1. COMP 14 Introduction to Programming Miguel A. Otaduy May 20, 2004

2. Writing Selection Statements What Type of Construct To Use? Only need to execute additional statements if condition is true Need to execute separate additional statements based on if the condition is true or false Need to execute different statements based on multiple conditions Need to execute different statements based on an expression that evaluates to a char or int if if-else nested if-else switch

3. Writing Selection Statements Write the outline of the selection statement –keywords –curly braces –case and break statements (for switch ) Write the expression –boolean expression, or condition –expression or variable evaluating to char or int Write statements that execute based on the condition

4. Example Write a selection statement that sets grade to 'F' if score is less than 60 and sets grade to 'P' if score is greater than or equal to 60. 1) Choose construct if-else 2) Write outline if ( ) { } else { }

5. Example Write a selection statement that sets grade to 'F' if score is less than 60 and sets grade to 'P' if score is greater than or equal to 60. 3) Add condition if (score < 60) { } else { } 4) Add statements if (score < 60) { grade = 'F'; } else { grade = 'P'; }

6. Questions What type of selection statement should be used? 1. Print "Male" if gender is 'M' and "Female" if gender is 'F'. 2. Print the state associated with areaCode, where areaCode is an int. 3. Print the maximum of three integers. if-else switch nested if-else

7. Question Assume that the lengths of 3 sides of a triangle are given in the integer variables side1, side2, and side3. Write code that will print: –"Equilateral" if all three sides are equal –"Isosceles" if only two sides are equal –"Scalene" if no sides are equal equilateral isosceles scalene First, what selection construct should be used? nested if-else

8. Answer // side1, side2, and side3 are lengths // of the sides of a triangle if (side1 != side2 && side2 != side3 && side1 != side3) // no sides are equal System.out.println ("Scalene"); else if (side1 == side2 && side2 == side3) // all sides are equal System.out.println ("Equilateral"); else // two sides are equal System.out.println ("Isosceles");

9. Today Repetition statements while loops for loops

10. Why Is Repetition Needed? Want to add 5 integers to find their average? –declare a variable for each integer –initialize the sum –read in the user input one at a time –sum the numbers –take the average Want to add 1000 integers to find their average? repeat steps 3 and 4 1000 times 1. declare one variable for input 2. initialize the sum 3. read in one line of user input 4. add to the sum 5. take the average

11. Repetition Statements Allow us to execute a statement multiple times Often referred to as loops Controlled by boolean expressions –like selection, or conditional, statements Java has three kinds of repetition statements: –the while loop –the for loop –the do loop

12. Loops Must use a loop control variable –controls how many times to loop 4 Parts to Every Loop –initialization - set loop control variable before condition –condition - when to stop –update - change the loop control variable –body - actions to repeat

13. Typical Uses of Loops Repeat a section of code a specified number of times - counter-controlled Repeat a section of code (reading input) until a specific value is read - sentinel-controlled Repeat a section of code (reading input) until a valid value is entered - input validation Repeat a section of code (reading from a file) until the end of the file is reached - EOF-controlled Repeat a section of code until a boolean variable becomes false - flag-controlled

14. The while Loop Syntax while ( condition ) { loop body; } while is a reserved word If the condition is true, the loop body is executed. Then the condition is evaluated again. The loop body is executed repeatedly until the condition becomes false.

15. The while Loop Syntax while (expression) statement Expression is always true in an infinite loop Statements must change value of expression to false

16. The while Loop Example final int LIMIT = 3; int count = 0; while (count < LIMIT) { System.out.println (count); count++; } System.out.println (“All done!”); boolean condition loop body update initialization Output: 0 1 2 All done!

17. The while Loop What's Going On? final int LIMIT = 3; int count = 0; while (count < LIMIT) { System.out.println (count); count++; } System.out.println (“All done!”); LIMIT count Output: 3 0 1 0 1 2 All done! 2 3

18. The while Loop final int LIMIT = 3; int count = 0; while (count < LIMIT) { count++; System.out.println (count); } System.out.println (“All done!”); Output: 1 2 3 All done!

19. The while Loop final int LIMIT = 3; int count = 0; while (count < LIMIT) { System.out.println (count); } System.out.println (“All done!”); Output: 0...

20. The while Loop final int LIMIT = 3; int count = 0; while (count <= LIMIT) { System.out.println (count); count++; } System.out.println (“All done!”); Output: 0 1 2 3 All done!

21. The while Loop If the condition of a while statement is false initially, the loop body is never executed The body of a while loop will execute zero or more times

22. Counter-Controlled while Loop Used when we know exactly the number of times to execute the loop Basic Form: counter = 0; while (counter < N) {... counter++;... } N is the number of times the loop should execute

23. Example final int NUM_STUDENTS = 100; int sum = 0; double average; int i = 0; // initialize loop control var while (i < NUM_STUDENTS) { sum += Integer.parseInt(inFile.readLine()); i++; // update loop control variable } average = (double) sum / NUM_STUDENTS; Often we use i, j, k as counter variable names.

24. Reading From a File How do you know how many items there will be? 1.ask the user 2.arrange the data file so that the first item is the number of items in the file Other ways to read all of the items: –use sentinel value –read until end-of-file (EOF)

25. Sentinel-Controlled while Loop Used when exact number of entry pieces is unknown but last entry (special / sentinel value) is known Basic Form: input the first data item into variable while (variable != sentinel) {... input a data item into variable }

26. Example int i = 0, sum = 0, score; final int SENTINEL = -99; double average; // initialize the loop control variable score = Integer.parseInt(inFile.readLine()); while (score != SENTINEL) { sum += score; // update the loop control variable score = Integer.parseInt(inFile.readLine()); } average = (double) sum / NUM_STUDENTS;

27. Average.java Example Read in integers and print their sum until the user enters 0 Print the average of the numbers entered

28. Input Validation while Loop Used to ensure that the user enters valid input Basic Form: input the first data item into variable while (variable is not in valid range) { ask the user for valid input input a data item into variable }

29. Example int num; // initialize loop control variable System.out.print (“Enter a number [0-100]: "); num = Integer.parseInt(keyboard.readLine()); while ((num 100)) { System.out.println (num + " is not [0-100]"); System.out.print (“Enter a number: “); // update the loop control variable num = Integer.parseInt(keyboard.readLine()); } System.out.println (“You entered “ + num);

30. Flag-Controlled while Loop Boolean value used to control loop Basic Form: boolean found = false; while (!found) {... if(expression) found = true;... }

31. EOF-Controlled while Loop Used when input is from a file Sentinel value is not always appropriate Basic Form: inputLine = inFile.readLine(); while (inputLine != null) {... inputLine = inFile.readLine(); } null is a reserved word

32. Average.java Example Modify example to read from a file

33. To do Finish assignment 3 Bring laptop. Study for Quiz! –Writing a whole program –Variables and classes –String, StringTokenizer –GUI –Relational and boolean operators –if, if-else, switch while, for loops  not in the quiz!!!