A Quick Shuffle method.. - GameCreators Forum

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David Gervais

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Posted: 1st Dec 2006 10:59 Edited at: 1st Dec 2006 11:05

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Hi there,

I'm back to doing some coding and needed to remember how to shuffle a deck of cards. I finally remembered how from my old Atari Basic days. Here is a commented code snippet explaining the process. (Note this is not original code by me, but rather something from the good old days and I do not remember the original programmer.)

REM Let's build and shuffle a deck of cards..

DIM deck(53)                          `First we create an array with 52 spaces (+1 to avoid using '0' as the first card).

for cards=1 to 52                   `this little for/next loop fills the deck(xx) with values 1 to 52
     deck(cards)=cards
  next cards

REM Now that we have a deck and it has 52 cards we will shuffle it

for s=52 to 1 step -1
     random_index=rnd(s)+1            `this gives us a random number between 1 and 52
     tmp=deck(random_index)           `we pull that randon card and store it in tmp
     deck(random_index)=deck(s)       `we then transfer the '52nd' card to the random_index position..
     deck(s)=tmp                      `then store the tmp card in the '52nd' slot
  next s                              `Since we now process the for/next, the 52nd slot is 'protected'.
                                      `The next loop will give a rnd # between 1 and 51

Rem There you have it, a deck of 52 cards all shuffled up.

+ Code Snippet

REM Let's build and shuffle a deck of cards..

DIM deck(53)                          `First we create an array with 52 spaces (+1 to avoid using '0' as the first card).

  for cards=1 to 52                   `this little for/next loop fills the deck(xx) with values 1 to 52
     deck(cards)=cards
  next cards

REM Now that we have a deck and it has 52 cards we will shuffle it

  for s=52 to 1 step -1
     random_index=rnd(s)+1            `this gives us a random number between 1 and 52
     tmp=deck(random_index)           `we pull that randon card and store it in tmp
     deck(random_index)=deck(s)       `we then transfer the '52nd' card to the random_index position..
     deck(s)=tmp                      `then store the tmp card in the '52nd' slot
  next s                              `Since we now process the for/next, the 52nd slot is 'protected'.
                                      `The next loop will give a rnd # between 1 and 51

Rem There you have it, a deck of 52 cards all shuffled up.

I hope this code snippet helps. Oh, and the 'size' of the deck can be mush larger if needed.

Cheers!

P.S. here is a small program with the routine as a function to test the shuffle code.

+ Code Snippet

REM Let's build and shuffle a deck of cards..

DIM deck(53)                          `First we create an array with 52 spaces (+1 to avoid using '0').

  for cards=1 to 52                   `this little for/next loop fills the deck(xx) with values 1 to 52
     deck(cards)=cards
  next cards

do
  text 0,0,"Each press of the left Mouse button will shuffle and view the results."
  if mouseclick()=1
    cls
    shuffle_deck()
    show_results()
    while mouseclick()=1
      donothing=0
    endwhile
  endif
loop

function show_results()

  c=1
  for x=0 to 12
    for y=0 to 3
      text (x*32)+32,(y*32)+32,str$(deck(c))+" "
      c=c+1
    next y
  next x

endfunction

function shuffle_deck()

  for s=52 to 1 step -1
     random_index=rnd(s)+1
     tmp=deck(random_index)
     deck(random_index)=deck(s)
     deck(s)=tmp
  next s

endfunction

have a great day.

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TEH_CODERER

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Posted: 4th Dec 2006 16:50

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Simple when you know how but can be quite the puzzler when you try to get it for the first time. I'm sure this will help some people.

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Mr Tank

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Posted: 5th Dec 2006 04:54

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Nice example. Sorry to be a pedant, but shouldn't it be s=52 to 2, and rnd(s-1)+1?
At the moment the first random index you generate is between 1 and 53. I'm likely wrong and just need more sleep though.

You'll be able to click on this someday.

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David Gervais

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Posted: 5th Dec 2006 12:25 Edited at: 5th Dec 2006 12:34

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it took me awhile to grasp the quirks in most programming languages regardin '0' (zero) as the 'first number'. I make a habbit of always DIM'ing the number I want +1. this way I can avoid the use of '0'

ie:

Dim deck(53) this sets up a deck with yes 53 cards, but they are numbered from 0-52. programs like using '0' as the first number. I try to avoid it, because it confuses me. So, I then use 1-52 as my pointers and ignore the '0' slot.

and in the same way rnd(52) gives a number between 0 and 51. so by adding +1 it makes the number between 1 and 52.

anyways, I hope this helps clear up the code.

Cheers!

Edit, to clarify your question.. "but shouldn't it be s=52 to 2, and rnd(s-1)+1?"..

in your example rnd(s-1) would be rnd(51) which would be a number between 0 and 50 adding +1 gets it to 1-51.. you skip the 52nd slot. Do the math. and trust me. I've been all over this function. my first error was doing a DIM deck(52) it always gave me a 'out of range' error. it was trying to address deck(52) which did not exist due to the 'evil' Zero as the first digit in the array. LOL

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Mr Tank

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Posted: 5th Dec 2006 17:58

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I don't know about other languages, but i'm fairly sure that in DBP (at least version 1.06 that i'm using) you can use the nth slot- for example, you can

dim myarray(0)

and use it OK.

This code example should illustate my point. I declare an array of dimension 9, so it has 10 slots, running from 0 to 9. Note that in the second for/next loop, it definitely goes through all the numbers 0 to 9.
The program should hopefully show that the rnd(n) function generates a random integer from 0 to n.

+ Code Snippet

dim occurance(9)

for n=1 to 2000
chosen=rnd(9)
inc occurance(chosen)
next n

for n=0 to 9
text 0,n*10,str$(n)
box 20,n*10,20+occurance(n),n*10+9
next n

text 200,200,"press escape to exit"

do:loop

You'll be able to click on this someday.

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David Gervais

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Posted: 5th Dec 2006 21:54

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Ouch, now my brain is imploding. I knew the first element in an array was '0' (zero) and had always taken a DIM xxx(10) to be 10 total elements numbered 0-9, now you say DIM xxx(10) is actually 11 elements?? that's just sick. it makes for very confusing array definitions.

I guess to preserve my sanity I now have to ignore both ends of an array lol.

Nuf said, Cheers!

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Mr Tank

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Posted: 5th Dec 2006 22:00

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If it makes your brain hurt, then you're in good company.

I only recently found out that if you create a bitmap measuring n by m, it has pixel coordinates ranging from (0,0) to (n-1,m-1), which is equivalent to what you reasonably assumed was going on with arrays.

You'll be able to click on this someday.

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gearce

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Posted: 21st Dec 2006 08:14

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Sorry David Gervais, but the programme as written causes Syntax Errors and will not run in DB Classic. Rem statements must be on a separate line. Whilst I acknowledge your comments "this is not original code by me, but rather something from the good old Atari Basic days", it is my understanding that anything appearing here should be written in DB language.

gearce

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Robot

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Posted: 23rd Dec 2006 20:46

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@Gearce It was put together for DBP but here's a what you would do for DBC. Corrected the arrays as well I think.

REM Let's build and shuffle a deck of cards..

DIM deck(52)                          :`First we create an array with 52 spaces (+1 to avoid using '0' as the first card).

for cards=1 to 52                   :`this little for/next loop fills the deck(xx) with values 1 to 52
     deck(cards)=cards
  next cards

REM Now that we have a deck and it has 52 cards we will shuffle it

for s=52 to 1 step -1
     random_index=rnd(s-1)+1          :`this gives us a random number between 1 and 52
     tmp=deck(random_index)           :`we pull that randon card and store it in tmp
     deck(random_index)=deck(s)       :`we then transfer the '52nd' card to the random_index position..
     deck(s)=tmp                      :`then store the tmp card in the '52nd' slot
  next s                              :`Since we now process the for/next, the 52nd slot is 'protected'.
                                      `The next loop will give a rnd # between 1 and 51

Rem There you have it, a deck of 52 cards all shuffled up.

+ Code Snippet

REM Let's build and shuffle a deck of cards..

DIM deck(52)                          :`First we create an array with 52 spaces (+1 to avoid using '0' as the first card).

  for cards=1 to 52                   :`this little for/next loop fills the deck(xx) with values 1 to 52
     deck(cards)=cards
  next cards

REM Now that we have a deck and it has 52 cards we will shuffle it

  for s=52 to 1 step -1
     random_index=rnd(s-1)+1          :`this gives us a random number between 1 and 52
     tmp=deck(random_index)           :`we pull that randon card and store it in tmp
     deck(random_index)=deck(s)       :`we then transfer the '52nd' card to the random_index position..
     deck(s)=tmp                      :`then store the tmp card in the '52nd' slot
  next s                              :`Since we now process the for/next, the 52nd slot is 'protected'.
                                      `The next loop will give a rnd # between 1 and 51

Rem There you have it, a deck of 52 cards all shuffled up.

and

+ Code Snippet

REM Let's build and shuffle a deck of cards..

DIM deck(53)                          :`First we create an array with 52 spaces (+1 to avoid using '0').

  for cards=1 to 52                   :`this little for/next loop fills the deck(xx) with values 1 to 52
     deck(cards)=cards
  next cards

do
  text 0,0,"Each press of the left Mouse button will shuffle and view the results."
  if mouseclick()=1
    cls
    shuffle_deck()
    show_results()
    while mouseclick()=1
      donothing=0
    endwhile
  endif
loop

function show_results()

  c=1
  for x=0 to 12
    for y=0 to 3
      text (x*32)+32,(y*32)+32,str$(deck(c))+" "
      c=c+1
    next y
  next x

endfunction

function shuffle_deck()

  for s=52 to 1 step -1
     random_index=rnd(s-1)+1
     tmp=deck(random_index)
     deck(random_index)=deck(s)
     deck(s)=tmp
  next s

endfunction

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David Gervais

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Posted: 23rd Dec 2006 23:03

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Sorry my bad, should have mentioned it was adapted to DBPro and not DBC. Oh and since I compiled and ran it no problem in DBPro I consider this code to be in the proper place. Also, I do not have DBC so I could not even try if it ran in that version of DB.

In My defense, this type of code is generic enough that any new programmer could easily adapt it to DBC, Playbasic or any other language.

Nuf said, Cheers!

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gearce

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Posted: 25th Dec 2006 07:12

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To David Gervais, I apologise. Like you don't have DB Classic and couldn't test the programme, I don't have DB Pro and couldn't test it. I moved the rem statements to individual lines and made a minor adjustment to the arrays and it works fine. Certainly a neater card shuffle than the one I had been using. Again, my apologies.

To Robot, thanks for your input. I didn't know rem statements could be written like this but I prefer to write them on individual lines. To each his own as the saying goes.

Let's leave it at that.

gearce

LANG MAY YER LUM REEK

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Code Snippets / A Quick Shuffle method..