Building Java Programs

Transcription

1 Building Java Programs Chapter 6: File Processing Lecture 6-2: Advanced file input reading: self-check: #7-11 exercises: #1-4, 8-11 Copyright 2009 by Pearson Education

2 Hours question Given a file hours.txt with the following contents: 123 Susan Brad Jenn Consider the task of computing hours worked by each person: Susan (ID#123) worked 31.4 hours (7.85 hours/day) Brad (ID#456) worked 36.8 hours (7.36 hours/day) Jenn (ID#789) worked 39.5 hours (7.9 hours/day) Can we produce this data token by token? Copyright 2009 by Pearson Education 2

3 Hours program strategy 123 Susan Brad Jenn We want to process the tokens, but we also care about the line breaks (they mark the end of a person's data). A better solution: First, break the overall input into lines. Then break each line into tokens. What if you tried it token by token first? Copyright 2009 by Pearson Education 3

4 Hours answer (flawed) import java.io.*; import java.util.*; // for File // for Scanner public class HoursWorked { // a non-working solution public static void main(string[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("hours.txt")); while (input.hasnext()) { // process one person int id = input.nextint(); String name = input.next(); double totalhours = 0.0; int days = 0; while (input.hasnextdouble()) { totalhours += input.nextdouble(); days++; System.out.println(name + " (ID#" + id + ") worked " + totalhours + " hours (" + (totalhours / days) + " hours/day)"); Copyright 2009 by Pearson Education 4

5 Flawed output Susan (ID#123) worked hours (97.48 hours/day) Exception in thread "main" java.util.inputmismatchexception at java.util.scanner.throwfor(scanner.java:840) at java.util.scanner.next(scanner.java:1461) at java.util.scanner.nextint(scanner.java:2091) at HoursWorked.main(HoursBad.java:9) The inner while loop is grabbing the next person's ID. Hard to tell it where to stop! The line breaks indicate the end of the data Much easier and more natural to process by asking for a line of input at a time, then only concentrating on that line Copyright 2009 by Pearson Education 5

6 Tokenizing lines of a file A Scanner object can also be used to process a String Scanner linescan = new Scanner(string); A String Scanner can tokenize each line of a file. Scanner input = new Scanner(new File("file name")); while (input.hasnextline()) { String line = input.nextline(); Scanner linescan = new Scanner(line); process the contents of this line... Copyright 2009 by Pearson Education 6

7 Hours program solution // Processes an employee input file and outputs each employee's hours data. import java.io.*; // for File import java.util.*; // for Scanner public class Hours { public static void main(string[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("hours.txt")); while (input.hasnextline()) { String line = input.nextline(); Scanner linescan = new Scanner(line); int id = linescan.nextint(); // e.g. 456 String name = linescan.next(); // e.g. "Brad" double sum = 0.0; int count = 0; while (linescan.hasnextdouble()) { sum = sum + linescan.nextdouble(); count++; double average = sum / count; System.out.println(name + " (ID#" + id + ") worked " + sum + " hours (" + average + " hours/day)"); Copyright 2009 by Pearson Education 7

8 File Processing Summary Token-based file processing: Breaks up entire file into tokens to process data Scanner input = new Scanner(new File("midterm.txt")); while(input.hasnext()) {... (use methods from input to process file) Line-based file processing: Breaks up file into lines to process data Scanner input = new Scanner(new File("hours.txt")); while(input.hasnextline()) { String line = input.nextline(); Scanner linescan = new Scanner(line);... (use methods from linescan to process file) Copyright 2009 by Pearson Education 8

9 Confusion w/ nextline Using nextline in conjunction with the token-based methods on the same Scanner can cause odd results Joe "Hello world" You'd think you could read 23 and 3.14 with nextint and nextdouble, then read Joe "Hello world" with nextline. System.out.println(input.nextInt()); // 23 System.out.println(input.nextDouble()); // 3.14 System.out.println(input.nextLine()); // But the nextline call produces no output! Why? Copyright 2009 by Pearson Education 9

10 Mixing lines and tokens Don't read both tokens and lines from the same Scanner: Joe "Hello world" input.nextint() // 23 23\t3.14\nJoe\t"Hello world"\n\t\t \n ^ input.nextdouble() // \t3.14\nJoe\t"Hello world"\n\t\t \n ^ input.nextline() 23\t3.14\nJoe\t"Hello world"\n\t\t \n ^ // "" (empty!) input.nextline() // "Joe\t\"Hello world\"" 23\t3.14\nJoe\t"Hello world"\n\t\t \n ^ Copyright 2009 by Pearson Education 10

11 Line-and-token example Scanner console = new Scanner(System.in); System.out.print("Enter your age: "); int age = console.nextint(); System.out.print("Now enter your name: "); String name = console.nextline(); System.out.println(name + " is " + age + " years old."); Log of execution (user input underlined): Enter your age: 12 Now enter your name: Sideshow Bob is 12 years old. Why? User's overall input: After nextint(): After nextline(): 12\nSideshow Bob 12\nSideshow Bob ^ 12\nSideshow Bob ^ Copyright 2009 by Pearson Education 11

12 Array traversals, text processing reading: 7.1, 4.4 self-check: Ch. 7 #8, Ch. 4 #19-23 Copyright 2009 by Pearson Education

13 Text processing text processing: Examining, editing, formatting text. Often involves for loops to break up and examine a String Examples: Count the number of times 's' occurs in a file Find which letter is most common in a file Count A, C, T and Gs in Strings representing DNA strands Copyright 2009 by Pearson Education 13

14 Strings as arrays Strings are represented internally as arrays. Each character is stored as a value of primitive type char. Strings use 0-based indexes, like arrays. We can write algorithms to traverse Strings. Example: String str = "Mr. E."; index value 'M' 'r' '.' ' ' 'E' '.' Copyright 2009 by Pearson Education 14

15 Recall: type char char: A primitive type representing a single character. Values are surrounded with apostrophes: 'a' or '4' or '\n' Access a string's characters with its charat method. String word = console.next(); char firstletter = word.charat(0); if (firstletter == 'c') { System.out.println("That's good enough for me!"); Copyright 2009 by Pearson Education 15

16 String traversals traversal: An examination of each element of an array. for (int i = 0; i < array.length; i++) { do something with array[i]; Use for loops to examine each character. for (int i = 0; i < string.length(); i++) { do something with string.charat(i); Copyright 2009 by Pearson Education 16

17 Section attendance problem Consider an input file of course attendance data: week1 week2 week3 week4 week5 week6 week7 week8 week week2 student1 student2 student3 student4 student Each line represents a section (5 students, 9 weeks). 1 means the student attended; 0 means absent. Copyright 2009 by Pearson Education 17

18 Section attendance problem Write a program that reads the preceding section data file and produces the following output: Section #1: Sections attended: [9, 6, 7, 4, 3] Student scores: [20, 18, 20, 12, 9] Student grades: [100.0, 90.0, 100.0, 60.0, 45.0] Section #2: Sections attended: [6, 7, 5, 6, 4] Student scores: [18, 20, 15, 18, 12] Student grades: [90.0, 100.0, 75.0, 90.0, 60.0] Section #3: Sections attended: [5, 6, 5, 7, 6] Student scores: [15, 18, 15, 20, 18] Student grades: [75.0, 90.0, 75.0, 100.0, 90.0] Copyright 2009 by Pearson Education 18

19 Data transformations In this problem we go from 0s and 1s to student grades This is called transforming the data. Often each transformation is stored in its own array. We must map between the data and array indexes. Examples: by position (store the i th value we read at index i ) tally (if input value is i, store it at array index i ) explicit mapping (count 'M' at index 0, count 'O' at index 1) Copyright 2009 by Pearson Education 19

20 Section attendance answer // This program reads a file representing which students attended // which discussion sections and produces output of the students' // section attendance and scores. import java.io.*; import java.util.*; public class Sections { public static void main(string[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("sections.txt")); while (input.hasnextline()) { // process one section String line = input.nextline(); int[] attended = countattended(line); int[] points = computepoints(attended); double[] grades = computegrades(points); results(attended, points, grades); // Produces all output about a particular section. public static void results(int[] attended, int[] points, double[] grades) { System.out.println("Sections attended: " + Arrays.toString(attended)); System.out.println("Sections scores: " + Arrays.toString(points)); System.out.println("Sections grades: " + Arrays.toString(grades)); System.out.println();... Copyright 2009 by Pearson Education 20

21 Section attendance answer 2... // Counts the sections attended by each student for a particular section. public static int[] countattended(string line) { int[] attended = new int[5]; for (int i = 0; i < line.length(); i++) { char c = line.charat(i); // c == '1' or c == '0' if (c == '1') { // student attended their section attended[i % 5]++; return attended; // Computes the points earned for each student for a particular section. public static int[] computepoints(int[] attended) { int[] points = new int[5]; for (int i = 0; i < attended.length; i++) { points[i] = Math.min(20, 3 * attended[i]); return points; // Computes the percentage for each student for a particular section. public static double[] computegrades(int[] points) { double[] grades = new double[5]; for (int i = 0; i < points.length; i++) { grades[i] = * points[i] / 20.0; return grades; Copyright 2009 by Pearson Education 21

22 File input/output reading: Copyright 2009 by Pearson Education 22

23 Prompting for a file name We can ask the user to tell us the file to read. The file name might have spaces: use nextline() // prompt for the file name Scanner console = new Scanner(System.in); System.out.print("Type a file name to use: "); String filename = console.nextline(); Scanner input = new Scanner(new File(filename)); What if the user types a file name that does not exist? Copyright 2009 by Pearson Education 23

24 Fixing file-not-found issues File objects have an exists method we can use: Scanner console = new Scanner(System.in); System.out.print("Type a file name to use: "); String filename = console.nextline(); File file = new File(filename); while (!file.exists()) { System.out.print("File not found! Try again: "); String filename = console.nextline(); file = new File(filename); Scanner input = new Scanner(file); // open the file Output: Type a file name to use: hourz.text File not found! Try again: h0urz.txt File not found! Try again: hours.txt Copyright 2009 by Pearson Education 24

25 Output to files PrintStream: An object in the java.io package that lets you print output to a destination such as a file. System.out is also a PrintStream. Any methods you have used on System.out (such as print, println) will work on every PrintStream. Do not open a file for reading (Scanner) and writing (PrintStream) at the same time. You could overwrite your input file by accident! The result can be an empty file (size 0 bytes). Copyright 2009 by Pearson Education 25

26 Printing to files, example Printing into an output file, general syntax: PrintStream name = new PrintStream(new File("file name"));... If the given file does not exist, it is created. If the given file already exists, it is overwritten. PrintStream output = new PrintStream(new File("output.txt")); output.println("hello, file!"); output.println("this is a second line of output."); Can use similar ideas about prompting for file names here. Copyright 2009 by Pearson Education 26

27 PrintStream question Modify our previous Sections program to use a PrintStream to output to the file section_output.txt. Section #1: Sections attended: [9, 6, 7, 4, 3] Student scores: [20, 18, 20, 12, 9] Student grades: [100.0, 90.0, 100.0, 60.0, 45.0] Section #2: Sections attended: [6, 7, 5, 6, 4] Student scores: [18, 20, 15, 18, 12] Student grades: [90.0, 100.0, 75.0, 90.0, 60.0] Section #3: Sections attended: [5, 6, 5, 7, 6] Student scores: [15, 18, 15, 20, 18] Student grades: [75.0, 90.0, 75.0, 100.0, 90.0] Copyright 2009 by Pearson Education 27

28 PrintStream answer // Section attendance // This version uses a PrintStream for output. import java.io.*; import java.util.*; public class Sections { public static void main(string[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("sections.txt")); PrintStream out = new PrintStream(new File("section_output.txt")); while (input.hasnextline()) { // process one section String line = input.nextline(); int[] attended = countattended(line); int[] points = computepoints(attended); double[] grades = computegrades(points); results(attended, points, grades, out); // Produces all output about a particular section. public static void results(int[] attended, int[] points, double[] grades, PrintStream out) { out.println("sections attended: " + Arrays.toString(attended)); out.println("sections scores: " + Arrays.toString(points)); out.println("sections grades: " + Arrays.toString(grades)); out.println();... Copyright 2009 by Pearson Education 28