public static void extractFeatures(File feaFile,File[] tweetFile,File[] outFile,String[] labels,boolean pos) throws Exception
	{
		//read in features into table
		feaNum=readFeaFile(feaFile);
		//for each tweet file, initiate a vector to maintain the labeled feature vectors
		Vector<Vector<String>> feaVec=new Vector<Vector<String>>();
		for(int i=0;i<tweetFile.length;i++)
			feaVec.add(new Vector<String>());
		//call unigram for each tweet and put the labeled fature vector into the correct vector
		for(int i=0;i<tweetFile.length;i++)
		{
			String temp="";
			BufferedReader in=new BufferedReader(new FileReader(tweetFile[i]));
			while((temp=in.readLine())!=null)
			{
				//take off the tweet ID
				String split[]=temp.split(ConstantClass.delimiter);
				feaVec.elementAt(i).add(split[0]+ConstantClass.delimiter+unigram(split[1],labels[i],pos).toString());
			}
			in.close();
		}
		//write the result out
		for(int i=0;i<tweetFile.length;i++)
		{
			BufferedWriter out=new BufferedWriter(new FileWriter(outFile[i]));
			for(String l : feaVec.elementAt(i))
				out.write(l+"");
			out.flush();
			out.close();
		}
		//write the feature out
		writeFeaFile(feaFile);
	}