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Term frequency/Inverse document frequency implementation in C#

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4.87 (55 votes)
28 Oct 2005
Text statistical-based measuring of similarity between two documents in a corpora.


This code implements the Term Frequency/Inverse Document frequency (TF-IDF). The TF-IDF is a text statistical-based technique which has been widely used in many search engines and information retrieval systems. I will deal with the documents similarity problem in the next section. To understand the theory, please see this article: Wiki definition of TF/IDF for more details.


Assume that you have a corpora of 1000 documents and your task is to compute the similarity between two given documents (or a document and a query). The following describes the steps of acquiring the similarity value:

Document pre-processing steps

  • Tokenization: A document is treated as a string (or bag of words), and then partitioned into a list of tokens.
  • Removing stop words: Stop words are frequently occurring, insignificant words. This step eliminates the stop words.
  • Stemming word: This step is the process of conflating tokens to their root form (connection -> connect).

Document representation

  • We generate N-distinct words from the corpora and call them as index terms (or the vocabulary). The document collection is then represented as a N-dimensional vector in term space.

Computing Term weights

  • Term Frequency.
  • Inverse Document Frequency.
  • Compute the TF-IDF weighting.

Measuring similarity between two documents

  • We capture the similarity of two documents using cosine similarity measurement. The cosine similarity is calculated by measuring the cosine of the angle between two document vectors.

Using the code

The main class is TFIDFMeasure. This is the testing code:

void Test (string[] docs, int i, int j)
// docs is collection of parsed documents
    StopWordHandler stopWord=new StopWordsHandler() ;
    TFIDFMeasure tf=new TFIDFMeasure(doc) ;
    float simScore=tf.GetSimilarity( i, j);
      // similarity of two given documents at the 
      // position i,j respectively


This library also includes stemming (Martin Porter algorithm), and N-gram text generation modules. If a token-based system did not work as expected, then you can make another choice with N-gram based. Thus, instead of expanding the list of tokens from the document, we will generate a list of N-grams, where N should be a predefined number. That means we will hash into a table to find the counter for the N-gram, but not words (or tokens).

The extra N-gram based similarities (bi, tri, quad...-gram) also help you compare the result of the statistical-based method with the N-gram based method. Let us consider two documents as two flat texts and then run the measurement to compare.

Example of some N-grams for the word "TEXT":

  • uni(1)-gram: T, E, X, T
  • bi(2)-gram: T, TE, EX, XT, T
  • tri(3)-grams: TE, TEX, EXT, XT, T
  • quad(4)-grams: TEX, TEXT, EXT, XT, T

A string of length k, will have k+1 bi-grams, k+1 tri-grams, k+1 quad-grams, and so on.

Point of interest

No complex technique was used, I only utilized the hashtable indexing, and array binary search to solve this problem. The N-gram based text similarity also gives us interesting results.

Articles worth reading


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here


About the Author

Thanh Dao
Software Developer
Vietnam Vietnam
I'm still alive...but temporarily moved to work on mobile & web stuffs(j2me/brew/php/flash...something not M$). things have just been very busy, and probably will don't have chance to maintain & respond. Hope will have time to try to write again, because many ideas with WPF &silver light are waiting. wish me luck Smile | :)

- MESHSimPack project(c# library for measuring similarity among concepts of the MESH ontology):

Comments and Discussions

QuestionVector space Pin
Member 1052649623-Jan-14 13:01
memberMember 1052649623-Jan-14 13:01 
AnswerAlternative version of TF*IDF code Pin
kbomb98711-Sep-13 10:10
memberkbomb98711-Sep-13 10:10 
Questionhelp for run program Pin
mostafamosakhany15-Jul-13 20:27
membermostafamosakhany15-Jul-13 20:27 
Questiontext categorization Pin
arjuncent11-Feb-13 15:43
memberarjuncent11-Feb-13 15:43 
QuestionGood program but complex structure Pin
falah ukm14-Apr-12 2:48
memberfalah ukm14-Apr-12 2:48 
Questionstopword removal issue Pin
sianiparmaruli24-Jul-11 22:44
membersianiparmaruli24-Jul-11 22:44 
AnswerRe: stopword removal issue Pin
falah ukm7-May-12 3:45
memberfalah ukm7-May-12 3:45 
Questioni have a question pls read this!!!! Pin
dyeingbreed8-Jul-11 21:22
memberdyeingbreed8-Jul-11 21:22 
Questionneed help for visual basic coding for tf-idf Pin
fifi fadli6-Jul-11 23:42
memberfifi fadli6-Jul-11 23:42 
GeneralMy vote of 1 Pin
Libra85224-Oct-10 22:23
memberLibra85224-Oct-10 22:23 
No description how to run the code..
GeneralMy vote of 3 Pin
Mustafa M. Gamal29-Aug-10 19:47
memberMustafa M. Gamal29-Aug-10 19:47 
Question"Term frequency/Inverse document frequency implementation in C#" Pin
Kasunmit9-May-10 16:15
memberKasunmit9-May-10 16:15 
QuestionHow to run the code Pin
aurangzeb khan27-Sep-09 18:48
memberaurangzeb khan27-Sep-09 18:48 
GeneralRelevance article Pin
Adrian Pirvu4-Jun-09 21:13
memberAdrian Pirvu4-Jun-09 21:13 
QuestionPerformance Pin
tohi21-May-09 3:57
membertohi21-May-09 3:57 
QuestionHow to use this code Pin
Nanomom24-Sep-08 21:11
memberNanomom24-Sep-08 21:11 
GeneralSir very thanx for this code at code project but would you like to help me Pin
chancomsats21-May-08 9:35
memberchancomsats21-May-08 9:35 
QuestionStill maintained? Pin
Marakai27-Dec-07 17:47
memberMarakai27-Dec-07 17:47 
AnswerRe: Still maintained? Pin
daisy0001bc3-Jun-08 0:47
memberdaisy0001bc3-Jun-08 0:47 
AnswerRe: Still maintained? Pin
Marakai3-Jun-08 15:40
memberMarakai3-Jun-08 15:40 
GeneralRe: Still maintained? Pin
Thanh Dao6-Jun-08 4:14
memberThanh Dao6-Jun-08 4:14 
GeneralRe: Still maintained? Pin
Marakai15-Jan-09 17:52
memberMarakai15-Jan-09 17:52 
GeneralRe: Still maintained? Pin
me_project12-Jan-09 18:49
memberme_project12-Jan-09 18:49 
GeneralRe: Still maintained? Pin
Marakai15-Jan-09 17:50
memberMarakai15-Jan-09 17:50 
GeneralRe: Still maintained? Pin
me_project18-Jan-09 19:45
memberme_project18-Jan-09 19:45 

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Posted 28 Oct 2005


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