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by Matthew D Moss
Time to release your inner nerd.
The task for this Ruby Q. is to write a dice roller. You should write
a
program that takes two arguments: a dice expression followed by the
number of
times to roll it (being optional, with a default of 1). So to calculate
those
stats for your AD&D character, you would do this:
> roll.rb "3d6" 6
72 64 113 33 78 82
Or, for something more complicated:
> roll.rb "(5d5-4)d(16/d4)+3"
31
[NOTE: You’ll usually want quotes around the dice expression to hide
parenthesis
from the shell, but the quotes are not part of the expression.]
The main code of roll.rb should look something like this:
d = Dice.new(ARGV[0])
(ARGV[1] || 1).to_i.times { print "#{d.roll} " }
The meat of this quiz is going to be parsing the dice expression (i.e.,
implementing Dice.new). Let’s first go over the grammar, which I present
in a
simplified BNF notation with some notes:
<expr> := <expr> + <expr>
| <expr> - <expr>
| <expr> * <expr>
| <expr> / <expr>
| ( <expr> )
| [<expr>] d <expr>
| integer
* Integers are positive; never zero, never negative.
* The "d" (dice) expression XdY rolls a Y-sided die (numbered
from 1 to Y) X times, accumulating the results. X is optional
and defaults to 1.
* All binary operators are left-associative.
* Operator precedence:
( ) highest
d
* /
+ - lowest
[NOTE: The BNF above is simplified here for clarity and space. If
requested, I
will make available the full BNF description I’ve used in my own
solution, which
incorporates the association and precedence rules.]
A few more things… Feel free to either craft this by hand or an
available
lexing/parsing library. Handling whitespace between integers and
operators is
nice. Some game systems use d100 quite often, and may abbreviate it as
“d%”
(but note that ‘%’ is only allowed immediately after a ‘d’).
