answered Jan 20 '20 at 05:13

Sitesh

1,816
1
18
25

It's a better tool to accomplish the goal, but doesn't use `rand`. – user4581301 Jan 20 '20 at 05:32
Maybe that's only half of the answer. We might produce all permutations with and then either just `std::shuffle` them or we might produce an array from all of these and pick arbitrary element from based on random number – depending on needs of user. There's yet some clarification necessary, though... – Aconcagua Jan 20 '20 at 07:11

Prabin Shrestha · Answer 3 · 2020-01-20T05:41:21.980

it's pretty easy to do.. just compare the number you with every occupied element in the array. if it is not in the array, add to array. else, try again.

I have done similar code check this out

    #include <iostream>
#include <ctime>
#include <cstdlib>
using namespace std;
void displayArray(int randNum[], int elements);
void randomNum(int randNums[], int elements);
int main ()
{
    //declare array
    int numbers[999] = {0};
    //random number generator
    srand(static_cast<int>(time(0)));

    randomNum(numbers, 999);
    displayArray(numbers, 999);

system("pause");
return 0;
}
void randomNum(int randNums[], int elements)
{
    for (int i = 0; i < elements; i++)
    {
        bool same;
        do
        {
            same = false;
            randNums[i] = rand() % 999 + 100;
            // Check if the newly generated number is a duplicate:
            for (int check = 0; check < i; check++)
            {
                if (randNums[i] == randNums[check])
                {
                    same = true;
                    break;
                }
            }
        } while (same);
    }
}
void displayArray(int randNum[], int elements)
{
    for (int sub = 0; sub < elements; sub ++)
    {
        cout << "Unique Numbers: " << randNum[sub] << endl;
    }
}

Modulo on random number generator's results produce rather bad distribution. And danger of collisions rises the more elements you have already filled in. There isn't even a guarantee that you ever get your array filled completely. Apart from bad distribution, `%999 + 100` can produce unwanted sequences like 497, 101, 1098, ... — Aconcagua, Jan 20 '20 at 07:20
About [using namespace std](https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice) – and `system("pause")` or similar stuff (`getc()`, checking with `kbhit` from `conio.h`) is bad practice as well. It's *not* the task of a console programme to keep the console window open! You only prevent your programme being usable from scripts. — Aconcagua, Jan 20 '20 at 07:27

(C++) Making a random sequence of a set of numbers 1-3

3 Answers3