Thread: Card Shuffle Help

I really see nothing strange with mike's deck of cards, though. the indices are 0 to 51. On the last iteration of his shuffling loop, i is 52, which would break it properly.

And you can't exclude cards in the modern algorithm, otherwise your shuffle has a bias towards the card placement. Your generator could still select any of 52 indices to swap the last one with.

Incidently I don't really understand why C starts it's array indexes with 0 rather than 1, however I think I have mentioned that before, it would certaintly have saved me a lot of grief in the past (and present and future no doubt!!).

mike, i get a lot of errors with this..

Code:

#include <stdio.h>
#include <stdlib.h>
#include <stdlib.h>
#include <time.h>
#include <conio.h>


int main()
{
	
    srand(time(NULL)); //Seed randomisation
	/*char* GetSuit(Uint8 card){

	char *suit[] = {"Clubs", "Diamonds", "Hearts", "Spades"};
	 return suit[card/13];
	}
	char* GetRank(Uint8 card)
	{
	char *rank[] = {"Ace", "Two", "Three", "Four", "Five", 

        "Six", "Seven", "Eight", "Nine", "Ten", "Jack", "Queen", "King"};
	return rank[card%13];
	}*/
	    int deck[52];
		
    int i, temp, random;
    // Init deck
    for(i=0; i<52; i++)
         deck[i] = i;
    //Shuffle deck
    for(i=0; i<52; i++)
    {
         random = rand() % 52;
         temp = deck[i];
         deck[i] = deck[ random ];
         deck[ random ] = temp;		 
		
    }
		char* GetSuit(Uint8 card){

	char *suit[] = {"Clubs", "Diamonds", "Hearts", "Spades"};
	 return suit[card/13];
	}
	char* GetRank(Uint8 card)
	{
	char *rank[] = {"Ace", "Two", "Three", "Four", "Five", 

        "Six", "Seven", "Eight", "Nine", "Ten", "Jack", "Queen", "King"};
	return rank[card%13];
	}
	printf("%d", deck[i]);
_getche();
return 0;
}

The main problem is that you have some functions written multiple times as well as inside of main(). Don't do that - write them once, somewhere else in the file.

the first section was commented out..but unfortunetley anyway i write this code i get errors. One beings that card is not defined..what is card supposed to be defined as? Here is code without the extra junk.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <stdlib.h>
#include <time.h>
#include <conio.h>


int main()
{
	
	char *suit[] = {"Clubs", "Diamonds", "Hearts", "Spades"};
	 return suit[card/13];
	 char *rank[] = {"Ace", "Two", "Three", "Four", "Five", 

        "Six", "Seven", "Eight", "Nine", "Ten", "Jack", "Queen", "King"};
	return rank[card%13];
	
    srand(time(NULL)); //Seed randomisation

	    int deck[52];
		
    int i, temp, random;
    // Init deck
    for(i=0; i<52; i++)
         deck[i] = i;
    //Shuffle deck
    for(i=0; i<52; i++)
    {
         random = rand() % 52;
         temp = deck[i];
         deck[i] = deck[ random ];
         deck[ random ] = temp;		 
		
    }
	
	
	printf("%d", deck[i]);
_getche();
return 0;
}

Oh, yeah you want to remove the line that start with 'return' from main, but leave the return 0 at the end. Returning from main will basically quit your program.

I find it simpler just to provide my own example than try to fix your version. The problem is that card is supposed to be a number in the pack. In your code, there used to be functions for finding out the pips on a card, and the suit of the card. Like this:

Code:

#include <stdlib.h>
#include <time.h>
#include <stdio.h>

const char * what_pip( int this_card );
const char * what_suit( int this_card );
void make_deck( int * this_deck );
void shuffle( int * this_deck );

int main ( void )
{
    int mypack[52];
    long card;

    make_deck( mypack );
    srand( (unsigned )time( NULL ) );
    shuffle( mypack );
    
    for ( card = 0; card < 52; card += 4 ) {
        printf( "&#37;s%s %s%s %s%s %s%s\n", 
            what_pip( mypack[card] ), what_suit( mypack[card] ),
            what_pip( mypack[card + 1] ), what_suit( mypack[card + 1] ), 
            what_pip( mypack[card + 2] ), what_suit( mypack[card + 2] ), 
            what_pip( mypack[card + 3] ), what_suit( mypack[card + 3] ) );
    }
    return 0;
}

const char * what_pip( int this_card )
{
    const char * pips[] = { 
        "A", "2", "3", "4", "5", "6", "7", "8", "9",
        "T", "J", "Q", "K"
    };
    return pips[ this_card % 13 ];
}

const char * what_suit( int this_card )
{
    const char * stacks[] = { "S", "H", "C", "D" };
    return stacks[ this_card / 13 ];
}

void make_deck( int * this_deck )
{
    long idx;
    if ( this_deck == NULL ) return ;

    for ( idx = 0; idx < 52; idx++ ) {
        this_deck[idx] = idx;
    }
}

void shuffle ( int * this_deck )
{
    long idx;
    if ( this_deck == NULL ) return ;

    for ( idx = 0; idx < 52; idx++ ) {
        int rnd = rand() / RAND_MAX / 52;
        int save = this_deck[idx];
        this_deck[idx] = this_deck[rnd];
        this_deck[rnd] = save;
    }
}

/** my output - 
TS 7D 5H JS
AD JD 7S 3D
QC 7C JH JC
KS 6H TD KD
2S 8H 9H 3C
QS QH TC 5C
TH QD 8S AH
AS 4H 4S 5D
4C 6S KH KC
6D 9C 5S 8D
7H 3S 6C 2H
8C 2C 9D 9S
3H 4D AC 2D
**/

Nice and shuffled.

the only problem with what you have done is that the cards dont print out randomely

Dude, just remove the return statements from your code

And the only problem with what your post says is I'm not sure what the problem is.

If you're running the executable multiple times in quick succession expecting a very different result each time, then rand() has a seed which isn't very good. If you're shuffling the same deck multiple times and the results aren't very different, you need a different approach to calling rand(), because the card selection is suffering from bias. You might want to read using rand() effectively to get some ideas on what to do about this (and how a PRNG will affect your card games in general).

If you are trying to write a casino program and this isn't some academic venture, then you will have to write a stronger PRNG anyway, since rand() is not cryptographically random and wouldn't do for like, online poker. If you're looking for something with that kind of effectiveness, you need to learn and weigh these approaches.

If my code is wrong because I chose to print out the whole deck in four columns so you could see the result of the shuffle, I'm sorry. You'll have to work out how to deal cards for the game you are playing yourself.