Programming with Loops

When to Use a Loop  Whenever you have a repeated set of actions, you should consider using a loop.  For example, if you have to read in the exam scores for all students in the class, you should write code to read in just one score and repeat it for all students in the class.  If we needed the sum of all the scores we could add each new score as it was entered to a variable containing the sum.

Initializing Statements  An initializing statement just sets a beginning or initial value for a variable to be used later.  If we don’t initialize a variable before it is used, we can’t be sure what value it contains.  For example, in order to be able to sum values into a variable, that variable must be initialized to zero before we begin.

Initializing Statements (cont’d)  You don’t always initialize variables to zero.  For example, for (count = 1,; count <= n; count++ ) { Read a number into the variable next. product = product * next; }

Ending a Loop  If you know the number of iterations required before entering the loop you can use a for loop. Such loops are called count-controlled loops.  One way to end the loop is to ask the user if it’s time to end the loop. This is called ask-before-iterating.  A third way to end a loop is by using a sentinel value.

Ending a Loop Using a Sentinel Value  A sentinel value is a special input value, different from valid input values, that is used to signal the end of the loop.  For example, if you are reading in exam scores and you don’t know how many you have, if you know that no score can be less that zero, you can use a negative value as the sentinel.  Your condition can just say to keep reading scores until you read a negative score. Then you’re done.

Example of Using a Sentinel Value to End a Loop System.out.println(“Enter scores for all students.”); System.out.println(“Enter a negative number after”); System.out.println(“you have entered the scores.”); Scanner keyboard = new Scanner(System.in); double max = keyboard.nextDouble(); double min = max;//The max and min so far are the first score. double next =keyboard.nextDouble(); while (next >= 0) { if (next > max) max = next; if (next < min) min = next; next = keyboard.nextDouble(); } System.out.println(“The highest score is “ + max); System.out.println(“The lowest score is “ + min);

Nested Loop Example do { System.out.println(); System.out.println(“Enter all the scores to be averaged.”); System.out.println(“Enter a negative number after”); System.out.println(“you have entered all the scores.”); sum = 0; numberOfStudents = 0; next = keyboard.nextDouble(); while (next >= 0) { sum = sum + next; numberOfStudents++ next = keyboard.nextDouble(); } if (numberOfStudents > 0) System.out.println(“The average is “ + (sum/numberOfStudents)); else System.out.println(“No scores to average.”); System.out.println(“Want to average another exam?”); System.out.println(“Enter yes or no.”); answer = keyboard.next(); }while (answer.equalsIgnoreCase(“yes”));

Loop Bugs  Programs with loops are more likely to have bugs than simpler programs.  The two most common bugs associated with loops are: Unintended infinite loops Off-by-one errors

Unintended Infinite Loops  If the condition tested to end the loop is incorrectly written, e.g. (a 0), the loop may never end if a starts off with a positive value and during each iteration a is increased.  Be aware that a loop may work correctly for some input values, but become an infinite loop for others. Running one test case may not be enough to insure correctness.

Off-By-One Bugs  These errors are the ones where your loop iterates one to many or one to few times. You may have written the condition as (number < 0) when you really meant (number <= 0).  To reduce the possibility of these errors you should always test at and near the values used in the condition controlling the loop.

Tracing Variables  If your program does not work correctly and it is not clear what the problem is, it is probably a good idea to trace some key variables. That is to watch what happens to them as they are changed when the program executes.  Many systems have built in utilities for tracing variables. If not you can trace them by printing them out from the program, or you can trace your algorithm by hand using pencil and paper.