Text Summarization using Vector Space Model

The rapid growth of World Wide Web has resulted in information overload. One cannot possibly read every news article, document, paper, etc that is out there on the web. That is why text summarization has become essential in this information age. Even the search engines show the summary of each of the search results to help users select the most appropriate link.

However, summarizing any document automatically is not an easy task. It is one of the challenging Natural Language Processing (NLP) problems. In this post, I will show you how to create a summary from a web document using Vector Space Model.

We will first extract the sentences from the document. The summary generated for each document will actually consist of sentences that are extracted from it. We will try to pick the sentences are more representative of the document than others using the VSM.

In the previous examples, there were two main components: list of documents and the query. We then assumed that there exist a string array "docs" which contains the query's (at zero index) and the documents' text. We are going to use the same data structure ("docs") but this time instead of the query we will have the document's text (which we are trying to summarize) at the zero index and the various sentences that we extracted from the document in the remaining indices.

We will then rank the sentences using the VSM according to their similarity with the document. The similarity will decide whether we want to include the sentence in our summary or not.

The code from VSM example is almost the same, the only new function is the sentence extractor that I had to write for this example. The sentence extractor is created using the following heuristics:

1. If the character is "?" or "!" then mark this as the end of the sentence
2. If the character is "." the check that it is not preceded by "Mr", "Mrs", "Dr", etc

• if Yes, then don't mark this as the end of the sentence
• if No, then check that "." is not followed by a number
• • if Yes, then don't mark this as the end of the sentence
  • if No, then mark this as the end of the sentence
    
  
  

The program requires a valid url address from which it will read the item and display the Top 5 sentence that it thinks best summarizes the document. For this simple program, I assume that "p" tags are used to display paragraphs.

As can be seen from the image above, the summary produced by this simple method is not too bad :)

Here is the code.

Extracting sentences from a document

Extracting words from a document is quite common and a fairly easy task. And using variations of Regex.Split(documentText, @"\W") one can easily do that. However, for a spam filtering project, I wanted to extract sentences from a document. I actually just wanted to extract the first and the last sentence but I thought it would be easier just to split the entire document into sentences at first using split function as follows:

char[] delimiter={'.','?','!'};
string[] sentences = document.Split(delimiter);

These two lines of code will work on most of the cases since '?','!' and '.' are usually the characters that represent the end of a sentence. However, using the period (.) can lead to erroneous results since dot '.' can be used to represent a decimal number ( 3.5 , 100.03, etc) or abbreviations (Mr., Mrs., Dr., etc).

Unfortunately, the corpora that I was using has a lot of instances of decimal numbers and abbreviations. So I have to come up with a solution. After unsuccessfully trying to Google (or Bing) for the some ideas, I came up with my own functions to solve this problem. The functions worked perfectly fine on my corpora but it may require some modifications to work for other types document.

The main function is 'parseSenteces()' that splits the document into sentences. It requires that the entire document text be sent as a string parameter.

The 'checkInitils()' function is called when the the program comes across a period. This function checks whether the character before the period represent any initial (like Dr., Miss., Mrs., Mr.) or not and returns a boolean value upon its finding.

The 'checkNum' function checks whether the character before the encountered period are numbers or not.

If the 'checkInitials()' and 'checkNum()' returns false then that represent end of sentence, and that sentence is added in a List. If any of them returns true, then the period is considered a part of sentence and the counter moves on to check the next character.

Here is the code:

private void parseSentences(string document)
{
string para = document;
string sentence = ""; //keeps track of current sentence
List words = new List(); //stores different sentences
int counter = 0; //keeps track of index of the character being read

CharEnumerator charEnum = para.GetEnumerator();//Enumerator to read each character
while (charEnum.MoveNext())
{
char currChar = para[counter]; //holds the current character being read

if (currChar == '.')
{
//check if the character before period are numbers
if (checkNum(para, currChar, counter))
{
//if not then period represents a number and not end of sentence
sentence += currChar.ToString();
}

//check if the character before period are initials of some sort
else if (checkInitial(para, currChar, counter))
{
sentence += currChar.ToString();
}
else
{
//add the sentence to the list
words.Add(sentence);
sentence = "";
}
}
else if (currChar == ' ')
{

sentence += currChar.ToString();

}

else if ((currChar == '!') || (currChar == '?'))
{
words.Add(sentence);
sentence = "";

}

else
{ sentence += currChar.ToString(); }
counter++;
}

if (sentence != "")
{ words.Add(sentence); }

string list = "";

foreach (string s in words)
{
Console.WriteLine(s);
}
}

private bool checkNum(string para, char currChar, int counter)
{ //space means not a decimal number

try
{
if (para[counter + 1] == ' ')
return false;
} catch(Exception)
{ return false; }

char currNext= para[counter+1];

if ((currNext >= '0' & currNext <= '9'))
{ char currPrev=para[counter-1];

if ((currPrev >= '0' & currPrev <= '9'))
return true;
}
return false;
}

private bool checkInitial(string para, char currChar, int counter)
{
string word1 = ""; string word2 = "";

for (int i = 3; i > 0; i--)
{
try
{
if (i < 3)
{
word1 += para[counter - i].ToString();
}
word2 += para[counter - i].ToString();
}
catch(Exception e)
{
Console.WriteLine(e.toString());
}

}

if ((word1.ToLower() == "mr") || (word1.ToLower() == "dr") || (word2.ToLower() == "miss") || (word2.ToLower() == "mrs"))
{
return true;
}
return false;
}