On Sat, Feb 28, 2009 at 12:22, <r...@goto10.org> wrote:
> Rob Dixon said :
>> r...@goto10.org wrote:
>> > hi,
>> >
>> > i am finding something couter intuative about randomness and search for a
>> > solution.
>> >
>> > i have a bit of code that randomly selects a number from an array and then
>> > adds
>> > it to the previous number. I have two positive numbers and their
>> > negitives qw(1 2 -1 -2)
>> >
>> > i expected the below code to hover around 0 up and down a bit in each
>> > direction
>> > - but it very quickly shoots of into big numbers.
>> >
>> > i would like to have a way to contain the output range to between -45 and
>> > 45 for example.
>> >
>> > anyone have a clue how to do this?
>> >
>> > thanks
>> >
>> > rob
>> >
>> > #!/usr/bin/perl
>> > use strict;
>> > use warnings;
>> >
>> > my @intervalset = qw(1 2 -1 -2);
>> >
>> > my $current = 0;
>> >
>> > my $t = 1;
>> >
>> > while ($t==1){
>> >
>> > my $rndi = $intervalset[rand @intervalset];
>> >
>> > $current = ($rndi+$current);
>> >
>> > print $current,"\n";
>> >
>> > }
>>
>> You are falling for the fabled 'law of averages' that tells people they are
>> more
>> likely to win the lottery after losing many times. The sum will not hover
>> around
>> its starting value as each random number is independent. If your total has
>> reached, say, 100 then you are still no more likely to roll negative numbers
>> than positive ones.
>>
>> The expected, or mean value of the rolls is zero, but the variance of the
>> total
>> increases with the number of rolls and so you will see it stray further and
>> further from zero as your program continues.
>
> so i guess what i would need to do would be:
> after each positive number weight, the probability in favour of the next
> number
> being negative and vica versa. then maybe things wont stray so far from zero.
>
>
>> Please explain what numbers you are trying to generate. If you want an even
>> distribution of integers from -45 to 45 then simply use
>>
>> my $num = int(rand 91) - 45, "\n";
>
> basically the numbers i am generating are musical intervals which sum together
> to give a string of pitches (melody) - things straying to far from 0 which in
> this case could be the middle key of a piano means that the melody soon runs
> outside the range of the instrument unless i intervene. at the moment i
> restrict
> the movement to one octave using modulus % 12 - but this is not the solution
> i need.
>
> i think i need to figure out how to change the probablities on each iteration.
>
>
>> Also, please indent your code inside a block to make it more legible.
>
> sorry, yes i need to learn the right way of doing this - will consult my
> o'reilly perl books.
>
>> And the
>> usual way of writing an indefinite loop is
>>
>> while (1) {
>> :
>> }
>>
> ah yes - i have more stuff in my code - i pulled out this "bleeding chunk" to
> try and keep my mail clear - nevertheless its always good to know these things
> as i'm a real newb and most of my code is still cobbled together in a
> frankenstein like manner :)
>
>
> many thanks,
>
> rob c
>
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>
This seems close to what you want, but it needs work to make it take
steps larger than 1:
#!/usr/bin/perl
use strict;
use warnings;
# 12345678901
# *
# 5/11 chance of moving right or left
# 1/11 chance of staying still
#
# 12345678901
# *
# 4/11 chance of moving left
# 6/11 change of moving right
# 1/11 chance of staying still
#
# and so on
sub random_move {
my ($pos, $max) = @_;
my $left = ($pos - 1)/$max;
my $nomove = 1/$max;
my $rand = rand;
return $pos - 1 if $rand < $left;
return $pos if $rand < $left + $nomove;
return $pos + 1; #must be move right;
}
my $pos = 20;
my $max = 41;
while (1) {
print " " x ($pos - 1), "*\n";
$pos = random_move($pos, $max);
select undef, undef, undef, .25;
}
--
Chas. Owens
wonkden.net
The most important skill a programmer can have is the ability to read.
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