What is Wrong With This Script!?

Falcata · 2008-06-25 12:13:15

I've been trying to write a script to calculate the odds of getting a particular number when rolling a pair of 10-sided dice and adding them together. The ruby script I've been trying to write simply does not work. I don't know if I'm overlooking something or stumbled into a bug with the ruby interpreter. Could someone take a look at it and tell me if anything's wrong?

Here's the offending section of the script:

$ary = Array.new(20, 0)
$x, $y, $z = 0, 0, 0

10.times do
    $x += 1
    10.times do
        $y += 1
        $z = $x + $y
        $ary[$z] += 1    #This is the line causing problems
    end
end

And here's the error message I'm getting:

Dice.rb:9: undefined method `+' for nil:NilClass (NoMethodError)
        from Dice.rb:6:in `times'
        from Dice.rb:6
        from Dice.rb:4:in `times'
        from Dice.rb:4

sniffles · 2008-06-25 13:40:45

I don't /speak/ ruby, but looking at your code it seems that `y` will take all values between 1 and 100 thus making z take all values between 2 and 110. This is probably problematic since it seems to me your array only has 20 elements.

Last edited by sniffles (2008-06-25 13:46:31)

xaph · 2008-06-25 13:51:30

$ary = Array.new(20, 0)
$x, $y, $z = 0, 0, 0
10.times do
    $x += 1
    $y += 1
    $z = $x + $y
    $ary[$z] += 1    #This is the line causing problems
end

try this

Jessehk · 2008-06-25 13:55:48

Is there a particular reason you're using global variables in Ruby (prefixed with '$')? Variable names with no prefix are local to whatever scope they are in.

Last edited by Jessehk (2008-06-25 13:56:04)

Falcata · 2008-06-25 13:59:36

sniffles wrote:

I don't /speak/ ruby, but looking at your code it seems that `y` will take all values between 1 and 100 thus making z take all values between 2 and 110. This is probably problematic since it seems to me your array only has 20 elements.

Now I see the problem. I had forgotten to reset `y` to 0 at the end of each iteration of `x`. Honestly, I should have used a for loop, and I'm not sure why.

And the global variables were just being used because I wasn't sure.

sniffles · 2008-06-25 14:09:04

BTW, is this a ruby task or a math task? If it's a math task, it's not so difficult as to need ruby.

dtw · 2008-06-25 14:12:02

You're also "wasting" 2 elements in your array as you will never get z = 0 or 1. If these is an academic exercise i.e. you are doing it for practice not for purpose, you might consider creating a hash instead of an array with keys 2 through 20 to refer to each result.

Falcata · 2008-06-25 14:15:29

Well, I was just being lazy, because I didn't want to do math on all 100 combinations. Shortly after my initial post I opened up OpenOffice.org and used a table to figure it out. Not much math was needed, because there was a very noticable linear pattern (line 1 was 2, 3, 4, 5, 6, 7, 8, 9, 10, 11; line 2 was 3, 4, 5, 6, 7, 8, 9, 10, 11, 12; and so on) The odds were also had a very linear pattern:

 2:  1%
 3:  2%
 4:  3%
 5:  4%
 6:  5%
 7:  6%
 8:  7%
 9:  8%
10:  9%
11:  10%
12:  9%
13:  8%
14:  7%
15:  6%
16:  5%
17:  4%
18:  3%
19:  2%
20:  1%

Falcata · 2008-06-25 16:38:17

dtw wrote:

You're also "wasting" 2 elements in your array as you will never get z = 0 or 1. If these is an academic exercise i.e. you are doing it for practice not for purpose, you might consider creating a hash instead of an array with keys 2 through 20 to refer to each result.

I was doing it for a purpose, but wasted elements are not a concern for me since this was just a script I would be using only once. I'm in the process of designing a tabletop RPG. One of the mechanics in the game is the use of 2d10, which are rolled for resolving actions. I needed to know the odds, as they would affect the game mechanics.

sniffles · 2008-06-25 17:29:36

Note: you don't have "100 cases". You have just 19, because you are only ref. to the sum, not to individual dice numbers. So it's rather:

 2:  1/19 * 100 %
 3:  2/19 * 100 %
 4:  3 / 19 * 100 %
etc.

There are 19 "possible cases" (outcomes): mainly the sum is one of 2, 3, ..., 20. The probability to get the sum "2" is 1 / 19 (one favorable case: 1-1 on the dices); the probability to get the sum "3" is 2 / 19 (two favorable cases: 1-2 or 2-1 on the dices); etc.

Last edited by sniffles (2008-06-25 17:31:11)

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#1 2008-06-25 12:13:15

What is Wrong With This Script!?

#2 2008-06-25 13:40:45

Re: What is Wrong With This Script!?

#3 2008-06-25 13:51:30

Re: What is Wrong With This Script!?

#4 2008-06-25 13:55:48

Re: What is Wrong With This Script!?

#5 2008-06-25 13:59:36

Re: What is Wrong With This Script!?

#6 2008-06-25 14:09:04

Re: What is Wrong With This Script!?

#7 2008-06-25 14:12:02

Re: What is Wrong With This Script!?

#8 2008-06-25 14:15:29

Re: What is Wrong With This Script!?

#9 2008-06-25 16:38:17

Re: What is Wrong With This Script!?

#10 2008-06-25 17:29:36

Re: What is Wrong With This Script!?

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