[QUIZ] Dice Roller (#61)

[Ruby Quiz]

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by Matthew D Moss

Time to release your inner nerd.

The task for this Ruby Quiz 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').

 

[Gregory Seidman]

]
} [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.]

I would appreciate the full BNF, please.

--Greg

 

[Jacob Fugal]

Or, for something more complicated:

31

I assume integer arithmetic? So if, for example, a 3 comes up on your
d4, 16/d4 would be 5?

Jacob Fugal

 

[J. Ryan Sobol]

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Please don't take this the wrong way, but I've never played D&D.
Would someone mind explaining the math that went into the command
below to generate it's result?

~ ryan ~

Time to release your inner nerd.

The task for this Ruby Quiz is to write a dice roller...

72 64 113 33 78 82

--Apple-Mail-15--645237476--

 

[Austin Ziegler]

Please don't take this the wrong way, but I've never played D&D.
Would someone mind explaining the math that went into the command
below to generate it's result?

I suspect user error.

The correct answer will always be between 3 and 18 for 3d6.

-austin

 

[James Edward Gray II]

Please don't take this the wrong way, but I've never played D&D.
Would someone mind explaining the math that went into the command
below to generate it's result?

~ ryan ~

Hmm, that example looks wrong now that you mention it. It should be
6 numbers between 3 and 18 (the roll of 3 six-sided dice).

James Edward Gray II

 

[Matthew Moss]

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Sticking with typical integer division (ie, round-down) is fine.

If you wanted to extend the syntax to support round-up division (using '\'
perhaps) or other options, feel free. Extra credit.

A lot of extra credit if you add syntax to support some RPGs/home rules
where you might want 3d6, but you'll actually roll 4d6 and toss the lowest.

I assume integer arithmetic? So if, for example, a 3 comes up on your
d4, 16/d4 would be 5?

Jacob Fugal

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[Matthew Moss]

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Ha ha... Must have copied the wrong line when writing up the quiz
description.

That should look like this:

roll.rb "3d6" 6

18 18 18 18 18 18

=3D)

Please don't take this the wrong way, but I've never played D&D.
Would someone mind explaining the math that went into the command
below to generate it's result?

~ ryan ~

------=_Part_10191_15694013.1136576379698--

 

[Matthew D Moss]

I'm not sure whether this made it to the list first time I sent it, or
is just delayed. Here it is again in case folks missed it...

I would appreciate the full BNF, please.

Okay, this is what I've done in my current version that takes care of
basic precedence and associativity.

INTEGER = /[1-9][0-9]*/

expr: fact
| expr '+' fact
| expr '-' fact

fact: term
| fact '*' term
| fact '/' term

term: unit
| [term] 'd' dice

dice: '%'
| unit

unit: '(' expr ')'
| INTEGER


Actually, this is slightly different than my current version, which
after reexamining to extract this BNF, I found a minor error (in
handling of the term rules and handling of the optional arg). My own
code has a morphed version of this BNF in order to code up a recursive
descent parser, but this BNF shows one way to handle the
precedence/association rules.

 

[Will Shattuck]

Ha ha... Must have copied the wrong line when writing up the quiz
description.

That should look like this:

18 18 18 18 18 18

Actually 3d6 means roll a 6 sided die 3 times so you would have a result of=
3-18

so this:

roll.rb "3d6" 6

Would actully be: (RND =3D Random)

RND(3-18) RND(3-18) RND(3-18) RND(3-18) RND(3-18) RND(3-18)

Below is 3d6 from the DnD Dice Roller on Wizards.com. The +0 would be
a modifier from depending if it was an attack roll or a defense roll.=20
For our purposes you would remove the +0

Roll(3d6)+0:
1,6,6,+0
Total:13

DnD Dice Roller:
http://www.wizards.com/default.asp?x=3Ddnd/dnd/20040517a


Will

 

[Austin Ziegler]

Ha ha... Must have copied the wrong line when writing up the quiz
description.

That should look like this:

18 18 18 18 18 18

Just don't tell me that the first one is 18/00.

-austin

 

[James Edward Gray II]

Just don't tell me that the first one is 18/00.

<dies laughing> They all were, of course.

James Edward Gray II

 

[Matthew Moss]

Actually 3d6 means roll a 6 sided die 3 times so you would have a result =

of 3-18

Actually, you're right, but actually my post was a half-joke. The
munchkin players seem to roll 18's every time.

 

[J. Ryan Sobol]

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I guess that must be a D&D inside half-joke because I'm totally
confused.

Don't worry about explaining it as I just needed to know what that
command, roll.rb "3d6" 6, did.

~ ryan ~

18 18 18 18 18 18

Just don't tell me that the first one is 18/00.

<dies laughing> They all were, of course.

Actually, you're right, but actually my post was a half-joke. The
munchkin players seem to roll 18's every time.

--Apple-Mail-17--642119500--

 

[James Edward Gray II]

I guess that must be a D&D inside half-joke because I'm totally
confused.

18 was the best stat a starting character could have. If you got
one, they let you roll d% and put it after the slash (the higher the
better). 00 == 100. So characters with 18/00 had some damn lucky
die rolls.

James Edward Gray II

 

[Austin Ziegler]

18 was the best stat a starting character could have. If you got
one, they let you roll d% and put it after the slash (the higher the
better). 00 =3D=3D 100. So characters with 18/00 had some damn lucky
die rolls.

In most versions of D&D/AD&D, this was also limited to Strength attributes =
only.

This may have changed recently.

-austin

 

[James Edward Gray II]

In most versions of D&D/AD&D, this was also limited to Strength
attributes only.

This may have changed recently.

Ah, yeah, you're right. It's been too long.

Actually, I believe 3rd Edition and up did away with the extra
percentile roll altogether.

James Edward Gray II

 

[Matthew Moss]

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I would appreciate the full BNF, please.

Okay, this is what I've done in my current version that takes care of basic
precedence and associativity.

INTEGER =3D /[1-9][0-9]*/

expr: fact
| expr '+' fact
| expr '-' fact

fact: term
| fact '*' term
| fact '/' term

term: unit
| [term] 'd' dice

dice: '%'
| unit

unit: '(' expr ')'
| INTEGER


Actually, this is slightly different than my current version, which after
reexamining to extract this BNF, I found a minor error (in handling of the
term rules and handling of the optional arg). My own code has a morphed
version of this BNF in order to code up a recursive descent parser, but thi=
s
BNF shows one way to handle the precedence/association rules.

------=_Part_10155_29622363.1136575984791--

 

[Will Shattuck]

forgive my ignorance... BNF?

w

I would appreciate the full BNF, please.

Okay, this is what I've done in my current version that takes care of bas= ic
precedence and associativity.

INTEGER =3D /[1-9][0-9]*/

expr: fact
| expr '+' fact
| expr '-' fact

fact: term
| fact '*' term
| fact '/' term

term: unit
| [term] 'd' dice

dice: '%'
| unit

unit: '(' expr ')'
| INTEGER


Actually, this is slightly different than my current version, which after
reexamining to extract this BNF, I found a minor error (in handling of th= e
term rules and handling of the optional arg). My own code has a morphed
version of this BNF in order to code up a recursive descent parser, but t= his
BNF shows one way to handle the precedence/association rules.

 

[Bill Kelly]

forgive my ignorance... BNF?

http://www.garshol.priv.no/download/text/bnf.html
http://en.wikipedia.org/wiki/Backus-Naur_form


HTH,

Bill