How to do random shuffling of an array

Question

I want to generate numbers between range of 1 to n to create test cases for my program. So i created an array of size n and then stored it in array then usedrandom_shuffle But it's actually not random it always gives same series of number. it changes series like after 3 or 4 shuffling.

So is there any better way to do it. My basic problem is just to create a series of number from [1, n] in any random manner.

int n = 9;

   int arr[9] = {1,2,3,4,5,6,7,8,9};

   int k = 2;

   while(k--)
    random_shuffle(arr, arr+ n);

    for(auto x :arr)
        cout << x <<" ";

Output : 5 4 8 9 1 6 3 2 7 
So for either k = 1 or k = 2 it gives same output.

`random_shuffle`, or now just `shuffle` is the way to go. Can you share a [mre]? — NathanOliver, May 06 '20 at 14:35
I recommend adding a [mcve]. Without one there best we can do is make guesses. — user4581301, May 06 '20 at 14:35
Also note that `std::random_device` is [broken](https://stackoverflow.com/questions/18880654/why-do-i-get-the-same-sequence-for-every-run-with-stdrandom-device-with-mingw) on some platforms. — Evg, May 06 '20 at 14:36

Yksisarvinen · Accepted Answer · 2020-05-06T14:47:59.787

2

According to cppreference, std::random_shuffle commonly uses std::rand() as randomizer, which would require setting seed with srand() first.

However, std::shuffle accepts the generators from <random> library (which are superior to rand() unless you are using MinGW older than 9.3)

Example of usage from cppreference (linked above):

#include <random>
#include <algorithm>
#include <iterator>
#include <iostream>

int main()
{
    std::vector<int> v = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

    std::random_device rd;
    std::mt19937 g(rd());

    std::shuffle(v.begin(), v.end(), g);

    std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, " "));
    std::cout << "\n";
}

edited May 06 '20 at 14:47

answered May 06 '20 at 14:38

Yksisarvinen

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`std::random_device rd;` as a seed is problematic. Not all implementations give you a non-deterministic source. – NathanOliver May 06 '20 at 14:39
1

`random_device` has a usable implementation in MinGW's port of gcc9.3. – user4581301 May 06 '20 at 14:39
@NathanOliver I think it is more accurate to say that MinGW is a problematic compiler because its `std::ramdom_device` is broken. Though I believe they have finally fixed it. – Galik May 06 '20 at 14:44
This works for me as it seems to gives new combination of array. – Deepanshu Singh May 06 '20 at 14:45

eerorika · Answer 2 · 2020-05-06T14:53:24.973

Computers cannot generate true random numbers. Although there are techniques to generate something close to true randomness by for example measuring background radiation, generally random numbers generated by programs are pseudo random numbers.

A pseudo random sequence starting from some state is always identical to another pseudo random sequence starting from the same state, and using the same algorithm. In order to generate a different sequence, you need to start with a different state. The ability to reproduce a random sequence repeatedly is actually quite a useful thing to do in some cases.

In case of std::random_shuffle, the initial state is set with the std::srand function. If you pass a different number to std::srand, then you get a different pseudo random sequence and thus a different shuffle. A typical simple approach is to use the current time of day using std::time function. But take into consideration that the time changes only once every second.

How to do random shuffling of an array

2 Answers2