I am trying to make a random number generator between 5 and 8 in C++

Question

I need this code to generate a number between 5 and 8 and then assign it to a variable. However when i make it give me only one number instead of multiple, it only gives me the same number. While when i have it set to give e multiple it gives me random numbers.

#include <random>
#include <iostream>

int main()
{
    std::random_device rd;
    std::mt19937 gen(rd());
    std::uniform_int_distribution<> dis(5, 8);

    for (int n=0; n<1; ++n)
        std::cout << dis(gen) << ' ';
    std::cout << '\n';
}

It looks like you need to set the seed of the `random_device` to a different value each time. `random_device` is allowed to be a PNRG. You can check it by calling it many times, e.g by putting `rd()` in the `cout` loop. — juanchopanza, Jun 05 '14 at 05:42
duplicated: http://stackoverflow.com/questions/22890738/seeding-the-random-number-generator/22891670#22891670 — harper, Jun 05 '14 at 05:44
FWIW in my implementation, I get different numbers each time. — juanchopanza, Jun 05 '14 at 05:48
@harper `rand` and `srand` are (thankfully) entirely separate from the C++11 RNGs. — , Jun 05 '14 at 05:51
Can you clarify your question? Do you get the same sequence when looping more than once or not? And what compiler/platform are you using? — juanchopanza, Jun 05 '14 at 05:51
http://codepad.org/ZSo2fVxU I managed to find this and it works except i need a way for it only to give me one. — user3317455, Jun 05 '14 at 06:02
@delnan Despite the implementation in C++11 is completely different the statement about pseudo random numbers and seeding applies entirely. — harper, Jun 05 '14 at 06:02
@user3317455 Give you only one what? Random number? Call the distribution once, and it'll give you a single random number. And what compiler/version/platform etc. are you using? — Praetorian, Jun 05 '14 at 06:21

quantdev · Answer 1 · 2014-06-05T06:47:12.093

1 - You have "only one number" :

Because your for-loop does only one iteration.

for (int n=0; n<1; ++n)
                ^
               Here

2 - If you always see the same pseudo random sequence, this is because the seed is itself pseudo random: for a std::random_device, this is implementation specific :

Note that std::random_device may be implemented in terms of a pseudo-random number engine if a non-deterministic source (e.g. a hardware device) is not available to the implementation.

Note that your example on ideone is not pseudo random and works fine: http://ideone.com/lysKrt

Alternatively to rd, you can use the system time as a seed :

    #include <chrono>
    // ...

    unsigned seed = std::chrono::system_clock::now().time_since_epoch().count();
    std::mt19937 gen(seed);

-1. You have to explain why the seed "won't change" (see my first comment.) OP is setting the seed using `random_device`. — juanchopanza, Jun 05 '14 at 05:49
Sure: edited with the explanation (rd might be pseudo random) — quantdev, Jun 05 '14 at 05:59

score 0 · Answer 2 · answered Jun 05 '14 at 05:42

0

It is because the executable always starts from the same seed. If you want different numbers in each run then you need to seed the generator. Say, with the current time.

answered Jun 05 '14 at 05:42

LaszloLadanyi

973
4
12

-1. You should explain how it can be that the same seed is being used. There is an attempt to seed the generator each time. – juanchopanza Jun 05 '14 at 05:54

I am trying to make a random number generator between 5 and 8 in C++

2 Answers2