Shuffle and Deal cards in C

[jamdonley]

I'm trying to write C code to shuffle a 52 card deck with 2 character arrays. One for the suit and the other for the value. The arrays are really killing me and I've searched for about 2 hours now and just not finding what I need. I'm not sure how to shuffle the cards within the array. Professor said to try switching 2 numbers within the array by running a for loop 1000 times, but just confused. I'm guessing just initialize the arrays like this:

char number[] = { '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', 'J', 'Q', 'K', 'A' };
char suit[] = { 'c', 'd', 'h', 's' };

I've read about pointers? Not sure exactly what that means and saw others with "const" in front of the arrays but haven't learned anything about those yet. I honestly don't even know where to go from there. I just need to write a couple functions to shuffle and deal the cards and I can go from there. This is what I have so far:

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

int shufflenumber(int[]);
char shufflesuit(char[]);
int dealnumber(int[]);
char dealsuit(char[]);

int main(void)
{
char number[] = { '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', 'J', 'Q', 'K', 'A' };
char suit[] = { 'c', 'd', 'h', 's' };
    
  
  system("PAUSE");    
  return 0;
}

int shufflenumber(int number)
{
int i=0;
srand(time(NULL));
for(i=0; i<1000; i++)

I'm not sure what to do within the for loop which is why it isn't completed

[oogabooga]

You can't do it with the two arrays you have. You need a single 52 element array. You could load it with the numbers 0 to 51 and interpret the card value as n % 13 + 1 and the suit as n / 13.

E.g., the number 21 gives
21 % 13 + 1 = 9
21 / 13 = 1 (remember integer division truncates any fractional part)
So the value is 9 and the suit is 1.

You could order the suits where C, D, H, S are 0, 1, 2, 3 respectively.

With that suit ordering, the card with the value 21 is the 9 of Diamonds.

To shuffle an array you could certainly loop a thousand times picking two random numbers 0 to 51 and swap those elements, but that's inefficient. Another way is to simply loop through the deck once, picking a random number 0 to 51 and swapping the current index with the random one.

[MK27]

You have to break this down and work on one issue at a time. A standard way to shuffle (randomize) an array is the Fisher-Yates shuffle, which is fairly simple, but first you need a deck to shuffle.

It's not clear if using those two arrays is a constraint in the assignment. If not, why do you think this is a way to proceed (notice, oogabooga's idea does not require that)?

In any case, a deck of 52 is a deck of 52. You could use those two arrays in creating it, but the deck itself is a third entity, eg:

#include <stdio.h>
#include <string.h>

const char suits[4] = { 'D', 'C', 'H', 'S' };
const char values[13] =
	{ '2', '3', '4', '5', '6', '7', '8', '9', 'X', 'J', 'Q', 'K', 'A' };

int main(void) {
	char deck[52][3];
	int s, v, c = 0;

/* create the deck */
	for (s = 0; s < 4; s++) {
		for (v = 0; v < 13; v++) {
			sprintf(deck[c], "%c%c", suits[s], values[v]);
			printf("%s\n", deck[c]);
			c++;
		}
	}

	return 0;
}

[oogabooga]

Reading up on the Fisher–Yates shuffle in Wikipedia I see that the algorithm I gave above is subtly incorrect, and in a fairly interesting way too. You shouldn't select the random element from the entire range every time, but only from what you might call the "unshuffled" range, which decreases by 1 each iteration. See the implementation in the Wikipedia article under "The modern algorithm" for the correct algorithm.

[jamdonley]

having the two arrays isn't necessary, but that's what he suggested. I'm still lost with the whole "const" before declaring the arrays, and also wondering why you added "c++" on line 17?

[jamdonley]

also is it supposed to be "sprintf" and not "printf" or is that a mistake in line 15?

[jamdonley]

is there a way you could write that in code? I am looking at the wikipedia page and it seriously looks like jibberish to me and I don't understand it at all

[quzah]

is there a way you could write that in code?

It wouldn't really do you any good for someone here to just do all the work for you. Oh sure, you'd get a good grade, but you'd still be a lousy programmer with no problem solving skills.


Quzah.

[jamdonley]

I'm doing this as my major. Trust me my grade is the last thing I'm worried about if I plan on doing this for a living. I need to be able to actually see and understand the code in order to do it.

[phantomotap]

By that same logic you'd better learn mechanical engineering by watching NASCAR events.

Soma

[jamdonley]

All I'm asking is to help understand what this means in code

To shuffle an array a of n elements (indices 0..n-1):
for i from n − 1 downto 1 do
j ← random integer with 0 ≤ j ≤ i
exchange a[j] and a[i]

I don't understand 0 ≤ j ≤ i and what it means and how exactly you do exchangea[j] and a[i]. I understand what it's doing by reading the page, but I don't know what to do with it. I understand where you guys are coming from, but how exactly would I understand how to do it if I don't have any code whatsoever to follow

[rags_to_riches]

don't understand 0 ≤ j ≤ i and what it means

No math background? It means j is between 0 and i inclusive.

how exactly you do exchangea[j] and a[i]

Exchange the values held in those indices (j and i)?

[jamdonley]

Yes i understand what it means, I'm saying in terms of c code. how do you exchange them? and why do you need 0 ≤ j ≤ i, what is its purpose

[MK27]

having the two arrays isn't necessary, but that's what he suggested. I'm still lost with the whole "const" before declaring the arrays

"const" just indicates a value can't be changed, eg:

const int X = 666;
X = 0;  // compiler will not like this.

and also wondering why you added "c++" on line 17?

It's the equivalent of:

c = c + 1;

and is performed after the rest of the operation (aka, post-increment).

also is it supposed to be "sprintf" and not "printf" or is that a mistake in line 15?

No mistake. If you look up sprintf(), you'll get it. Hopefully...

is there a way you could write that in code? I am looking at the wikipedia page and it seriously looks like jibberish to me and I don't understand it at all

That's actually why I didn't post a link to wikipedia, lol. I have to run, but here's a "fisher-yates.c" example:

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

void FYshuffle (int *ray, int len) {
	int i, tmp, x;
	for (i=len-1; i>1; i--) {
		x = rand()%i; 
		if (x==i) continue;
	// now swap
		tmp = ray[i];
		ray[i] = ray[x];
		ray[x] = tmp;
	}
}

int main(void) {
	int ray[10] = {0,1,2,3,4,5,6,7,8,9}, i;

	srand(time(0));
	FYshuffle(ray,10);
	for (i=0;i<10;i++) printf("%d\n",ray[i]);

	return 0;
}

[whiteflags]

The whole purpose is just to write down an algorithm in a concise way without assuming what language the reader cares about.