Chapter 5 – Part 3 Conditionals and Loops

© 2004 Pearson Addison-Wesley. All rights reserved2/19 Outline The if Statement and Conditions Other Conditional Statements Comparing Data The while Statement Other Repetition Statements Iterators

© 2004 Pearson Addison-Wesley. All rights reserved3/19 The do Statement A do statement has the following syntax: do { statement; } while ( condition ) The statement is executed once initially, and then the condition is evaluated The statement is executed repeatedly until the condition becomes false

© 2004 Pearson Addison-Wesley. All rights reserved4/19 Logic of a do Loop true condition evaluated statement false Note: no ‘pretest.’ Statement will at least be executed on time!

© 2004 Pearson Addison-Wesley. All rights reserved5/19 The do Statement An example of a do loop: The body of a do loop executes at least once See ReverseNumber.javaReverseNumber.java int count = 0; do { count++; System.out.println (count); } while (count < 5);

© 2004 Pearson Addison-Wesley. All rights reserved6/19 Comparing while and do statement true false condition evaluated The while Loop true condition evaluated statement false The do Loop

© 2004 Pearson Addison-Wesley. All rights reserved7/19 The for Statement – VIP! A for statement has the following syntax: Remember: every loop has some kind of initialization, test to continue/end the loop, and a body of the loop. for ( initialization ; condition ; increment ) statement; The initialization is executed once before the loop begins The statement is executed until the condition becomes false The increment portion is executed at the end of each iteration

© 2004 Pearson Addison-Wesley. All rights reserved8/19 Logic of a for loop statement true condition evaluated false increment initialization Does initializing, pretest, increment and posttest …..

© 2004 Pearson Addison-Wesley. All rights reserved9/19 The for Statement A for loop is functionally equivalent to the following while loop structure: initialization; while ( condition ) { statement; increment; } Know how to write the same functionality in ALL loops. For example, I will give you an exercise and require you to write the solution using a do…while, while loop, and a for loop.

© 2004 Pearson Addison-Wesley. All rights reserved10/19 The for Statement An example of a for loop: for (int count=1; count <= 5; count++) System.out.println (count); The initialization section can be used to declare a variable Like a while loop, the condition of a for loop is tested prior to executing the loop body Therefore, the body of a for loop will execute zero or more times

© 2004 Pearson Addison-Wesley. All rights reserved11/19 The for Statement The increment section can perform any calculation A for loop is well suited for executing statements a specific number of times that can be calculated or determined in advance Used a lot where ‘counting’ is needed See Multiples.javaMultiples.java See Stars.java next slide.Stars.java for (int num=100; num > 0; num -= 5) System.out.println (num);

© 2004 Pearson Addison-Wesley. All rights reserved12/19 public class Stars { public static void main (String[] args) { final int MAX_ROWS = 10; for (int row = 1; row <= MAX_ROWS; row++) { for (int star = 1; star <= row; star++) System.out.print (“*”); System.out.println(); } // end for } // end main() } // end class Here we have an example of nested-for loops.

© 2004 Pearson Addison-Wesley. All rights reserved13/19 The for Statement  Each expression in the header of a for loop is optional If the initialization is left out, no initialization is performed  (This had better be performed outside / prior to the loop) If the condition is left out, it is always considered to be true, and therefore creates an infinite loop  But the loop must terminate. So some specific code within the loop had better constitute a test so that the loop may terminate. If the increment is left out, no increment operation is performed  Again, if left out, something within the body of the loop had better increment or adjust the increment/decrement counter.  You may omit these at your own peril. It is actually okay, but you’d better be VERY careful!!!!!

© 2004 Pearson Addison-Wesley. All rights reserved14/19 Iterators An iterator is an object that allows you to process a collection of items one at a time It lets you step through each item in turn and process it as needed hasNext methodAn iterator object has a hasNext method that returns true if there is at least one more item to process The next method returns the next item Iterator objects are defined using the Iterator interface, which is discussed further in Chapter 6

© 2004 Pearson Addison-Wesley. All rights reserved15/19 Iterators Several classes in the Java standard class library are iterators The Scanner class is an iterator  the hasNext method returns true if there is more data to be scanned  the next method returns the next scanned token as a string The Scanner class also has variations on the hasNext method for specific data types (such as hasNextInt ) Essentially, an iterator is an object that points to a collection of objects. Within the iterator are methods that the service provider provides, such as hasNext(), next() and others as appropriate. The iterator object’s methods allow a client to process a collection of objects without getting into the collection methods themselves… That is, the iterator’s methods are data independent. Will explain more later. Very useful for processing arrays, linked lists, and more.

© 2004 Pearson Addison-Wesley. All rights reserved16/19 Iterators – I couldn’t resist… Example: I have stored a collection of numbers in an ‘array.’ I want to add all of them up or develop the ‘average.’ So, I must first see if there is another number, then fetch it, then add it, and repeat until there are no more numbers. To do this, I must initialize a variable to accumulate the sum and then do all above. If an iterator is defined on this collection of numbers, I don’t have to worry about if there are more, indexing to point to the next one, fetch it, add it and continue. I only need to determine if there are more (hasNext()) and issue a next() and add. I don’t have to worry about what the data looks like (integers, floats, objects, etc.). The iterator, properly designed, will accommodate all this for me. Sample: int [ ] myGrades = {92, 85, 72, 98, 80}; Interpret as: myGrades [0] = 92; myGrades [1] = 85, etc…..

© 2004 Pearson Addison-Wesley. All rights reserved17/19 Iterators The fact that a Scanner is an iterator is particularly helpful when reading input from a file Suppose we wanted to read and process a list of URLs stored in a file One scanner can be set up to read each line of the input until the end of the file is encountered Another scanner can be set up for each URL to process each part of the path See URLDissector.java on the next slide.URLDissector.java

© 2004 Pearson Addison-Wesley. All rights reserved18/19 // URLDissector.java Author: Lewis/Loftus import java.util.Scanner; import java.io.*; public class URLDissector { // Reads urls from a file and prints their path components. public static void main (String[] args) throws IOException { String url; Scanner fileScan, urlScan; // Here we have two Scanner references to objects to be created. // so far, we’ve only read from the keyboard; here, we read from a file fileScan = new Scanner (new File ("urls.input")); // creates Scnner object which points to file named urls.input… // Read and process each line of the file while (fileScan.hasNext()) // as long as there is one more ‘record’, we will continue… // hasNext() is an inherited method that all Scanner objects have. This is how the Scanner //class is defined – it already contains these methods… { url = fileScan.nextLine(); // retrieves next string from urls.inp file. System.out.println ("URL: " + url); // prints it. urlScan = new Scanner (url); // Here we create new Scanner object; feed it String retrieved // from the most recent fileScan (three lines above) urlScan.useDelimiter("/"); // Print each part of the url while (urlScan.hasNext()) System.out.println (" " + urlScan.next()); System.out.println(); }// end outer while } // end main() } //end class URLDissector much more later….

© 2004 Pearson Addison-Wesley. All rights reserved19/19 Iterators and for Loops Recall that an iterator is an object that allows you to process each item in a collection A variant of the for loop simplifies the repetitive processing the items For example, if BookList is an iterator that manages Book objects, the following loop will print each book: array of Books for (Book myBook : BookList) System.out.println (myBook); Very important concept. Will hit this harder later. But learn the basics here.

© 2004 Pearson Addison-Wesley. All rights reserved20/19 Iterators and for Loops This style of for loop can be read "for each Book in BookList, …" Therefore the iterator version of the for loop is sometimes referred to as the foreach loop It eliminates the need to call the hasNext and next methods explicitly It also will be helpful when processing arrays, which are discussed in Chapter 7

© 2004 Pearson Addison-Wesley. All rights reserved21/19 Questions I like to ask… Write a loop to add the first 100 positive integers (1-100) using  A while loop  A for loop  A do…while loop Do all loops test first? If so, agree; if not, who?? Which loop is best for testing some general boolean condition? Which loop is best for counting? Which loop will always execute first before testing?