Skip to content

Instantly share code, notes, and snippets.

Last active Sep 16, 2021
What would you like to do?
A simple bare-bones dice probability calculator framework, compatible with both Python 2 and Python 3
def dice_roll(die, count = 1, select = None):
"""Generate all possible results of rolling `die` `count` times, sorting
the results (according to the order of the sides on the die) and selecting
the first `select` elements of it.
The yielded results are tuples of the form `(roll, prob)`, where `roll` is a
sorted tuple of `select` values and `prob` is the probability of the result.
The first argument can be either a custom die, i.e. a tuple of `(side, prob)`
pairs, where `prob` is the probability of rolling `side` on the die, or just
a simple integer, which will be passed to `make_simple_die`.
Keyword arguments:
die -- a custom die or an integer
count -- the number of dice to roll (default 1)
select -- the number of results to select (set equal to count if omitted)
# cannot select more dice than there are in the pool
if select is None or select > count:
select = count
# for convienience, allow simple dice to be given as plain numbers
die = make_simple_die(die) if isinstance(die, int) else tuple(die)
if len(die) == 1:
# base case: a one-sided die has only one possible result
yield ((die[0][0],) * select, die[0][1]**count)
elif len(die) > 1:
# split off the first side of the die, normalize the rest
side, p_side = die[0]
rest = tuple((side, prob / (1-p_side)) for side, prob in die[1:])
p_sum = 0 # probability of rolling this side less than select times
for i in range(0, select):
# probability of rolling this side exactly i times
p_i = binomial(count, i) * p_side**i * (1-p_side)**(count-i)
p_sum += p_i
# recursively generate combinations
for roll, p_roll in dice_roll(rest, count-i, select-i):
yield ((side,) * i + roll, p_i * p_roll)
# final case: all selected dice (and possibly more) roll this side
yield ((side,) * select, 1-p_sum)
_factorials = [1]
def binomial(n, k):
"""Helper function to efficiently compute the binomial coefficient."""
while len(_factorials) <= n:
_factorials.append(_factorials[-1] * len(_factorials))
return _factorials[n] / _factorials[k] / _factorials[n-k]
def make_simple_die(n):
"""Generate a simple n-sided die with sides listed in decreasing order."""
return tuple((i, 1.0/n) for i in range(n, 0, -1))
def explode(die, count=2):
"""Make an "exploding die" where the first (=highest) side is rerolled up to
count times.
die = make_simple_die(die) if isinstance(die, int) else tuple(die)
exploded = die
for i in range(count):
top, p_top = exploded[0]
exploded = tuple((side + top, prob * p_top) for side, prob in die) + exploded[1:]
return exploded
def sum_roll(die, count = 1, select = None, ascending=False):
"""Convenience function to sum the results of `dice_roll()`. Takes the same
parameters as `dice_roll()`, returns a list of `(sum, prob)` pairs sorted in
descending order by sum (and thus suitable for use as a new custom die). The
optional parameter `ascending=True` can be used to change the sort order.
from collections import defaultdict
summary = defaultdict(float)
for roll, prob in dice_roll(die, count, select):
summary[sum(roll)] += prob
return tuple(sorted(summary.items(), reverse = not ascending))
Copy link

posita commented Aug 18, 2021

Credit (see note with your name) where credit is due. I even have a function named after you. Let me know if you'd like me to add/adjust anything here.

Thanks for all the guidance and the great stuff! 😊

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment