CSCI 161 Introduction to Computer Science

Transcription

1 CSCI 161 Introduction to Computer Science Department of Mathematics and Computer Science Lecture 7 File Input/Output

2 Motivation for File I/O So far: Just relying on the user for data input/output (I/O)... Problem 1: Today's applications are mostly data-driven Many programs require large amount of data to be input into a program - Inputting data by hand is very error-prone and tedious - If program crashes... need to start over - Need: to input data from a file to a program 2

3 Motivation (Cont.) Examples of Input Files: Enter the height of all known Sitka Spruce in Olympic National Park Enter the U.S. census data Enter our customers' data Enter all the synonyms and root words for a chat bot... 3

4 Motivation (Cont.) Problem 2: Output can be large (or even continuous)! Many programs run continuously, generating output - Health-monitor (Fitbit, etc.), for instance. But we only know how to output to the screen - How do you make sense of data that is streaming past your eyes at insane rates? Need: to output to a file, and analyze the data in that file later! 4

5 Outline File Input/Output Reading from Files: Scanner Writing to Files: PrintWriter Parsing Input Splitting/Tokenizing Type Conversion of Strings Putting It All Together Tweet Processing 5

6 Assume... We have a file, tree_height.txt, that is in the BlueJ project folder 6

7 Opening Files Content of tree_height.txt (each line is a tree's height) Want: TreeAnalyzer class A constructor that inputs the name of the file, stores its data in an ArrayList Methods that operate over the data now stored in the ArrayList 7

8 Opening Files (Cont.) Include at the top of the class: import java.io.file; import java.io.filenotfoundexception; import java.util.scanner; How to open a file for reading (need a Scanner object): Important! The methods and/or constructors that opens the file needs to indicate that it may throw an exception. public void somemethod(string filename) throws FileNotFoundException { //open the file Scanner file = new Scanner(new File(filename)); } //file is open, now what? Let's take a peek at Scanner's API 8

9 Review: Java's Scanner API Method public Scanner(File source) public Scanner(InputStream source) public boolean hasnext() Descrip-on Constructor. Creates a scanner on the given file. Constructor. Creates a scanner on the given stream (later). Returns true if the scanner has another token. public String nextline() Returns an entire line of input as a String. public void close() Closes the scanner. 9

10 Let's Write a TreeAnalyzer Class TreeAnalyzer Class A constructor that inputs a file name. Opens the file and reads its contents into an ArrayList. public void open(string filename). Reads file contents into an ArrayList. public double gettallest() that returns the max tree height. public ArrayList<Double> gettreeslessthan(double limit) that returns the list of trees shorter than the given limit. public void report() that prints the following info on the screen: File name: (name of file you opened) Number of trees: (number of trees in the file) Tallest tree height: (tallest tree found in file) Code is posted on course page! 10

11 TreeAnalyzer public class TreeAnalyzer { private ArrayList<Double> listofheights; private String thefile; //stores a list of tree heights //name of the file that we opened public TreeAnalyzer(String filename) throws FileNotFoundException { //instantiate an empty arraylist listofheights = new ArrayList<Double>(); thefile = filename; open(thefile); } } //other methods omitted 11

12 TreeAnalyzer public class TreeAnalyzer { private ArrayList<Double> listofheights; private String thefile; //stores a list of tree heights //name of the file that we opened public TreeAnalyzer(String filename) throws FileNotFoundException { //instantiate an empty arraylist listofheights = new ArrayList<Double>(); thefile = filename; open(thefile); } public void open(string filename) throws FileNotFoundException { //clear out the ArrayList, which holds data from the previously opened file listofheights.clear(); } //open the given file for reading Scanner file = new Scanner(new File(filename)); while (file.hasnext()) { String line = file.nextline(); //grab the next line from the file double treeheight = Double.parseDouble(line); //grab the next line from the file listofheights.add(treeheight); //add it to my list } file.close(); } //other methods omitted 12

13 Outline File Input/Output Reading from Files: Scanner Writing to Files: PrintWriter Parsing Input Splitting/Tokenizing Type Conversion of Strings Putting It All Together Tweet Processing 13

14 Writing to Files? TreeAnalyzer Class We know how to read from files... how about writing to files? public void reporttofile(string filename) that writes the file's info into a file, in the following format: File name: tree_height.txt Number of trees: Tallest tree height: 14

15 Writing to Files Include at the top of the class: import java.io.printwriter; import java.io.filenotfoundexception; How to open a file for writing: Again, methods using a PrintWriter requires you to throw an Exception //constructor public void somemethod(string filename) throws FileNotFoundException { //open the file for writing PrintWriter fileout = new PrintWriter(new File(filename)); } //file is open, now what? Let's take a peek at PrintWriter's API 15

16 Java's PrintWriter API Method Descrip-on public PrintWriter(File destination) public void close() public void print(string s) public void println(string s) Constructor. Opens the given file for writing. Closes the file after you're done writing to it. (Must be done!) Prints the given string to the opened file Prints the given string and a newline to the opened file Back to this: public void reporttofile(string filename) that writes the file's info into a file, in the following format: Code is posted on course page! 16

17 Still in TreeAnalyzer /** * Writes a summary of the data to the file indicated by the given filename filename the name of file to write into */ public void reporttofile(string filename) throws FileNotFoundException { PrintWriter fileout = new PrintWriter(new File(filename)); fileout.println(" "); fileout.println("file name: " + thefile); fileout.println(" "); fileout.println("total number of trees: " + listofheights.size()); fileout.println("tallest tree height: " + gettallest()); fileout.close(); } 17

18 Outline File Input/Output Reading from Files: Scanner Writing to Files: PrintWriter Parsing Input Splitting/Tokenizing Type Conversion of Strings Putting It All Together Tweet Processing 18

19 Dealing with More Complicated Input Most files are tab- or comma-delimited (separated) per line Let's say each line holds data about a Robot, and we want to create a RobotArmy: - Each line contains the Robot's (x,y) coordinates, whether it's visible, whether it's colorful, respectively. # x,y,visible,colorful (just a comment -- skip!) 0,50,true,false 200,150,true,true 100,250,true,false 300,150,false,false 240,0,true,true 200,350,true,true Want: Add a method to RobotArmy, public int addrobots(string filename), that adds Robots from the given file, and returns the number of Robots added. 19

20 Dealing with More Complicated Input (Cont.) Most files are tab- or comma-delimited (separated) per line: - Each line contains the Robot's coordinates, whether it's visible, whether it's colorful, respectively. Recall the Robot API: # x,y,visible,colorful (just a comment -- skip!) 0,50,true,false 200,150,true,true 100,250,true,false 300,150,false,false 240,0,true,true 200,350,true,true Method public Robot() public void moveto(int x, int y) public void makecolorful() public void makemonochrome() Descrip-on Constructor. Creates a new Robot object Moves a robot to the given location Colors the robot Changes the robot to black and white 20

21 Tokenizing Strings There are two common ways to tokenize a String: StringTokenizer class (You know this one from HW5 TweetParser) String's split() method (example below) Also from the String API: Method public String[] split(string delim) Descrip-on Tokenizes the current String by the given delimiter. Return an array of tokens. 21

22 Using the split() Method Example String str = "Hello world! 1 2 3"; String[] words = str.split(" "); //tokenizes by whitespace 22

23 Using the split() Method Example String str = "Hello world! 1 2 3"; String[] words = str.split(" "); //tokenizes by whitespace System.out.println(words.length); //how many tokens? > 5 (int) 23

24 Using the split() Method Example String str = "Hello world! 1 2 3"; String[] words = str.split(" "); //tokenizes by whitespace System.out.println(words.length); //how many tokens? > 5 (int) System.out.println(words[0]); > "Hello" (String) System.out.println(words[1]); > "world!" (String) System.out.println(words[2]); //String with a value of "1" (not an int!!) > "1" (String) System.out.println(words[3]); > "2" (String) System.out.println(words[4]); > "3" (String) How do we convert these to ints?! Can't cast from String 24

25 Converting Strings to Other Types Important! We still need to convert Strings holding numbers to an int, or double. Know these! From the Boolean wrapper class: Method public static boolean parseboolean(string s) Descrip-on Returns a boolean given the String representation From the Double wrapper class: Method public static double parsedouble(string s) Descrip-on Returns a double given the String representation From the Integer wrapper class: Method public static int parseinteger(string s) Descrip-on Returns a int given the String representation 25

26 Using the split() Method Example String str = "Hello world! 1 2 3"; String[] words = str.split(" "); //tokenizes by whitespace System.out.println(words.length); //how many tokens? > 5 (int) System.out.println(words[0]); > "Hello" (String) System.out.println(words[1]); > "world!" (String) System.out.println(Integer.parseInt(words[2])); //now it's an int! > 1 (int) System.out.println(Integer.parseInt(words[3])); > 2 (int) System.out.println(Integer.parseInt(words[4])); > 3 (int) 26

27 Putting It All Together File Contents - Each line contains the Robot's coordinates, whether it's visible, whether it's colorful, respectively. # x,y,visible,colorful (just a comment -- skip!) 0,50,true,false 200,150,true,true 100,250,true,false 300,150,false,false 240,0,true,true 200,350,true,true Add a method to RobotArmy, public int addrobots(string filename), that adds Robots from the given file, and returns the number of Robots added. - Problem 1: Need to extract "tokens" from each line of input (use split() or tokenizer) - Problem 2: Need to parse tokens into ints and booleans (use parse* methods from the wrapper classes) <<Let's implement it>> - Code on course page: RobotArmy_FileIO.zip 27

28 Outline File Input/Output Reading from Files: Scanner Writing to Files: PrintWriter Parsing Input Splitting/Tokenizing Type Conversion of Strings Putting It All Together Tweet Processing 28

29 Processing Real Data Let's put your TweetParser to use! Recall the following API you wrote for HW 5 Method public TweetParser() Descrip-on Constructor. Creates a default TweetParser public boolean ishashtag(string s) public boolean ismention(string s) Returns true if the given string is a #hashtag Returns true if the given string is public void processtweet(string tweet) public void printreport() public void reset() Parses the given tweet and updates statistics Prints out the statistics that were collected from the processed tweets Resets all statistics 29

30 Example Usage of Current Class TweetParser tp = new TweetParser(); tp.processtweet("@erin and the Lady Logs vie for the D3 championship! tp.processtweet("what a send-off! Good luck, Loggers! Logger Sports Network: Good #LoggerUP"); tp.printreport(); > Number of Tweets: 2 > Number of Terms: 27 > Number of Hashtags: 2 > Number of Mentions: 4 Wouldn't it be great if, instead of calling processtweet() by hand on each tweet, we input a file instead? Write a class, TweetsAnalyzer with a single method, - public void processtweetsfromfile(string filename) 30

31 Working with Real Twitter Data We collected data from the Twitter feed for a week straight in Stored it in tweets.txt Tab-Delimited Format: date [tab] handle [tab] tweet_content :52: BrandonChild21 Why do I always have to miss track on the nice days :58: Xo_laj Oh my goooodddd Paul Wesley ï ßòâ ï ßòªï ßòé :00: KoBAEne_Kist Eyes of the world - Dillon Cooper ft azizi Gibson/denzel curry :05: carizzzzle why not ï ßòõ (lots more) Idea: Open the file, and for each line in the file, - Tokenize it: Split line up by a tab. A tab in Java is: "\t" - Throw out first two tokens - Call processtweet() on the final token Report the results! Code Provided on course page: Lec6_TweetProcessor_FileIO.zip 31

32 Code for TweetsAnalyzer public class TweetsAnalyzer { private TweetProcessor tweetprocessor; public TweetsAnalyzer() { tweetprocessor = new TweetProcessor(); } public void processtweetsfromfile(string filename) throws FileNotFoundException { Scanner file = new Scanner(new File(filename)); while (file.hasnext()) { //grab a tweet from the file (one per line) String tweet = file.nextline(); } //the tweet content "payload" is the third token! String[] tokens = tweet.split("\t"); tweetprocessor.processtweet(tokens[2]); } } //print out the report! tweetprocessor.printreport(); 32

33 Administrivia 4/5 Some reminders: Lec 6 (Scanner, HashMap) posted Exam II next week!!! Lab 9 Smarter Bot Things worth mentioning: - People not using/bringing their API handouts - Finding something in a HashMap does not require loops Unlike arrays and ArrayLists 33

34 Administrivia 4/8 Posted Solution to Hwk 6 Homework 7 (Due Wed, 4/17) Reminder: Review Wednesday, exam Thursday Practice old lab: ComboGuesser (hits Arrays and loops hard) - Solution is given Talk tonight! Windows Internals: Multi-Core, Virtualization, and Persistent Memory - Matt Holle, Group Manager, Microsoft - Where: Thompson Hall Where: Monday, April 8th, 4pm 34

35 Administrivia (Cont.) Need student volunteers for faculty search Professor Chambers 35