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Generating unique random numbers

Posted on 1998-01-31
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Last Modified: 2008-02-01
The following code will generate nine random numbers, but how can I change the code so that it will generate nine unique random numbers?

srand (time|$$);
$square=1;
while ($square != 10) {
$random=int(rand(50))+1;
$word{$square}=$random;
$square++
}
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Question by:bazzle
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4 Comments
 
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icd earned 400 total points
ID: 1209713
First of all any computer program cannot generate true random numbers, only pseudo random. That is they are deterministic if you know the algorithm but they have certain properties of truly random numbers.

To solve your problem, there are two ways.

1. You keep a record of all the previous random numbers and compare each new number against the list. This is a fairly trivial approach but it might not be practical for very large sequencies of numbers and it will get slower as you generate more and more numbers. If this is OK then I will post a solution for you.

2. Use another random number generator that has the properties you want. I will post this answer in a following comment (when I have had time to look it up!)

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LVL 5

Expert Comment

by:icd
ID: 1209714
OK. Here is an alternative that will always give a pseudo random number sequence, without repeats (upto a maximum of 50 numbers!).

$random = (time|$$);
$square=1;
while ($square != 10) {
$random = ($random * 213 + 11) % 50;
$word{$square}=$random;
$square++
}

The sequence will give you ten numbers, guaranteed to be different and showing pseudo random characteristics. Given the same seed value the generator will always generate the same sequence, but that  is true of any computed random number sequence.

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LVL 84

Expert Comment

by:ozo
ID: 1209715
(time+$$) is more likely to give a unique seed than (time|$$)

 $random = ($random * 213 + 11) % 50;
Might as well be
 $random = ($random * 13 + 11) % 50;
And there's correlation in the way different batches of 10 overlap each other.
It's also weak in that there are only 50 distinct seeds.

One way to implement suggestion 1. could be:
 1 while( $previous{$random=int(rand(50))+1} )
 $previous{$random}=1;

But for large sequences, the approach from the FAQ may be better:
 srand;
 @new = ();
 @old = 1 .. 50;
 for( @old ){
   my $r = rand @new+1;
   push(@new,$new[$r]);
   $new[$r] = $_;
 }
#or
 @word=();
 @old = 1 .. 50;
 while( @word < 10 ){
  my $r = rand @old;
  push(@word,$old[$r]);
  $old[$r] = $old[-1];
  pop(@old);
 }
(since you're indexing %word with integers in sequence,
you might as well use an @word array instead of a hash)

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LVL 84

Expert Comment

by:ozo
ID: 1209716
Another problem with
 $random = ($random * 213 + 11) % 50;
is that with a seed of 22 (or 47) it generates only 2 unique values.
(there are also 8 seeds which generate only 4 unique values)
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