Deck Shuffle

**pjharris** · 01-05-2006

Originally Posted by Salem

> well with the way he uses srand(0)
Ok, so you can't read then, if you've got srand(0);
>> srand(time(0)); //seed random number generator

Don't try and insult me by saying I can't read. Granted I didn't type it correctly in my previous post, but in the code its right. I copied it straight from what he put.

**7stud** · 01-06-2006

That is all good and dandy. But I just ran your example and I got the same set of numbers each time a ran it, which was like 10 times.

Prove it.

**Salem** · 01-06-2006

RETURN VALUE
On success, the value of time in seconds since the Epoch is returned. On error, ((time_t)-1) is
returned, and errno is set appropriately.

Perhaps your time() is broken and it's always returning -1. That would generate the effect you see.
Investigate this.

**pjharris** · 01-06-2006

Originally Posted by 7stud

Prove it.

How would you like me to do that? Copy and paste? But then you could just say I copied it twice.

Salem:
I will investigate it to see if thats the case.

**pjharris** · 01-06-2006

I don't know whats going on, I'm sure the example works as it's suppose to. time() is working fine. I even tried it on another machine. It seems to me that srand() is not working seeing as I get the same numbers. Maybe it's not seeding the generator. And I checked if srand() had a return value and it doesn't so I can't check it.

**Salem** · 01-06-2006

So something really basic like

Code:

#include <iostream>
#include <cstdlib>
#include <ctime>
using namespace std;
      
int main()
{
    srand(time(0)); //seed random number generator
    for(int j = 0; j<10; j++)
    {
        cout << rand() << endl;
    }
    cin.get();
    return 0;
}

Produces constant output?

Puzzled...

**xhi** · 01-06-2006

i too was not getting random results when i tested the provided example.. not salem's but the earlier one.. so i was just about to post a little example when salem posted that one.. damn! 1 minute late..

anyhow.. i still have a little issue with the original code.. here is what i found when i tested it.... ?
is the array ever initialized.. ? not that i see...
mine was not working and i did this and now it does..
i have never messed with random_shuffle.. very good to know about.!

hth

Code:

#include <iostream>
#include <algorithm> //random_shuffle()
#include <ctime> //time()
#include <cstdlib> //srand()


using namespace std;

int main()
{
    srand(time(0)); //seed random number generator
    int arr[] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
    ////    HERE  vvvv
    for(int i=0; i<10; ++i){
        arr[i]=rand();
    }
    for(int j = 0; j<10; j++)
    {
        random_shuffle(arr, arr+9);

        for(int i=0; i<9; i++)
        {
            cout<<arr[i]<<endl;;
        }

        cout<<endl;
    }

    cin.get();
    return 0;
}

<edit> oh and btw.. the small example i used resembling salems last post.. works just fine.... </edit>

**pjharris** · 01-06-2006

Salem your example works fine for me. It must be random_shuffle() then.

**xhi** · 01-06-2006

nevermind.. i must be a moron.. let me check..

yes, yes i am

<edit>
hmm yes still am..

but i cant seem to make random_shuffle work either..
good thing i dont need it..

oh and yes i am leaving..
</edit>

**7stud** · 01-06-2006

is the array ever initialized.. ? not that i see...

Code:

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

If you aren't familiar with the STL, then the code may seem strange to you, but you have to provide random_shuffle() with the memory range where the values you want shuffled are located.

How would you like me to do that? Copy and paste? But then you could just say I copied it twice.

Yes, copy and paste the code your are running. Discussing errors in imaginary code is never very productive.

**xhi** · 01-06-2006

yes i know.. i over looked that somehow..

i do have a question though.. the results i was getting are what threw me off..

so i run the provided example and get this

Code:

no matter how many times i run it.. i get the same thing.. i see now what shuffle is doing.. and that the array was initialized.. duh! but i dont understand why the shuffling is the same everytime..

i would think that it would be different in different program executions also..

get what im saying??

**pjharris** · 01-06-2006

Code:

#include <iostream>
#include <algorithm> //random_shuffle()
#include <ctime> //time()
#include <cstdlib> //srand()
using namespace std;
      
int main() {
    srand(time(0));
    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;
}

Thats the code I'm running

Code:

[pjharris@rundi]$g++ board.cc -o board
[pjharris@rundi]$./board;sleep 2;./board
269543178
587942631
893761425
849256317
659381274
583127694
638412957
265381497
947168235
749263158

269543178
587942631
893761425
849256317
659381274
583127694
638412957
265381497
947168235
749263158

[pjharris@rundi]$

and thats the output

**dwks** · 01-06-2006

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]

**pjharris** · 01-06-2006

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.

Thats what the sleep does. Sleep 2 says wait two seconds then run it again And it's not the case about timing, because I ran it 10, 30, 60 mins after and I still got the same. I've been getting the same output for 2 days now.