# Random Number Question

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Hi,

Is there a way to repeat a set of code until a certain if statement is
satisfied.  If it is then to exit the loop and if not repeat the code?

Say I want to write a card game application in PHP and I want to chose a
card from the deck \$card1 = rand(1,52); gets me my first card.

I need to record which card has been dealt so I have a variable \$dealt which
starts out as a string of 52 zeros.  I update the position of \$card1 in the
\$dealt string to "1".

So the 5 of Spades makes \$dealt = "000010000000000 ..."

I want to choose another card so I use \$card2 = rand(1,52); and then need to
check if it has already been dealt.  If the position in \$dealt is a 1 then I
need to repeat the random number code until it is a 0 when I can update it
to a 1 and move on ...

Or is there a better way of doing this sort of thing ??

Help, as always, appreciated.

Paul.

## Re: Random Number Question

"Paul C-T" wrote:
> Hi,
>
> Is there a way to repeat a set of code until a certain if statement
is
> satisfied.  If it is then to exit the loop and if not repeat the
code?
>
> Say I want to write a card game application in PHP and I want to
chose
> a
> card from the deck \$card1 = rand(1,52); gets me my first card.
>
> I need to record which card has been dealt so I have a variable
\$dealt
> which
> starts out as a string of 52 zeros.  I update the position of
\$card1
> in the
> \$dealt string to "1".
>
> So the 5 of Spades makes \$dealt = "000010000000000 ..."
>
> I want to choose another card so I use \$card2 = rand(1,52); and
then
> need to
> check if it has already been dealt.  If the position in \$dealt is a
1
> then I
> need to repeat the random number code until it is a 0 when I can
> update it
> to a 1 and move on ...
>
> Or is there a better way of doing this sort of thing ??
>
> Help, as always, appreciated.
>
> Paul.

Paul, Yes, can be done.  You make an array of all the cards (n cards).
You pick a random number between 1 and n.  When a number is picked,
you simply take the card out of the array, so now you have one less
card in the array. So now you pick from 1 to (n-1) and so on.

You can use php’s rand function.

Also you can unset any item in the array.  The problem would be that
the indecis would not reorganize, and e.g. when you remove item a,
item (a+1) would still have the same index.

So even though there is probably a better solution (anyone?) you can
do:
\$my_array = array_merge( array(), \$my_array);
which reorganizes the index, and you can continue iterating.

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## Re: Random Number Question

This is generally called a while loop.  Depending on your exact
wording, you may need to negate the condition.

This can take a LOT of time especially when you get to the point
where you have only 2 cards left in the deck, and it gets even worse
when you use multiple "standard" 52-card decks.

Random shuffling is usually done like this:

Make an array with indexes 0 .. N-1 for N cards.  Populate the array.
Select a card by selecting a random integer between 0 and N-1 inclusive.
This is the index of your chosen card.  Now swap that card in the
array with card N-1, then decrement N.  Repeat to draw more cards until
you don't need any more, or you run out of cards (N == 0).

Gordon L. Burditt

## Re: Random Number Question

Burditt contained the following:

shuffle() and array_rand() also seem useful.
--
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## Re: Random Number Question

Shuffle() is the right one to use in this case. Just shuttle the array then
pop the items off one at a time with array_pop(). That mimicks exactly what
happens when you're dealing cards. Array_rand() wouldn't work since it
doesn't prevent the same item being picked again.

## Re: Random Number Question

Thanks for the advice ... seems like I need to do a bit more flowcharting of
the problem ... and investigate some new PHP function.

:-)

Much obliged.

Paul.

## Re: Random Number Question

Paul C-T wrote:

Yw. Some nice background on real-life issues for poker-sites (applies to any
similar gametype of course)
Quite a good explanation:

## Re: Random Number Question

Paul C-T wrote:

Ah, finally someone doing something I know a bit about, lol!
Paul, there are some things you may want to consider. First of all,
shuffling can be done easy using shuffle (), which auto-shuffles your array
of cards. Then just deal from the top or bottom. There is also the Mersenne
Twister Random function (quicker) you can use. Check php manual for that.
For home use the shuffle () function is very comfortable in use.

But there is a catch when dealing and shuffling cards. A deck of cards can
be stacked in 2^52 ways. The default randomizer function is NOT capable of
doing all these permutations, simply because its seed and reach is too low.
So unless you are doing this just for fun, beware your randomly shuffled
deck is not exactly like having a real deck of cards. Also be careful with
the predictability. Cardplaying sites use all kinds of true random input
(mouse movement etc) from their users to seed the randomizers.

There are better/faster randomizers available online, including ones hat
have a range broader then 2^52. Not sure if they are available for PHP btw,
but you could always use an external randomizer if need be using exec()
Need more, just holler!
Pjotr

## Re: Random Number Question

array
Mersenne
low.

Errr, isn't that as good as casinos asking their patrons to shuffle the
cards?

Modern CPUs have hardware random number generator built-in so it seems a
better idea to hit /dev/random when you need a truely random number.

## Re: Random Number Question

Chung Leong wrote:

True random generator in a computer ? Me thinks that's a bit of an
impossibility, unless it takes in analog signals from somewhere.
And the problem is the seeding of the (pseudo) randomizer plus the fact most
randomizers don't cover the full range to 2^52.

## Re: Random Number Question

I've found network traffic dumps to be more than random enough to seed the
randomizer ;)

--
Tim Van Wassenhove <http://home.mysth.be/~timvw

## Re: Random Number Question

wrote:

Hardware RNG's, at least on Intel chips, use "thermal noise" to generate their
values, making them pretty reasonably random. They're certainly not
pseudo-random based off an algorithm, they're based on a randomly fluctuating
physical property.

--
http://www.andyh.co.uk          / http://www.andyhsoftware.co.uk/space

## Re: Random Number Question

Andy Hassall wrote:

That's cool! (or hot). Indeed, if analog data is the randomizer source, true
randomness can be had. Any idea on the range of this randomizer ? Or links
to good in depth articles ? Thanks! Very happy with this, if I can use that
I can throw the soundcardnoise/input based randomizer out!
Pjotr

## Re: Random Number Question

Also, I didn't see this mentioned...

rather than doing rand(1,52) each time through the loop and subsequent
multiple attempts to "draw" a random number that hasn't already been
picked, why not do a "real world" approach and only pick a random num
from what is avail?

have a \$current_deck array or \$cards_left array..
after you "draw" a card:  unset(\$current_deck[card_x]);

you can get a random card from this deck with
\$rand_card  = array_rand(\$current_deck);

complete psudo code:

while ( sizeof(\$current_deck) > 0 )
{
// this code is executed exactly 52 times
\$rand_card  = array_rand(\$current_deck);
unset(\$current_deck[\$rand_card]);
}

## Re: Random Number Question

I can share with you the code i use for my random routine (isnt perfect but
works) ;)

here it goes.....

// FUNCTION TO TRY AND MAKE A BETTER RANDOM NUMBER
function make_seed() {
list(\$usec, \$sec) = explode(' ', microtime());
return (float) \$sec + ((float) \$usec * 100000);
}

\$solucao = 0
\$options = 1
srand((double)microtime()*intval(rand(1,1000000)));
\$respostas[1] = rand (1,52);
\$solucao++;
while (\$solucao <= \$options)
{
srand(make_seed());
\$pergunta = rand(1, 52);
if (!in_array(\$pergunta, \$respostas))
{
\$respostas[\$solucao] = \$pergunta;
\$solucao++;
}
}

***************************************************
explanation:
***************************************************
you say how many cards (\$options) you want to add to the already picked
cards in the example 1

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