Rolling a Die

Rolling a fair die is in some sense a generalization of a flipping a coin.  The regular six-sided die allows for many interesting questions:

  • What is the probability of rolling a 1?
  • What is the probability of rolling an even number?
  • If you roll a die 10 times, how many times do you expect to roll a 2?
  • If you roll a die 1000 times, how many times do you expect to roll a 1 or a 6?

There exist die with fewer as well as more than six faces. A 10-sided die shows the numbers from 1 to 10. We can ask similar questions:

  • What is the probability of rolling an 8, a 9, or a 10?
  • If you roll the die 500 times, how many times do you expect to get a 5?

In many games we don’t roll one die, but two or more. Assume we roll 2 die.

  • What is the probability that we roll a sum of 4?
  • If we roll two dice 100 times, how many times do we expect to get a pair?

The probabilities of some of the above questions may not be straightforward to determine mathematically. However, once we know how to simulate throwing a die, we answer these questions experimentally.

Programming Guidelines

For this assignment, you will simulate rolling a die. Before starting to program, think about the following:

  • How can you simulate rolling a regular six-sided die? A regular 10-sided die? Think about this in terms of flipping a coin; the randomness can be achieved the same way, but now there are more possible results.
  • How will you keep track of the results?
  • How could you model a weighted die that rolls a given number with a higher probability? (for more advanced students to consider)

Program

Start with your program for the Coin Toss Simulation and adjust it to make it about Rolling a 6-sided Die. Note the following changes:

  • Rolling a 6-sided die requires choosing a random number from 1 to 6
  • Keep track of the results either with six variables representing the six possible outcomes OR, more efficiently with a list with six elements

Adjusting your program to model a die with, for example, 10 sides, requires minimal changes:

  • Rolling now requires a random number from 1 to 10
  • Keeping track of the results will require either more variables OR a larger list.

NOTE: keeping track of results using a list will require significantly less code, as the result of the die-roll can be used to directly index the list. EX: Let result be the value of the roll, then updating the results list is simply: list[result -1] ++.

Sample Programs

Program 1 – Simulation of Die Throwing (using a list)

#Simulation of Die Throwing (using a list)
import random

test_data = [0, 0, 0, 0, 0, 0] #this will keep track of the results
n = 1000 # this can be easily changed for different sample sizes


for i in range(n):
  result = random.randint(1, 6)
  test_data[result - 1] = test_data[result - 1] + 1

for i in range(0, 6):
  print "Number of ", i+1, "'s: ", test_data[i]

#To change the number of sides on the die, change the size of
#test_data and the range for the random number.

Program 2 – Simulation of Tossing a Weighted Die

#Simulation of Tossing a Weighted Die

import random

n = 1000
test_data = [0, 0, 0, 0, 0, 0]

results = [1, 1, 1, 1, 1, 2, 3, 4, 5, 6] #die weighted to roll a 1 half the time

for i in range(n):
  index = random.randint(0, 9)
  result = results[index]
  test_data[result - 1] = test_data[result - 1] + 1

for i in range(0, 6):
  print "Number of ", i+1, "'s: ", test_data[i]

Program 3 – Simulation of Tossing a Die to find Average result

#Simulation of Tossing a Die to find Average result
#Uses a visual representation for the results

import matplotlib.pyplot as plt
import numpy as np
import random

def throw_die(data_list, times):
    total = 0
    for i in range(times):
        num = random.randint(1,6)
        total = total + num
        data_list[num-1] = data_list[num-1] + 1
    average = total * 1.0 / times
    return average  
    

test_data = [0, 0, 0, 0, 0, 0] #use a list to keep the times each number appears
times = 1000 # throw a die n times
average = throw_die(test_data,times)
print 'The average is',average

#Plot the result
ind = np.arange(6)  # the x locations for the groups
width = 0.5      # the width of the bars
fig, ax = plt.subplots()
result = ax.bar(ind+width/2, test_data, width,color='r')
ax.set_ylabel('Counts')
ax.set_title('Throw a Die')
ax.set_xticks(ind+width)
ax.set_xticklabels( ('1', '2', '3', '4', '5','6') )
plt.show()

Internet Resources

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