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Laboratory 5 Building an Oracle

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Computer Science CS134
Laboratory 5
Building an Oracle
Objective. To  nish a simple module that generates text in an intelligent (?) manner.
This week we’ll complete a simple little module, oracle, that can be trained to generate \readable”
random text. The module makes use of a technique that  ngerprints a source text by keeping track of a
distribution of combinations of n letters, called n-grams.
The Concept. The central component to this lab is the module oracle. It has the ability to scan and
internalize a source text and then, later, it can generate random text that is remarkably similar to the
source.
Internally, the oracle module keeps track of all the n-grams that appear in a text. For example, the
11-character text
‘hello world’
contains the following nine 3-grams:
‘hel’ ‘ell’ ‘llo’ ‘lo ‘ ‘o w’ ‘ wo’ ‘wor’ ‘orl’ ‘rld’
If a text contains m characters, it is made up of (m−n+1) n-grams. For a very large text, of course, some
n-grams appear quite frequently, while others do not. The power of the oracle module is the development
of a distribution of n-grams that, in a sense, acts like a \ ngerprint” for the author of the text. If we are
given (n − 1) letters, we can use the distribution to make an informed guess as to how the author would
add a  nal character to complete an n-gram. When we iterate this process, it is possible to generate text
that takes on some of the characteristics of the prose  ngerprinted by the oracle.
In this week’s module de nition, we’ll keep track of the distribution of n-grams using a dictionary.
Each key of the dictionary is an (n − 1)-character string that is the pre x for one or more n-grams
encountered in the text. The value associated with the key is a string of all the characters that, when
individually appended to the key, form an n-gram from the original text. Each character in the value
represents one of the n-gram occurrences and the distribution of characters that appear in the value
re ects the distribution of n-grams that begin with the (n − 1)-letter key. For example, the distribution
dictionary for the ‘hello world’ 3-grams would be:
{‘he’: ‘l’,
‘el’: ‘l’,
‘ll’: ‘o’,
‘lo’: ‘ ‘,
‘o ‘: ‘w’,
‘ w’: ‘o’,
‘wo’: ‘r’,
‘or’: ‘l’,
‘rl’: ‘d’}
Code Review. Let’s download and take a look at the code provided.
Download the repository for this package in the usual manner (use your name instead of 22xyz3):
git clone https://evolene.cs.williams.edu/cs134-labs/22xyz3/lab05.git ~/cs134/lab05
This repository contains a single Python module oracle.py and the pre-processed text from Jane Austen’s
novel, Pride and Prejudice. Acquaint yourself with the text and observe Austen’s writing style.
As usual, we begin by carefully reading through the existing code. (We hope to take advantage of work
that has already been done!)
 Run some simple experiments with the function random.choice. This function takes a sequence (a
list, tuple, or string) and chooses one of the elements randomly and uniformly:
>>> from random import choice
>>> choice(‘hello’) # ‘h’ (20%), ‘e’ (20%), ‘l’ (40%), ‘o’ (20%)
‘h’
>>> choice([‘hello’, ‘world’]) # ‘hello’ (50%), ‘world’ (50%)
‘world’
As we see, by repeating values in the sequence, we can simulate any distribution we wish. This
module makes use of random.choice in a number of places, so you should be familiar with how it
works.
 Familiarize yourself with the islice(seq, n) iterator from the package itertools. This function
limits the number of values delivered by iterating across seq to n. Here are some examples:
>>> from itertools import islice
>>> list( islice(range(10), 3) )
[0, 1, 2]
>>> ‘-‘.join( islice(‘jello world’, 5) )
‘j-e-l-l-o’
>>> for line in islice( open(‘PrideAndPrejudice.txt’), 2):
… print(line.strip())

pride and prejudice a novel by jane austen it is a truth universally
acknowledged that a single man in possession of a good fortune must be in want
This is especially useful when you are working with in nite iterators.
 Now, let’s examine the oracle module, in oracle.py. All the code we write will be included as part
of this module.
When completed, we may use the functions of oracle interactively in the following manner:
from oracle import *
from itertools import islice # see above
# read a source text as a long string:
with open(‘PrideAndPrejudice.txt’) as source:
text = ‘ ‘.join([line for line in source])
# analyze the distribution of n-grams
fingerprint(text, n=3)
# generate 20 lines of random text with width <= 70
for line in islice(lines(width=70),20):
print(line)
The oracle’s fingerprint(text, n) method scans a string from beginning to end, keeping track of
the frequency of each combination of n characters. The module uses this distribution to  ngerprint
the string.
The lines() method generates new lines of text from the distribution seen during the scanning
process. The new lines, though random, can be expected to have the same  ngerprint as the original
text.
 Near the top of the module, observe the three global variables, text, n, and dist. These variables
hold the state of the oracle’s text generator:
? The int n. This is an integer that describes the size of the n-gram window used in scanning
the text. It should be 2 or greater, and is set by the n parameter to fingerprint.
? The text string. This is a copy of the text used to develop the textual  ngerprint. It is a
string of at least n characters given to the fingerprint method.
? The dist dictionary. This keeps track of the distribution of n-grams encountered during the
scan of text. If the n-gram window width is n, the keys are the  rst n-1 characters of
any window seen, and the value is a string that contains all the possible single character
completions encountered. Because these completions are not uniformly distributed, there may
be many copies of the most common completion letters, and very few copies of the least common
completions.
Read through the fingerprint(text, n=4) method. This method takes a string, text, and records
the occurrences of n character combinations. This one method determines the value of each of the
global state variables, text, n, and dist and ensures they’re left in a consistent state. Since the
state variables begin with an underscore (_, meaning \private”) users of this module should avoid
manipulating these values directly.
 The module contains three private functions. Since these begin with an underscore, they will not
appear in the documentation for the module. The intent is that these functions are only available
for use within the module’s other methods.
? The randomChar() function returns a character at random from the text. While every character
of the text has equal opportunity for being picked, notice that the distribution of characters
returned re ects the distribution of the characters within the text string, itself.
? The randomKey() function returns an n-1 character key, randomly selected from the dist
dictionary. This function, by the way, might be simpli ed. Think about how you might do
that. How would you test the functionality of randomKey?
? The randomCompletion(key) function, given key of the  rst n-1 characters of an n-gram,
returns a random character that, according to the distribution, typically follows it. If the key
has never been seen before, it simply returns a random character from the text. (How might
we get into a situation where a key has never been seen before?)
Required Tasks. This week’s work harnesses the above tools to generate in nite streams of characters,
words, and lines that have the same n-gram distribution as the  ngerprinted text. Here’s what needs to
be done:
1. Please write the generator, chars(). Because it’s a generator, it can be used as the domain of
iteration in a for loop. It begins by selecting a random key. It then, using an in nite while loop
it repeatedly (1) extends the key to form an n-gram, (2) yields a character, and then (3) drops a
character from the n-gram to form a new key.
Before tackling a text the length of Pride and Prejudice, you can test chars() in interactive Python
by creating a  ngerprint from some text and then using a loop similar to the following:
>>> from oracle import *
>>> from itertools import islice
>>> fingerprint(“Hey diddle diddle, the cat and the fiddle.”,n=3)
>>> for c in islice(chars(), 10):
… print(c)

h
e
c
a
t
a
n
d
>>>
The characters generated should resemble the initially scanned text. Here, for example, the key is,
at some point, ’he’ which is then extended to include a space, then the key becomes ’e ’, extended
to become ’e c’, etc.
2. Please write the generator, words(maxlen=20). Using the chars() generator in a loop like:
for c in chars():

it collects non-space characters into words. A word is yielded whenever (1) a space is encountered
(check with str’s isspace() method), or (2) it becomes maxlen characters long.
As you create your generator, test what words would be generated by di erent input texts. What
words would be printed from input text t?
>>> t = ‘yaddayadda’
>>> fingerprint(t,n=3)
>>> for word in islice(words(maxlen=10), 5):
… print(word)

What if t = ‘yin yang yin’?
3. Write a  nal generator, lines(width=80). This generator yields lines of words from words() that
come close to but do not exceed width. We’ve given you a start: a list, wordList, that collects
words used to form the line. If the generator is carefully constructed, it will produce lines that
read consistently from one line to the next; it never drops any words from the underlying words()
generator. You should think about how to limit word lengths; source texts that have no spaces (like
’yaddayadda’, above) must be handled reasonably.
If your oracle module supports the lines method, you should now be able to use the following code
to generate an in nite amount of text (use Control-c if you’re stuck in an in nite loop):
for line in lines():
print(line)
To limit the number of lines generated to, say, 20 lines, use:
>>> for line in islice(lines(), 20):
… print(line)
Again: make sure you understand why this works!
4. Run your doctests, review your code, check your documentation, and sign the honorcode in honorcode.txt.
5. At this point you can turn in your work for grading. Please make sure you add, commit, and push
oracle.py and honorcode.txt.
Pushing onward. If you want to go a bit further in your investigation, you might try to tune the default
window size so that it generates reasonable text from typical sources. We’ve included Austen’s Pride and
Prejudice and Alcott’s Little Women as examples. Note that if the window size is too small, there’s
not enough context to re-generate text similar to the original. If the window size is too large, runs of
the original text are reconstructed, but the generator obviously \loses its way” from time to time. You’re
searching for a happy medium that works for several texts. If you do experiment with this, document your
ndings as a suggestion about how to set  ngerprint’s n parameter in the module’s documentation at the
top of the  le.
For the fullest credit, you might consider writing a helper function, entropy(string), that computes
the Shannon entropy1 associated with selecting a random character from string. The Shannon entropy
measures the \unexpectedness” of the character returned by random.choice(string).
You can compute the Shannon entropy, H, using the following formula:
H = −
N
X−1
i=0
pi
log2 pi
Here, N is the number of distinct characters in the string. The value 0 ≤ pi ≤ 1 is the probability that
you will pick character i.
Notice that if you are in a situation where exactly one character has a chance of being selected, N = 1,
p0 = 1 and log2 p0 == 0. The total entropy, H = 0. There is no surprise; no information is gained by
learning what the next character is; the result was obvious.
On the other hand, if there are two characters that might be selected with equal likelihood, we have
H = −
N
X−1
i=0
pi
log2 pi = −X
1
i=0
0.5 · −1 = −(0.5 · −1 + 0.5 · −1) = 1
In essence, the Shannon entropy tells us how many yes/no questions we would have to have answered
before we could identify the next randomly selected character. If you randomly pick a letter from Pride
and Prejudice, how many yes/no questions would you expect to have to ask to accurately guess the letter?
The Shannon entropy of the text will tell us that.
Given entropy(string), we could then write keyEntropy(key). This method measures the \unexpectedness” of the result of randomCompletion(key). Do you have choices? Then the entropy is greater
than zero. On the other hand, if the entropy of the key is zero, the text generator is reproducing the text,
verbatim.
?
1Shannon entropy was introduced by Claude Shannon in 1948. A mathematician working for AT&T, he was interested in
modeling how much information could be transmitted across a communication channel.

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