How can I print all the numbers from 1 to n in random order?

[vroom]

How can I print all the numbers from 1 to n in random order?

Originally posted as a Categorized Question.

[ChrisS]

I'd use the List::Util module.

use List::Util qw(shuffle);

my $n = 10;  # use $ARGV[0] or whatever to set $n

print join " ", shuffle(1 .. $n);

[btrott]

I don't see the original here, so I don't know if this has been suggested... but you could just stick the numbers in an array, then shuffle the array, then print them. Use the Fisher-Yates Shuffle, which avoids the splice quadratic algorithm bit.

    sub fisher_yates_shuffle {
        my $array = shift;
        my $i;
        for ($i = @$array; --$i; ) {
            my $j = int rand ($i+1);
            next if $i == $j;
            @$array[$i,$j] = @$array[$j,$i];
        }
    }

    my $N = 50;
    my @array = (1..$N);
    fisher_yates_shuffle( \@array );    # permutes @array in place

    print "@array";

From perlfaq4, How do I shuffle an array randomly?.

[chromatic]

Hmm, see splice for why that doesn't work. Here's the fix: print (splice @a, rand @a, 1);

[lanval]

$ cat numbers.pl


#!/usr/bin/perl

sub RandomiseArray {
        my (%b, $c);
        map { do { $c = rand } until(!exists $b{$c}); $b{$c} = $_ } @_
+;
        return values(%b);
}

my @a = RandomiseArray(1..10);
print "@a\n";

$ ./numbers.pl
8 5 1 3 10 2 6 4 9 7
$ ./numbers.pl
4 2 3 9 5 8 6 1 7 10
$ ./numbers.pl
8 10 7 6 5 9 2 3 4 1

Yeah?

[matija]

NO. (For an explanation, time it with RandomiseArray(1..1000) and RandomiseArray(1..100000) - it has exponential complexity in time)

Here is a better way to do it:

sub RandomiseArray {
  my @a=@_;
  my @b=();
  while (scalar @a) {
    push(@b,splice(@a,int(rand @a)));
  }
  return(@b);
}

As the array becomes longer, the probablility of your routine finding an unused space becomes smaller and smaller. With a 1..10 shuffle the probablility of finding the right element for the last space is one in ten - which means you will need 10 tries (on the average). Not too bad. But with a 1..100000 array, you will need 100_000 tries to fill in the last element. and 99_999 to find the next to last, and... you get the idea. It will take a LOOONG time.

Yeah, I could do it inplace (without the second copy of the array), but I'm going to save that for another time.

[ysth]

The above comment would be true if it did $c = int(rand @_), but it doesn't. It just uses the floating point value returned from rand as a hash key. There are unlikely to be any collisions, even shuffling 1000000 numbers. You can test this with:

perl -wle'undef $x{rand()} while keys(%x) == $x++; print $x'

which adds keys until it gets a collision. Be prepared to wait a very long time.