Week 4 Counter V Sentinel Loops.pdf

Counter Controlled vs Sentinel Controlled Loops Counter controlled repetition • A class of ten students take a test. The grades are in range 0 to 100. Determine the average mark for the test • The average is the sum of the grades divided by the number of students • Use counter controlled repetiti...

Counter Controlled vs Sentinel Controlled Loops Counter controlled repetition • A class of ten students take a test. The grades are in range 0 to 100. Determine the average mark for the test • The average is the sum of the grades divided by the number of students • Use counter controlled repetition to input the grades one at a time Counter controlled repetition • Use a variable called counter to specify the number of times a set of statements should execute • Repetition terminates when counter exceeds 10 • Normally we would use a for loop for this – but we have not looked at for loops yet • total is a variable used to accumulate the sum of a series of values. • counter is a variable used to count – in this case – used to count the number of grades entered – counter is used to control the number of repetitions Pseudocode SET total to zero SET counter to 1 WHILE counter is less than or equal to 10 INPUT next grade ADD the grade to the total ADD one to counter CALC average ( total divided by 10) DISPLAY average Sample program 1 – find average of 10 grades //program to calculate the average of 10 grades import java.util.*; public class AverageGrade { public static void main(String[] args) { Scanner keyboardIn = new Scanner(System.in); //declare variables int counter = 1, total = 0, grade; double average; //read in 10 grades while(counter <= 10) //while counter is less than 11 { System.out.print("Enter grade no " +counter +": " ); grade = keyboardIn.nextInt(); total = total + grade; ///add current grade to total counter++; } //end while //calc average average = (double)total/10; //note use of cast System.out.println("Average grade is } //end main } //end class " +average); Counter v Sentinel • In counter controlled repetition a control variable is used to count the number of times a group of instructions should be repeated • A sentinel value can be used to terminate the repetition when it is not known in advance how many times a set of statements will be repeated. • The sentinel value is used to indicate the end of the input data. Sentinel controlled repetition • Develop a class averaging program that will process an arbitrary number of grades each time the program is run • In this example there is no indication of how many grades are to be entered • One solution is to use a special value called a sentinel value to indicate “end of data entry”. Sentinel controlled repetition • User enters all grades and then enters the sentinel value to indicate that the last grade has been entered. • The sentinel value must be chosen so that it cannot be confused with an acceptable input value. • In this example grades will be in range 0 to 100 so –1 is an acceptable sentinel value pseudocode SET total to 0 SET counter to 0 INPUT first grade WHILE user has not entered sentinel ADD grade to running total ADD one to counter ///why do we still need counter? INPUT next grade (possibly sentinel) CALC average (total divided by counter) DISPLAY average //sentinal controlled loop import java.util.*; public class AverageGradeSentinalEg { public static void main(String[] args) { Scanner keyboardIn = new Scanner(System.in); //declare variables int counter = 0, total = 0, grade; double average; System.out.print("Enter grade no "+(counter+1) +": " ); grade = keyboardIn.nextInt(); //read in grades while(grade!= -1) { total = total + grade; ///add current grade to total counter++; //keep track of no of grades – WHY? System.out.print("Enter grade #"+(counter+1) +": " ); grade = keyboardIn.nextInt(); } //end while //calc average average = (double)total/counter; //note use of cast System.out.println("Average grade is } //end main } //end class " +average); Sentinel Example 2 • Use Sentinel value to allow a user to confirm if they want to continue playing a game or quit • Y to continue. N to quit • Counter controlled loop is no use as we don’t know how many games user wants to continue playing //sentinal controlled loop example 2 import java.util.*; public class GameSentinel { public static void main(String args[]) { Scanner keyboardIn = new Scanner(System.in); //declare variable char continueGame = 'Y'; while (continueGame == 'Y') //continue repeating if user enters { System.out.print("Enter Y to continue the game or N to quit:"); continueGame = keyboardIn.next().charAt(0); }//end while System.out.print("Game Over"); } //end main } //end class Y