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### Re^2: Is there a better way to generate unique set of random numbers ?

by JavaFan (Canon)
 on Jul 28, 2011 at 10:02 UTC ( #917237=note: print w/replies, xml ) Need Help??

If you need to generate many more than 10 distinct random numbers, this solution will be faster because the hash lookup works in constant time, whereas your solution has to iterate over the whole array for each number. For 10 there won't be a big difference.
But do note that the only reason your algorithm guarantees to terminate is because rand uses a pseudo random number generator. Were rand to be truly random, neither algorithm could guarantee to be finished before Perl 6 dominates the world.

Here's an algorithm that does terminate:

```use List::Util 'shuffle';
my @set = (shuffle(0..1184))[0..9];

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Re^3: Is there a better way to generate unique set of random numbers ?
by moritz (Cardinal) on Jul 28, 2011 at 10:09 UTC
Were rand to be truly random, neither algorithm could guarantee to be finished before Perl 6 dominates the world.

Though if you do the math and calculate the probability that either algorithm does not terminate with the given parameters in, let's say, 2 seconds on a modern machine, you'll probably find that the chance of computation errors in the CPU caused by cosmic rays is much higher.

Doing the strict theory only makes sense if the actual machine corresponds to the machine model that the theory assumes.

```use List::Util;

sub unique_random_list {
my (\$from, \$to, \$count) = @_;

\$count = List::Util::min (\$count, \$to - \$from);

my @result;
my @queue = [ \$from, \$to - \$from, \$count ];

while (my \$job = shift @queue) {
my (\$from, \$length, \$count) = @\$job;

if (\$count == \$length) {
push @result, \$from .. \$from + \$length;
} elsif (\$count == 1) {
push @result, \$from + int rand \$length;
} else {
my \$split_length = int (\$length / 2);
my \$split_count  = List::Util::min (int rand \$count, \$spli
+t_length);

my \$pad_length = \$length - \$split_length;
my \$pad_count = \$count - \$split_count;

unshift @queue, grep \$_->[2],
[ \$from, \$split_length, \$split_count ],
[ \$from + \$split_length, \$length - \$split_length, \$count
+ - \$split_count ];
}
}

@result;
}

my @list = unique_random_list (10, 100, 30);
Doing the strict theory only makes sense if the actual machine corresponds to the machine model that the theory assumes.
Sure, but what's the fun of that? That makes all algorithms either O(1) (aka, "it terminates") or they loop and never terminate.

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