COSC 236 Section 101 Computer Science 1 -- Prof. Michael A. Soderstrand

Transcription

1 COSC 236 Section 101 Computer Science 1 -- Prof. Michael A. Soderstrand

2 COSC 236 Web Site You will always find the course material at: or or From this site you can click on the COSC-236 tab to download the PowerPoint lectures, the Quiz solutions and the Laboratory assignments. 2

3 3

4 Review of Quiz 15 Quiz 15: This Quiz is aimed at using random numbers and printing out a list separated by commas using the fencepost algorithm discussed in Lecture 14. Write a complete program (class) that asks the user how many random numbers the user wants printed and then prints on a single line that number of random numbers. The random numbers will be integers in the range 2 through 12 inclusive. Each random number is separated by a comma and a space. Here is what typical output from your program will look like: NOTE: There should be no extra commas or spaces at the beginning or at the end of the list. 4

5 Review of Quiz 15 There are two issues addressed in Quiz 15: First is generating random integers between 2 and 12 inclusively The range is max min + 1 = = 11 The minimum number is 2 rand.nextint(11) + 2 The second is the fence-post algorithm to print out the random numbers with commas between each number Print one random number before the for loop and leading commas in the for loop Print following commas in the for loop and one random number after the for loop 5

6 Review of Quiz 15 import java.util.*; // for Scanner Boiler Plate public class Quiz15 { public static void main(string[] args) { Random rand = new Random(); Set up random numbers Scanner console = new Scanner(System.in); Set up scanner for input System.out.print("How many random numbers would you like? "); int number = console.nextint(); System.out.print(rand.nextInt(11) + 2); for (int n = 1; n <= number; n++) { System.out.print(", " + (rand.nextint(11) +2)); System.out.println(); Prompt and get input Print first fence post Print line of number preceded by comma and space Use println to go to next line at end of program 6

7 Review of Quiz 15 QUESTION about the random numbers generated in Quiz 15: Would these number represent the throw of two dice? The range is correct: min = 2, max = 12 The numbers are random Does rand.nextint(11) + 2 simulate the roll of two dice? NO: The numbers 2 through 12 are not equally likely! 7

8 Review of Quiz 15 QUESTION about the random numbers generated in Quiz 15: Would these number represent the throw of two dice? NO: The numbers 2 through 12 are not equally likely! rand.nextint(6) + rand.nextint(6) + 2 8

9 Chapter 6 File Processing (pp ) 6.1 File reading basics 6.2 Details of token based processing 6.3 Line based processing 6.4 Advanced file processing 6.5 Case study zip code lookup 9

10 Chapter 6 File Processing (pp ) In Chapter 3 we discussed how to construct a Scanner object to read input from the console. Now we will look at how to construct Scanner objects to read input from files. The idea is fairly straightforward, but Java does not make it easy to read from input files. This is unfortunate because many interesting problems can be formulated as fileprocessing tasks. Many introductory computer science classes have abandoned file processing altogether and left the topic for the second course because it is considered too advanced for novices. 10

11 Chapter 6 File Processing (pp ) There is nothing intrinsically complex about file processing Java was not designed for file processing The designers of Java have not been particularly eager to provide a simple solution. They did, however, introduce the Scanner class as a way to simplify some of the details associated with reading files. The result is that file reading is still awkward in Java But at least the level of detail is manageable. 11

12 Chapter 6 File Processing (pp ) Before we start writing file-processing programs, we have to explore some issues related to Java exceptions. Remember that exceptions are errors that halt the execution of a program. In the case of file processing: trying to open a file that doesn t exist trying to read beyond the end of a file 12

13 Chapter 6 File Processing (pp ) Types of files: Text files: Can be viewed and edited using simple text editors Binary files: Use internal format and require special software to read, write or view 13

14 Chapter 6 File Processing (pp ) Text Files 14

15 Chapter 6 File Processing (pp ) 15

16 Chapter 6 File Processing (pp ) // Report some basic information about a file. import java.io.*; // for File public class FileInfo { public static void main(string[] args) { File f = new File("Test.txt"); System.out.println("exists returns " + f.exists()); System.out.println("canRead returns " + f.canread()); System.out.println("length returns " + f.length()); System.out.println("getAbsolutePath returns " + f.getabsolutepath()); 16

17 Chapter 6 File Processing (pp ) 17

18 Chapter 6.1 File Reading Basics (pp ) Input/output (I/O) import java.io.*; Create a File object to get info about a file on your drive. (This doesn't actually create a new file on the hard disk.) File f = new File("example.txt"); if (f.exists() && f.length() > 1000) { f.delete(); Method name canread() delete() exists() getname() length() renameto(file) Description returns whether file is able to be read removes file from disk whether this file exists on disk returns file's name returns number of bytes in file changes name of file 18

19 Chapter 6.1 File Reading Basics (pp ) Reading files To read a file, pass a File when constructing a Scanner. Scanner name = new Scanner(new File("file name")); Example: File file = new File("mydata.txt"); Scanner input = new Scanner(file); or (shorter): Scanner input = new Scanner(new File("mydata.txt")); 19

20 Chapter 6.1 File Reading Basics (pp ) File paths absolute path: specifies a drive or a top "/" folder C:/Documents/smith/hw6/input/data.csv Windows can also use backslashes to separate folders. relative path: does not specify any top-level folder names.dat input/kinglear.txt Assumed to be relative to the current directory: Scanner input = new Scanner(new File("data/readme.txt")); If our program is in H:/hw6, Scanner will look for H:/hw6/data/readme.txt 20

21 Chapter 6.1 File Reading Basics (pp ) Compiler error w/ files import java.io.*; import java.util.*; // for File // for Scanner public class ReadFile { public static void main(string[] args) { Scanner input = new Scanner(new File("data.txt")); String text = input.next(); System.out.println(text); The program fails to compile with the following error: ReadFile.java:6: unreported exception java.io.filenotfoundexception; must be caught or declared to be thrown Scanner input = new Scanner(new File("data.txt")); ^ 21

22 Chapter 6.1 File Reading Basics (pp ) Exceptions exception: An object representing a runtime error. dividing an integer by 0 calling substring on a String and passing too large an index trying to read the wrong type of value from a Scanner trying to read a file that does not exist We say that a program with an error "throws" an exception. It is also possible to "catch" (handle or fix) an exception. checked exception: An error that must be handled by our program (otherwise it will not compile). We must specify how our program will handle file I/O failures. 22

23 Chapter 6.1 File Reading Basics (pp ) The throws clause throws clause: Keywords on a method's header that state that it may generate an exception (and will not handle it). Syntax: public static type name(params) throws type { Example: public class ReadFile { public static void main(string[] args) throws FileNotFoundException { Like saying, "I hereby announce that this method might throw an exception, and I accept the consequences if this happens." 23

24 Chapter 6.1 File Reading Basics (pp ) import java.io.*; // for File import java.util.*; // Java utilities public class FileRead { public static void main(string[] args) throws FileNotFoundException { File f = new File("Test.txt"); Scanner input = new Scanner(f); System.out.println("exists returns " + f.exists()); System.out.println("canRead returns " + f.canread()); System.out.println("length returns " + f.length()); System.out.println("getAbsolutePath returns " + f.getabsolutepath()); int count = 0; while (input.hasnext()) { String word = input.next(); count++; System.out.println("total words = " + count); 24

25 Chapter 6.1 File Reading Basics (pp ) 25

26 Chapter 6.2 Details of Token-Based Processing (pp ) Input tokens token: A unit of user input, separated by whitespace. A Scanner splits a file's contents into tokens. If an input file contains the following: "John Smith" The Scanner can interpret the tokens as the following types: Token Type(s) 23 int, double, String 3.14 double, String "John String Smith" String 26

27 Chapter 6.2 Details of Token-Based Processing (pp ) Files and input cursor Consider a file weather.txt that contains this text: A Scanner views all input as a stream of characters: \n \n\n \n ^ input cursor: The current position of the Scanner. 27

28 Chapter 6.2 Details of Token-Based Processing (pp ) Consuming tokens consuming input: Reading input and advancing the cursor. Calling nextint etc. moves the cursor past the current token \n \n\n \n ^ double d = input.nextdouble(); // \n \n\n \n ^ String s = input.next(); // "23.5" \n \n\n \n ^ 28

29 Chapter 6.2 Details of Token-Based Processing (pp ) File input question Recall the input file weather.txt: Write a program that prints the change in temperature between each pair of neighboring days to 23.5, change = to 19.1, change = to 7.4, change = to 22.8, change = to 18.5, change = to -1.8, change = to 14.9, change =

30 Chapter 6.2 Details of Token-Based Processing (pp ) // Displays changes in temperature from data in an input file. import java.io.*; // for File import java.util.*; // for Scanner public class Temperatures { public static void main(string[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("weather.txt")); double prev = input.nextdouble(); // fencepost for (int i = 1; i <= 7; i++) { double next = input.nextdouble(); System.out.println(prev + " to " + next + ", change = " + (next - prev)); prev = next; 30

31 Chapter 6.2 Details of Token-Based Processing (pp ) Reading an entire file Suppose we want our program to work no matter how many numbers are in the file. Currently, if the file has more numbers, they will not be read. If the file has fewer numbers, what will happen? A crash! Example output from a file with just 3 numbers: 16.2 to 23.5, change = to 19.1, change = -4.4 Exception in thread "main" java.util.nosuchelementexception at java.util.scanner.throwfor(scanner.java:838) at java.util.scanner.next(scanner.java:1347) at Temperatures.main(Temperatures.java:12) 31

32 Chapter 6.2 Details of Token-Based Processing (pp ) Scanner exceptions NoSuchElementException You read past the end of the input. InputMismatchException You read the wrong type of token (e.g. read "hi" as an int). Finding and fixing these exceptions: Read the exception text for line numbers in your code (the first line that mentions your file; often near the bottom): Exception in thread "main" java.util.nosuchelementexception at java.util.scanner.throwfor(scanner.java:838) at java.util.scanner.next(scanner.java:1347) at MyProgram.myMethodName(MyProgram.java:19) at MyProgram.main(MyProgram.java:6) 32

33 Chapter 6.2 Details of Token-Based Processing (pp ) Scanner tests for valid input Method hasnext() hasnextint() hasnextdouble() Description returns true if there is a next token returns true if there is a next token and it can be read as an int returns true if there is a next token and it can be read as a double These methods of the Scanner do not consume input; they just give information about what the next token will be. Useful to see what input is coming, and to avoid crashes. These methods can be used with a console Scanner, as well. When called on the console, they sometimes pause waiting for input. 33

34 Chapter 6.2 Details of Token-Based Processing (pp ) Using hasnext methods Avoiding type mismatches: Scanner console = new Scanner(System.in); System.out.print("How old are you? "); if (console.hasnextint()) { int age = console.nextint(); // will not crash! System.out.println("Wow, " + age + " is old!"); else { System.out.println("You didn't type an integer."); Avoiding reading past the end of a file: Scanner input = new Scanner(new File("example.txt")); if (input.hasnext()) { String token = input.next(); // will not crash! System.out.println("next token is " + token); 34

35 Chapter 6.2 Details of Token-Based Processing (pp ) File input question 2 Modify the temperature program to process the entire file, regardless of how many numbers it contains. Example: If a ninth day's data is added, output might be: 16.2 to 23.5, change = to 19.1, change = to 7.4, change = to 22.8, change = to 18.5, change = to -1.8, change = to 14.9, change = to 16.1, change =

36 Chapter 6.2 Details of Token-Based Processing (pp ) // Displays changes in temperature from data in an input file. import java.io.*; // for File import java.util.*; // for Scanner public class Temperatures { public static void main(string[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("weather.txt")); double prev = input.nextdouble(); // fencepost while (input.hasnextdouble()) { double next = input.nextdouble(); System.out.println(prev + " to " + next + ", change = " + (next - prev)); prev = next; 36

37 Chapter 6.2 Details of Token-Based Processing (pp ) File input question 3 Modify the temperature program to handle files that contain non-numeric tokens (by skipping them). For example, it should produce the same output as before when given this input file, weather2.txt: Tuesday 19.1 Wed 7.4 THURS. TEMP: <-- Marty here is my data! --Kim 14.9 :-) You may assume that the file begins with a real number. 37

38 Chapter 6.2 Details of Token-Based Processing (pp ) // Displays changes in temperature from data in an input file. import java.io.*; // for File import java.util.*; // for Scanner public class Temperatures2 { public static void main(string[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("weather.txt")); double prev = input.nextdouble(); // fencepost while (input.hasnext()) { if (input.hasnextdouble()) { double next = input.nextdouble(); System.out.println(prev + " to " + next + ", change = " + (next - prev)); prev = next; else { input.next(); // throw away unwanted token 38

39 Chapter 6.2 Details of Token-Based Processing (pp ) Election question Write a program that reads a file poll.txt of poll data. Format: State Obama% McCain% ElectoralVotes Pollster CT Oct U. of Connecticut NE Sep Rasmussen AZ Oct Northern Arizona U. The program should print how many electoral votes each candidate leads in, and who is leading overall in the polls. Obama : 214 votes McCain: 257 votes 39

40 Chapter 6.2 Details of Token-Based Processing (pp ) // Computes leader in presidential polls, based on input file such as: // AK Oct Ivan Moore Research import java.io.*; import java.util.*; // for File // for Scanner public class Election { public static void main(string[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("polls.txt")); int obamavotes = 0, mccainvotes = 0; while (input.hasnext()) { if (input.hasnextint()) { int obama = input.nextint(); int mccain = input.nextint(); int evotes = input.nextint(); if (obama > mccain) { obamavotes = obamavotes + evotes; else if (mccain > obama) { mccainvotes = mccainvotes + evotes; else { input.next(); // skip non-integer token System.out.println("Obama : " + obamavotes + " votes"); System.out.println("McCain: " + mccainvotes + " votes"); 40

41 Chapter 6.2 Details of Token-Based Processing (pp ) Hours question Given a file hours.txt with the following contents: 123 Kim Eric Stef Consider the task of computing hours worked by each person: Kim (ID#123) worked 31.4 hours (7.85 hours/day) Eric (ID#456) worked 36.8 hours (7.36 hours/day) Stef (ID#789) worked 39.5 hours (7.9 hours/day) Let's try to solve this problem token-by-token... 41

42 Chapter 6.2 Details of Token-Based Processing (pp ) Hours answer (flawed) // This solution does not work! import java.io.*; import java.util.*; // for File // for Scanner public class HoursWorked { 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)"); 42

43 Chapter 6.2 Details of Token-Based Processing (pp ) 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. 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 is a hybrid approach: First, break the overall input into lines. Then break each line into tokens. 43

44 Assignments for this week 1. Laboratory for Chapter 5 due TODAY (Monday 10/27) IMPORTANT: When you me your laboratory Word Document, be sure it is all in one file 2. Laboratory for Chapter 6 due NEXT MONDAY (Monday 11/3) IMPORTANT: When you me your laboratory Word Document, be sure it is all in one file 3. Read pp (Chapter 6) for Wednesday 4/9 4. Be sure to complete Quiz 16 before leaving class tonight This is another program to write You must demonstrate the program to me before you leave lab 44