Lab 13. Name: Checked:

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1 Lab 13 Name: Checked: Objectives: Learn how to input data from text files using Scanner. We will also use Scanner in other novel ways, including reading data directly from a webpage! Preparation: Processing a text file, line by line //*************************************************************** // FileInput.java Author: MAP // Demonstrates the use of Scanner to read text file input. //*************************************************************** import java.util.scanner; import java.io.*; public class FileInput // // Reads text from a file and prints it in uppercase. // public static void main (String[] args) throws IOException String line; Scanner filescan; File myfile = new File("sample.txt"); filescan = new Scanner (myfile); // Read and process each line of the file while (filescan.hasnext()) line = filescan.nextline(); System.out.println (line.touppercase()); //**** 1. Download and compile FileInput.java from the course website; create a small text file with a few lines of text and save as sample.txt in the lab 13 folder (use a plain text editor or use jgrasp: FileàNewà Otherà Plain text). Run FileInput what does it do? 2. Modify the code to use the parameter args[0] of main() as the file name. Do this as follows: Replace the use of File myfile = new File("sample.txt"); with File myfile = new File(args[0]); In jgrasp, select Run Arguments from the Build menu, and provide the file name as an argument (parameter) to main() by typing sample.txt in the box that appears above your program. Try running your program with different files, eg, the titanic.txt dataset from the course website. 3. Modify the code to incorporate a counter linenumber for the number of lines that are input in the while loop. Add some code at the end of the program to print the number of lines in the file. 4. Modify the code in the inner loop (marked //**** in the code above) filter the results: if (line.touppercase().contains("air")) System.out.println (linenumber + "\t" + line);

2 Part A. Using a dataset from a public repository 1. Make copies of the following files and save them in a separate subfolder of Lab 13 for part A of this lab. Then, review your work with your partner and sign each other s worksheet. FileInput.java (from preparation) TwoDArrayInputFromFile.java (from warm-down exercise for File input) Lab partner s signature (indicates approval): We will reuse our code with Wikipedia data obtained from the Stanford Network Analysis Project (SNAP) datasets: 1. Visit the SNAP website to locate and download the wiki-vote dataset. (Resist the urge to also download all the Facebook datasets!). Save in the same folder as the above files and unzip the file. 2. Open wiki-vote.txt in jgrasp to inspect the file s contents. The numbers encode anonymized Wikipedia users; each pair of numbers (A B) means that A voted for B to be promoted to admin. Note that several lines near the top are comment lines indicated by #. These lines are helpful to programmers and researchers working with the dataset. For the moment we will just ignore what this file is about and just run FileInput.java with it as input. How many lines in the file? How many times does AIR occur in this file? 3. We now prepare to run TwoDArrayInputFromFile.java on wiki-vote.txt. We make a couple of small changes: Delete the comment lines in wiki-vote.txt and save the file. It should now contain just the data, similar to the small input file likes.txt that we used to test TwoDArrayInputFromFile.java In TwoDArrayInputFromFile.java we need a larger 2D array of boolean in order to represent the voting data from this file. It should be large enough to fit every possible vote of the form x y. For example, the line containing represents the fact that user 43 has voted for user 5029, so it will be encoded by setting table[43][5029] = true. Inspect the file again and note that the largest numbers occurring are in the 8000 range. To be safe, you can set the size of the array to 9000x9000. Now we run TwoDArrayInputFromFile.java on wiki-vote.txt. If you are printing the array contents, this should result in A LOT of output, which is not so helpful. You can comment-out the code for printing the contents of the array and the matches found and try again. How many pairs of mutual votes (matches) did your program find?

3 Part B. Scanning from a String Just as we can use a Scanner to input from a file or from System.in, we can also use a Scanner to input from a String! 1) Try this code: ScanFromString.java //******************************************************************** // ScanFromString.java MA Papalaskari // Simple example: scanning from a String //******************************************************************** import java.util.scanner; public class ScanFromString public static void main(string[] args) Scanner scan = new Scanner(System.in); System.out.print("Please type 3 words: "); String line = scan.nextline(); Scanner scanline = new Scanner(line); String word1 = scanline.next(); String word2 = scanline.next(); String word3 = scanline.next(); System.out.println("Word 1: " + word1); System.out.println("Word 2: " + word2); System.out.println("Word 3: " + word3); scanning from a String Run : ScanFromString.java what does it do? Part C. Simple Calculator Next, we will create Calculator.java by modifying ScanFromString.java so that it does something more interesting with the input. Our new program will treat the input as a command for a simple numeric computation. For example, the input might be: 55 * 83 We want the program to compute and print the product First, run ScanFromString.java with this input and observe how it picks out the 55, *, 83 as word1, word2, and word3, respectively. Note that the code uses scanline.next() which produces String tokens and that was fine because word1, word2, and word3 are Strings. But now you want to use the values 55 and 83 as numbers, so the variables have to be of type double (we could use int, but double will allow you to

4 handle a wider range of values), and you need to obtain their values using scanline.nextdouble() instead of scanline.next(). Can you use these ideas to create a simple calculator? Change the prompt from Please enter 3 words to Calculate: Note that you can test the value of word2.charat(0) to see if it is equal to +, *, etc, and, accordingly, compute the result. (If you want to be able to handle more than 2 operators, it is best to use a switch statement.) Sample runs: ----jgrasp exec: java Lab13d Calculate: = jgrasp exec: java Lab13d Calculate: 55 * 83 = Part E: Scanning from a website We have seen how Scanners can be set up to scan from System.in (keyboard input), text files, or Strings. Here we see how to scan directly from a website. Try it with FileInput.java: 1. Add another import directive at the beginning or your program: import java.net.url; 2. Set up your Scanner to read from a url instead of a file. For example: String myurl = " InputStream instream = new URL(myurl).openStream(); Scanner webscan = new Scanner (instream); 3. Use webscan as any other Scanner to input from a webpage, as if it were a text file. This technique will work with most webpages, as long as they can be read as text (including html files).

5 Lab 13 Comments Name: Comments on this lab, please: What was the most valuable thing you learned in this lab? What did you like best about this lab? Was there any particular problem? Do you have any suggestions for improving this lab as an effective learning experience?