---
title: Ruby Math with Ranges
date: 2012-04-30
techne: :done
episteme: :believed
---

You're playing some D&D game, like you always do, and you find two weapons, the Sword of Burnination, 1d8+3 and 1d6 fire damage, and the [Axe of Castigation][Lecher Bitch], 2d6+3. And you're tired of doing this math in your head.

So you write a Ruby gem to do it for you.

First, you write a [simple gem][range_math] to do math with ranges. You wanted this since you read [Up and Down the Ladder of Abstraction][]. Now this works:

~~~
#!ruby
(1..6) * 3      # => (3..18)
(-5..3) + 7     # => (2..10)
(1..3) + (2..4) # => (3..7)
(4..12) / 2     # => (2.0..6.0)
~~~

You can do quick math in your irb sessions with ranges instead of numbers now. You're happy.

You do a `gem install range_math` on any system, and put this in your `.zshrc`:

~~~
#!sh
alias i="irb -rrange_math"
~~~

Then you write a [simple script][avg_dmg] to convert dice rolls into ranges, put it in your PATH and alias it like this:

~~~
#!sh
alias avg="noglob average_damage.rb"
~~~

Now this works:

~~~
#!sh
$ avg 1d8+3 + 1d6
range: 5..17, average: 11.0
~~~

Suffering ends.