Thread: Deck Shuffle

>> Since you're running the program a few seconds apart, random_shuffle() must use its own random number generator (and not rand()).


i was reading up on random_shuffle a little earlier and there are two different usages..

Code:

void random_shuffle (RandomAccessIterator first,
                          RandomAccessIterator last,
                          RandomNumberGenerator& rand);

void random_shuffle (RandomAccessIterator first,
                                 RandomAccessIterator last);

couldnt find anything on RandomNumberGenerator& rand though

and yes it does not depend on rand being initialized.. as it gives same results if you remove it...

also ..

Code:

#include <iostream>
#include <algorithm> //random_shuffle()

using namespace std;

int main()
{
    int arr[] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
    for(int j = 0; j<10; j++)
    {
        random_shuffle(arr, arr+9);
        for(int i=0; i<9; i++)
        {
            cout<<arr[i];
        }
        cout<<endl;
    }
    cin.get();
    return 0;
}

gives this

Code:

283467159
192345876
916472583
216895734
193624857
945128376
728134596
194625783
279613845
328956741

exact copy as if rand is in there ??

Originally Posted by dwks

I think it's a seed. Pass time(0) to it and see what happens.

Yeah but thats what I did I the code that I posted and it doesnt work.

i passed it like so

Code:

random_shuffle(arr, arr+9, time(0));

it doesnt like it one bit..

Code:

/usr/include/c++/3.3.6/bits/stl_algo.h: In function `void
   std::random_shuffle(_RandomAccessIter, _RandomAccessIter,
   _RandomNumberGenerator&) [with _RandomAccessIter = int*,
   _RandomNumberGenerator = time_t]':
7.cpp:14:   instantiated from here
/usr/include/c++/3.3.6/bits/stl_algo.h:1673: error: `__rand' cannot be used as
   a function
7.cpp: In function `int main()':
7.cpp:14: error: invalid initialization of non-const reference of type 'time_t&
   ' from a temporary of type 'time_t'
/usr/include/c++/3.3.6/bits/stl_algo.h:1666: error: in passing argument 3 of `
   void std::random_shuffle(_RandomAccessIter, _RandomAccessIter,
   _RandomNumberGenerator&) [with _RandomAccessIter = int*,
   _RandomNumberGenerator = time_t]'

Originally Posted by dwks

You should get the same thing if you run it twice in a second. Try running it a few seconds apart.

[edit]
Hey, you beat me to it.

Since you're running the program a few seconds apart, random_shuffle() must use its own random number generator (and not rand()).

Try this code:

Code:

#include <iostream>
#include <ctime>
#include <cstdlib>

int main(void) {
    srand(time(0));

    for(int x = 0; x < 10; x ++) {
        std::cout << rand() % 10;
    }

    std::cout << std::endl;

    return 0;
}

[/edit]

I tried something exactly like that example that Salem had me try and it worked fine.