Random generation number doubt

Question

Hi

Is there is any way to avoid duplication in random number generation .

I want to create a random number for a special purpose. But it's should be a unique value. I don't know how to avoid duplicate random number

ie, First i got the random number like 1892990070. i have created a folder named that random number(1892990070). My purpose is I will never get that number in future. I it so i have duplicate random number in my folder.

Answer 1

A random series of number can always have repeated numbers. You have to keep a record of which numbers are already used so you can regenerate the number if it's already used. Like this:

$used = array(); //Initialize the record array. This should only be done once.

//Do like this to draw a number:
do {
  $random = rand(0, 2000);
}while(in_array($random, $used));
$used[] = $random; //Save $random into to $used array

My example above will of course only work across a single page load. If it should be static across page loads you'll have to use either sessions (for a single user) or some sort of database (if it should be unique to all users), but the logic is the same.

Answer 2

This will generate a string with one occurence of each digit:

$randomcharacters = '0123456789';
$length = 5;
$newcharacters = str_shuffle($randomcharacters);
$randomstring = substr($newcharacters, 0, $length);

Answer 3

For what purpose are you generating the random number? If you are doing something that generates random "picks" of a finite set, like shuffling a deck of cards using a random-number function, then it's easiest to put the set into an array:

$set = array('one', 'two', 'three');

$random_set = array();

while (count($set)) {
    # generate a random index into $set
    $picked_idx = random(0, count($set) - 1);
    # copy the value out
    $random_set []= $set[$picked_idx];
    # remove the value from the original set
    array_splice($set, $picked_idx, 1);
}

If you are generating unique keys for things, you may need to hash them:

# hold onto the random values we've picked
$already_picked = array();

do {
    $new_pick = rand();
# loop until we know we don't have a value that's been picked before
} while (array_key_exists($new_pick, $already_picked));

$already_picked[$new_pick] = 1;

Answer 4

You can write a wrapper for mt_rand which remembers all the random number generated before.

function my_rand() {
        static $seen = array();
        do{                                                                                                                                                                                                    
                $rand = mt_rand();
        }while(isset($seen[$rand]));
        $seen[$rand] = 1;
        return $rand;
}

Answer 5

The ideas to remember previously generated numbers and create new ones is a useful general solution when duplicates are a problem.

But are you sure an eventual duplicate is really a problem? Consider rolling dice. Sometimes they repeat the same value, even in two sequential throws. No one considers that a problem.

If you have a controlled need for a choosing random number—say like shuffling a deck of cards—there are several approaches. (I see there are several recently posted answer to that.)

Another approach is to use the numbers 0, 1, 2, ..., n and modify them in some way, like a Gray Code encoding or exclusive ORing by a constant bit pattern.