Presidents

IntroductionIn this project, we’re going to be doing some text analysis. The purpose of this project is to build the computational machinery required to efficiently compare the language used in these documents. Ultimately,we’dliketobeable to characterize documents by the language theyuse, which means being able to measure if a document is similar to another.Here, you will be asked to characterize the most similar documents, and then to characterize some newtextbywhich document it is most similar to. The basis we will use for comparison is called the vector space model,and was originally introduced by Gerard Salton in 1975 — when there was precious little electronic text around to characterize! It is still one of the models used by information retrievaland/or text analysis systems eventoday,although there are more sophisticated methods available.
The basic idea is that each document can be represented by an (ordered) list of frequencies, where each frequencycorresponds to the number of times its corresponding word appears in the text. If you think of this list of N numbers as an N-dimensional vector,then the notion of a "vector space" makes more sense: adocument is just a vector in this N dimensional space. We’llworry about exactly which words are counted and appear in the vector in just a minute, but assuming that the vector is normalized so as to be 1 unit long, twovectors representing documents with very similar word content will have vectors that point in approximately the same direction.
Nowthis leavesalot of details to be pinned down. First, which words are included in the vector? Howdo we normalize vectors to unit length? And are these raw frequencies? Or are they also normalized in some way? HowdoIcompare twovectors? These details are really important, and we’ll have tosettle them before we can start coding.
Data
Ihav e obtained from an online archive the text of 894 presidential speeches. Some of these speeches are very short (President Reagan’saddress at MoscowState University on May 31, 1988 is only 72 words long) and some are very long (President Lincoln’sthree hour long speech arguing against the Kansas/Nebraska Act on October 16, 1854 clocks in at 32,991 words). I have put the text through a regex filter to remove most of the cruft, but there are still likely to be some unrecognized or missing characters (some of these files were generated by optical character recognition software, which can makethis kind of mistake).
Iwill provide for you a zip file containing all 511 speeches that exceed 2000 words. In addition, I will provide a set of 5 "unknown" speeches to identify by president.
Document Similarity
We will be using the tf/idf (term frequency/inverse document frequency) weighting scheme originally proposed by Salton and his students for our vectors. Each vector v can be represented as a list of N terms, where a term is essentially a word or word stem and N is a prespecified number of terms we will target in characterizing a document. In general, one picks the N most frequently used words or word stems over the entire document collection or corpus;as N gets larger,weget better performance, at least until the words become so infrequent as to add little to performance.
The tf/idf weighting scheme is defined as follows:
v = [w0, w2, w3 ...w(N−1)] and, for a givendocument, each weight wi is computed as follows:
wi = tfi *log
|D| 1 + {d ∈ D|ti ∈ d} Here, for term ti, tfi is related to the number of times the term appears in the current document, |D| is the number of documents in the entire corpus D,and {d ∈ D|ti ∈ d} is the number of documents that contain the term ti.The 1 in the denominator savesusfrom a divide-by-zero issue; the logarithm can be to anybase and serves only as a scaling factor.Note that tfi is not usually just a word count, but is normalized by the length of the document, hence:
tfi =
number of times ith term appearsindocument length of document
If we characterize each document by its vector v, we can compute a measure of similarity by simply taking the scalar product (also called the dot product) of the twovectors. The dot product of twovectors v1 and v2 represented as indexable tuples or lists is simply:
0≤i<N Σ v1[i] × v2[i]
Adot product of 1 means the twodocuments have exactly the same word frequencies, while a dot product of 0 means the twodocuments share no words in common.
To a first approximation, this model does a pretty good job of ranking similar documents. Of course, it does have its limitations. For example, the model loses anyidea of the order in which the words actually occur,which is whythis is sometimes called a bag-of-words approach.


Part 1: Preliminary Processing
Foreach of the documents provided, you will need to build a document vector to represent it. Tobuild the document vector,you will have toselect appropriate terms and, because we are using tf/idf, you will also need to knowthe frequencies for these terms both within a document and across all documents in the corpus.
Your first function:
de f ex t ractTe rms (fil e list , co r pusTe rms ) :
computes term frequency dictionaries (tfd) for each file in filelist, as well as a corpus frequencydictionary (the second argument) for the entire corpus of files. It should be invokedas, e.g.,
cfd = {} t fds=ex t ractTe rms ( [’fil e1’ , ’fi le2 ’ ,’fil e3’ ],cfd)
which will return a list of term frequencydictionaries (i.e.,dictionaries where the terms are keysand the values are counts), one per file in the input filelist, and update the corpus frequencydictionary (i.e. a dictionary where terms are keysand the values are the number of documents where that term occurs) that waspassed in as an argument.
More specifically,the function opens each file in the specified filelist, reads the contents, removesstop words (see list provided), applies a stemmer (see below) to each remaining word to produce a term, and then constructs a term frequencydictionary for that file while at the same time adding entries and updating counts in the corpus term frequencydictionary.Inthis way,once all the files are processed, the corpus frequencydictionary will accurately reflect, for each term, the number of files in which it occurs.
 
An example is givenlater in this document. Agood place to start arethe readInput() and parse() functions we wrote while studying War and Peace in class. You will want to handle punctuation, contractions and any other issues, such as dates and the like, that may arise when you examine the output of your extractTerms() function on the documents provided.
Your code should makeuse of a stemmer:
de f stemme r (word) :
to reduce each word to its basic form. Your stemmer should extend the stemmer used in class:
de f stemme r (word) : if word[ -3:] ==’ies’ and word[ -4:] not in [’eies’ , ’a i es’ ]: wo r d =word[ : -3] +’y’ if word [-2 :]==’es ’ andword[ -3:] not in [’ae s’, ’ee s’, ’oe s’] : wo r d =word[ : -1] if word [-1 :]==’s’ and word[ -2:] not in [’us ’,’ss ’,’ys ’]: wo r d =word[ : -1] re turn(wo r d )
which is based on Harman’s weak stemmer, to also remove the following endings from words: -able,-al, -ance,-ant, -ar,-ary,-ate,-ement, -ence,-ent, -er,-ess, -ible,-ic, -ify,-ine,-ion, -ism, -iti, -ity,-ive,-ize, -ly,-ment, -or,-ou, -ous, -th,and -ure. Youmay wish to rewrite/recast the stemmer to use regular expressions and incorporate additional features, but this isn’t required.

Part 2: Vector Models

Once you have created the word distributions for each of the documents provided, you can use the document dictionary to find the k=100 most frequently used words in the corpus; these will be the terms in your vector space. You are nowready to convert the list of dictionaries returned from extractWords() into a list of tuples representing the tf/idf encoding of each document according to the k most commonly used terms within the corpus (i.e.,the terms with the highest values in the corpus frequencydictionary).
de f createMo d el s(tfds , cfd, k):
should taketakethe list of term frequencydictionaries returned from extractWords(), the corpus frequencydictionary,aninteger k, and return twovalues: a k-tuple of word terms and a list of Nk-tuples, representing the vector model of each document built according to the word term k-tuple.
An example will help to clarify.Consider a corpus consisting of only three documents, file1, file2,and file3 containing, respectively,’aaabb’, ’b b b b b c c’, and ’a a b b b b b b b’. Here, we’ll assume each letter from [abc] represents a single word in the document, there are no stop words, and because stemming isn’tanissue, terms are simply single characters.
cfd={ } t fds=ex t ractTe rms ( [’fil e1’ ,’fi le2 ’ ,’fil e3’ ],cfd) cfd {’ a ’:2 ,’b’ :3,’c’ :1}
Note that the corpus frequencydictionary correctly states that ’a’ appears in 2 documents, ’b’ in 3 documents, and ’c’ in 1 document.
t fds[0] {’ a ’:3 , ’b’ :2} t fds[1] {’b’ :5, ’c’ :2}
3 Revised December 2, 2014
t fds[2] {’b’ :7, ’a’ :2}
Also showthe correct character counts from each file.
NowifIinv oke createModels() as follows:
( terms, mod els)=createMo d el s(tfds , cfd, 2)
the first value returned, terms, will be a list of the terms used to construct each document’svector model. Here, because k = 2, there will be exactly twoterms, and theywill correspond to the most terms with the highest values in cfd:
(′b′,′a′)
Note: this was the part that was totally screwed up the first time: Forthe first document, the tfd vector will be:
(
2 5
× log(
3 3
),
3 5
× log(
3 4
))
Forthe second document:
(
5 7
× log(
3 6
),
0 7
× log(
3 7
))
And for the third document:
(
7 9
× log(
3 8
),
2 9
× log(
3 3
))
Moreover, wewill want these vectors to be unit vectors, so they must be normalized accordingly.To normalize a vector [x1, x2, x3],you will want to divide each term in the vector by the sum of the terms squared. In other words, the normalized form of [x1, x2, x3] is:
[
x1 u
,
x2 u
,
x3 u
]
where u = √  x2 1 + x2 2 + x2 3.


Part 3: Presidential Similarity
Nowthat you have your machinery in place, for each of the 43 presidents provided, compute the 6 possible similarities between the 4 speeches provided for that president; that is, for each of the 4 speeches, compute the similarity with each of the other 3 speeches: when you ignore ordering, there will be 6 dot products to compute. Report the average dot product for each president, and plot, using matplotlib, a histogram of the 43 averages obtained.


Part 4: Attributing Unknowns
Forthe last part of the assignment, you will compute the vector model of the 5 unknown documents provided and compute their similarity with each of the presidential speeches in your corpus. For each unknown speech, report the 10 closest matches with existing speeches. Can you guess who wrote those speeches? (You will not be graded on accuracy, but once the assignment is complete, I will reveal the answers).
 
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