Preventing duplicate random numbers in small sample

[dalecosp]

Given this, or somthing like it:

for ($n=0; $n<6; $n++) {
srand((double) microtime() * 1000000);
$BookSelectNumber[$n]=rand(0, 65);
}

it seems as if I have a 1 in 11 chance of having
duplicate values within the $BookSelectNumber array.

Is there a way to prevent duplicate numbers in the
array, other than LOTS of comparison statements and
reselection of values....???

TIA,

dalecosp🙂

[Weedpacket]

I suppose you could use range() and shuffle()...

Or

$BookSelectNumber=array();
while(count($BookSelectNumber)<6 && !in_array($r=rand(0,65),$BookSelectNumber))
  $BookSelectNumber[]=$r;

Or am I just being too obfuscating for my own good, here?

[dalecosp]

Originally posted by Weedpacket
I suppose you could use range() and shuffle()...

Or

$BookSelectNumber=array();
while(count($BookSelectNumber)<6 && !in_array($r=rand(0,65),$BookSelectNumber))
  $BookSelectNumber[]=$r;

[/B]

in_array, indeed!! Wonderful!!!😃

Or am I just being too obfuscating for my own good, here?

Well, not necessarily! A direct paste into the script shows that this avoids duplicates---it also introduces an occasional null variable that I'm trying to track now... I'll probably get it sooner or later...

Anyway, many thanks, Weedpacket!!!!

P.S. I sincerely appreciate your help, and would loan you some hair if I could---assuming, of course, you even care, hehe..... :-)

[Weedpacket]

The problem is that if it does find the value of $r already in the array, it gives up trying to add any more - but we want it to try another value and keep going.

$BookSelectNumber = array();
while(count($BookSelectNumber)<6)
{	while(in_array($r=rand(0,65), $BookSelectNumber))
		/*empty loop*/;
	$BookSelectNumber[] = $r;
}
print_r($BookSelectNumber);

Warning: if the range of the random value is greater than the intended size of
the array then this loop will not terminate. The Right Thing would be to
either produce an undersized array, or complain about an error. So some
suitable argument-checking should be wrapped around this.

The empty while loop could be more conventionally written:

do
{	$r = rand(0,65);
}while(in_array($r,$BookSelectNumber));

[dalecosp]

Originally posted by Weedpacket
The problem is that if it does find the value of $r already in the array, it gives up trying to add any more - but we want it to try another value and keep going.

The Right Thing would be to either produce an undersized array, or complain about an error. So some suitable argument-checking should be wrapped around this.

The LORD desires that we do the Right Thing...

$BookSelectNumber=array();
while(count($BookSelectNumber)<6 && !in_array($s=rand(0,65),$BookSelectNumber))
        $BookSelectNumber[]=abs($s);

#########  IF WE CAN'T GET SIX RANDOM BOOKS (GET A DUPLICATE) WE START OVER.....

$Success=count($BookSelectNumber);
if ($Success !=6) { die("Sorry, I failed to draw six random books.  Press reload to try again, please."); }

I thank you most sincerely!!

dalecosp🙂

[ednark]

php already has an [man]array_rand[/man] function.... use it with rand() (basically weepackts suggestion)

it has some overhead (you are creating an array with range() just to pull numbers out :eek: )

<?php
/// gets 6 random items from an array: gets us 6 non-overlapping numbers from 0 through 65
$randoms_items = array_rand(range(0,65),6);
// joins 6 non-overlapping digits from 0 to 65 into one string
$random_string = join(array_rand(range(0,65),6),'');
// show our results
print "<pre>";
print_r($random_string);
print "\n";
print_r($randoms_items);
print "</pre>";
?>

also id like to point out for efficiency:

while(count($BookSelectNumber)<6 && !in_array($s=rand(0,65),$BookSelectNumber))

calling count() and in_array() each iteration is expensive.....
assuming that you never have a collision while asking for 6 items this will loop through ~7 items each iteration (42 total)... adding another ~5 items to loop through on each collision...

this is kinda expensive... for just getting 6 numbers

[dalecosp]

Originally posted by ednark
php already has an [man]array_rand[/man] function...

Thanks, I'll try that one also....

This is kinda expensive

...gee, and I thought php was open-source 🙂

might as well give weedpacket a break, as he said, "off the top of my head..." tee hee, (balding member)...

Thanks to both of you...
dalecosp

[ednark]

my post was just weedpackets original idea anyway just using a different function... he gets the props