Input and Output and Arrays Robots Learning to Program with Java - PowerPoint PPT Presentation

Input and Output and Arrays Robots Learning to Program with Java Byron Weber Becker chapter 9 and 10

Announcements (Nov 7 th )  Reading for next Monday  Ch 10 ( not covering 10.1.8, 10.7)  Program#7  Clarification  Driver  Exam#2 scheduled for Nov 9 th (Wed 8PM)  Sign-up for make-up exam (talk to the instructor by Monday)

Exam#2  Change of room  The 168 exam 2 next week will be in CVA ( Center for the Visual Arts) 149 instead of STV 101.  http://maps.illinoisstate.edu/locations/center_for_the_vis ual_arts_cva.shtml  Coverage (practically comprehensive)  ~ program6, ~ Ch7 (including console I/O using Scanner, Sytem.in, and System.out.print*), ~ Lab10  few questions (one or two) related to Robot class  Main focus: control structure, write your own classes , inheritance

Facebook using Java?  Let’s start with “Person” class  “String” name  Four “Person” objects to represent four friends  Implement toString()

Lab example  Person class

Review: Some Scanner Methods  String next () Finds and returns the next complete token from this scanner.  double nextDouble () Scans the next token of the input as a double – will produce an error if the token is not numeric.  int nextInt () Scans the next token of the input as an int – will produce an error if the token is not numeric.  String nextLine () Advances this scanner past the next line return and returns the data that was skipped.

Review: Some Scanner Predicates  boolean hasNext () Returns true if this scanner has another token in its input stream.  boolean hasNextDouble () Returns true if the next token in this scanner's input stream can be interpreted as a double value using nextDouble()  boolean hasNextInt () Returns true if the next token in the input stream can be interpreted as an integer value using nextInt()  boolean hasNextLine () Returns true if there is another line return in the input stream of this scanner.

Dumping the Input Buffer  When you combine the use of nextLine() and any of the other 3 next methods you must take care with new line characters.  Sometimes you need to dump the input buffer.  This can be done using scan.nextLine();

The Input Buffer  The input buffer holds the characters that are being read – whether from the keyboard or from a file.  In the Scanner class, next(), nextInt, and nextDouble read up to or over any new line character (ignoring it).  The nextLine() method reads through the next new line character and stops at the front of the next line. The new line character is dropped, not returned.

Practice**  Write a main method that will allow the user to enter the names (storing the full name) and ages of 5 students.  Assume we have a Student class with a constructor that takes a String and int: public Student(String name, int age)  Collect and store the data in 5 Student variables.

Review: File Reading  The Scanner class can be used to read from a file (instead of the keyboard) with a one small change.  Instead of declaring our Scanner as Scanner scan = new Scanner(System.in); We will declare it with a file.  First we create the File using File file = new File(“input.txt”); Scanner fScan = new Scanner(file);

Same Old Methods  Whether reading from a file or from a keyboard, the Scanner methods work the same.  The biggest difference is that from a file you have no user prompts…

Practice  Write a main method that will read the file below (alternating lines are integer or text) and echo it to the screen  Use nextLine to read the text lines and nextInt for lines with numbers only  File name is input.txt 1 this is a text line 2 this is another text line 3 this is the last text line

Writing to a File  There are a number of ways to write to a file  We will use a PrintWriter object  Once the file has been opened, the PrintWriter object works just like using System.out

Exception handeling  The try-catch block try { // attempt to do something that might error out } catch (ExeptionType excp) { // print some error message for easy debugging }

Opening a File for Output PrintWriter out = null; try { out = new PrintWriter(“fileName.txt"); System. out .println("file created and open"); } catch (FileNotFoundException excp) { System. out .println(" fileName.txt could not be opened for output"); excp.printStackTrace(); }

Practice** • Change the last practice problem so it first reads 2 lines from the input file, then switches their order to create an output file  File name is input.txt  File name is output.txt 1 this is a text line this is a text line 1 2 this is another text line this is another text line 2 3 this is the last text line this is the last text line 3

More Practice**  Write a main method that requests data from the user and writes it to a file – one data element per line.  The data will be  A person’s full name – which we will store in all uppercase letters regardless of how it is entered  A single word nickname  Their current age (as an integer)

Constructor that takes a Scaner**  When you are creating a class that will be read from a file it is typical to create a constructor that accepts a Scanner and reads a single record from an opened file  The constructor will NOT open or close the file – simply read the next record

Method that takes a Scanner  Instead of using the constructor you can create a method for reading a single record  When using such a method you must take care not to create aliases

Writing a Record  Regardless of which version of reading you use (in a constructor or a method) you need to create a method for writing a single record to an open file  This is basically the opposite of the reading method and should create a record in the same format  This method will not open or close the file

Tracing Practice Scanner infile = new Scanner(new File(“input.txt”)); int val1, val2; int curVal = infile.nextInt(); val1 = curVal; val2 = curVal; while (infile.hasNext()) { curVal = infile.nextInt(); if (val1 < curVal) { val1 = curVal; } else if (val2 > curVal) { val2 = curVal; } } System.out.println(“Val1: ” + val1 + “\nVal2: ” + val2);

Pseudocode Practice  Write pseudocode to read a file that contains course id, room capacity, and current enrollment, in that order, on each line of the file.  Read in each row from the file and display the data.  In addition, give the number of seats available and a message stating whether or not the class is full.  Process the entire file.  Example file: Itk168 40 27 Itk261 25 25 Itk150 220 219

Coding Practice  Write code to read integers from the keyboard and count the number of odd integers entered.  Input should stop when the user enters -1 .  Do not count the -1 as one of the odd integers that you’re counting.  Prompt the user for input.  Be sure to declare all needed variables.

Arrays ITK 168 Spring 2011 Robots Learning to Program with Java Byron Weber Becker chapter 10

Chapter Objectives  Store data in an array  Primatives  Objects  Access a single element  Process all elements  Search for a particular element  Insert element

Containers  There are a number of “containers” used in programming  An Array is one type of container  Can contain primitive or objective data  Elements are stored consecutively and numbered – starting at zero and counting to one less than the capacity  Has a maximum capacity and current size

Declaring and Filling Arrays  Declaring an array with no initial elements int[] integers = new int[5]; double[] prices = new double[10]; Person[] people = new Person[25];  Filling an array at declaration int[] numbers = {1, 2, 3, 4, 5};

Walking Through an Array  Suppose we have an array of up to 100 integers (of unknown contents) originally declared as int[] scores = new int[100]; int numRecords = 0;  Suppose we want to  Print the contents  Average the contents  Print the largest and smallest value  Find the first location of a value

For-each  Typically used to walk through the entire array – assumes the array has been filled completely  Take care with arrays of objects! for(Person person : addressBook) { System. out .println(person); }

Practice  Declare an array of integers to hold up to 100 scores  Code a loop to get scores from the user until they enter -99 to quit  Be sure not to process -99  Print the scores in 4 columns across the rows  Print the highest score, lowest score, and average

More Practice  Declare an array to hold up to 10 characters  Get the 10 characters from your user – requiring all 10 to be entered  Print the characters on one row in the order entered  Print the characters on one row in the opposite order entered  Look for the letter ‘s’ – if entered, return the first index location, if not return -1