# Introduction to Computers and Programming Lecture 8: More Loops New York University.

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Introduction to Computers and Programming Lecture 8: More Loops New York University

Road Map Sentinels Sentinel Controlled Loops Case Study: Sentinel Controlled Loop Case Study: Nested Control Structures do/while Loop Summary of looping covered so far Reading –Chapter 3: 3.3.1, 3.3.2

review What are three ways to rewrite: x = x + 1; What are the three elements of a while loop? What is the difference between pre and post increment operators? Are you guaranteed to execute the body of a while loop at all? What is an infinite loop? True or False: You never want to use an infinite loop?

4 Review Given: char c = 'a'; c++; What is the value of variable c? What method do you use to extract characters from a String? –How would you use that method to get the first character from a String? How are characters stored in memory? What is the difference between the character '0' and the integer 0?

Sentinels

Sentinel-controlled repetition Counter Controlled Repetition: –Simply means that we use a counter to tell you when to stop repeating a statement or group of statements –The examples from last class were counter-controlled repetition However, what if we want the user or the input to decide when to end the program? –Use a sentinel

Understanding Sentinels Sentinel: a special value that indicates the “end of data entry.” Also known as signal value, dummy value, or flag value For example: –-1 means end of data. –0 means end of data. –"END" means ends of data –Depends on the specific application you are building. With a sentinel, we have an indefinite repetition, because the number of repetitions is unknown at the time we write the program (or start the loop).  2000 Prentice Hall, Inc. All rights reserved. Modified by Evan Korth

Using Sentinels How are they used? –Programmer picks a value that would never be encountered for normal data –User enters normal data and then when done, enters the unusual value –The loop will stop when seeing the unusual value

Using Sentinels cont’d For example, if entering age for people, could pick a sentinel of –1 No one would expect to be –1 year old. Good practice is to remind the user in each iteration of the loop what the sentinel value is –For example, System.out.println (" Enter age of current resident or –1 to end" );

/* A sentinel controlled loop */ /* A simple census taker */ import javax.swing.JOptionPane; public class Sentinel { public static void main(String [] args) { int currentAge = 0 ; String currentAgeAsString; /* priming */ currentAgeAsString = JOptionPane.showInputDialog ("Enter age of resident: ") ; currentAge = Integer.parseInt (currentAgeAsString); /* testing: keep going until input is sentinel value */ while (currentAge != -1) { /* do some calculations with age, e.g. AVERAGE */ /*updating: get the next value from the user */ currentAgeAsString = JOptionPane.showInputDialog ("Enter age of resident: ") ; currentAge = Integer.parseInt (currentAgeAsString); } System.exit (0); }

Good Programming tips Pick a sentinel value that you are CERTAIN will never be confused with normal data Style: Remind user each iteration what the sentinel is Y2K-like problem –Programmers often used 9999 as a sentinel to end a loop –Worry that on September 9, 1999 (sometimes abbreviated 9999) programs would erroneously stop executing before they were supposed to.

Case Study: Using Sentinel Controlled Loops

Formulating Algorithms with Top-Down, Stepwise Refinement Problem becomes: Develop a class-averaging program that will process an arbitrary number of grades each time the program is run. –Unknown number of students –How will the program know to end? Use sentinel value –Loop ends when user inputs the sentinel value –Sentinel value chosen so it cannot be confused with a regular input (such as -1 in this case)  2000 Prentice Hall, Inc. All rights reserved.

Formulating Algorithms with Top-Down, Stepwise Refinement Top-down, stepwise refinement –Begin with a pseudocode representation of the top: Determine the class average for the quiz –Divide top into smaller tasks and list them in order: Initialize variables Input, sum and count the quiz grades Calculate and print the class average Many programs have three phases: –Initialization: initializes the program variables –Processing: inputs data values and adjusts program variables accordingly –Termination: calculates and prints the final results  2000 Prentice Hall, Inc. All rights reserved.

Formulating Algorithms with Top-Down, Stepwise Refinement Refine the initialization phase from Initialize variables to: Initialize total to zero Initialize counter to zero Refine Input, sum and count the quiz grades to Input the first grade (possibly the sentinel) While the user has not as yet entered the sentinel Add this grade into the running total Add one to the grade counter Input the next grade (possibly the sentinel)  2000 Prentice Hall, Inc. All rights reserved.

Formulating Algorithms with Top-Down, Stepwise Refinement Refine Calculate and print the class average to If the counter is not equal to zero Set the average to the total divided by the counter Print the average else Print “No grades were entered”  2000 Prentice Hall, Inc. All rights reserved.

17 Initialize total to zero Initialize counter to zero Input the first grade (possibly the sentinel) While the user has not as yet entered the sentinel Add this grade into the running total Add one to the grade counter Input the next grade (possibly the sentinel) If the counter is not equal to zero Set the average to the total divided by the counter Print the average else Print “No grades were entered” Fig. 4.8 Class-average problem pseudocode algorithm with sentinel-controlled repetition.  2003 Prentice Hall, Inc. All rights reserved.

 2003 Prentice Hall, Inc. All rights reserved. Outline 20 Average2.java 60 else // if no grades entered, output appropriate message 61 JOptionPane.showMessageDialog( null, "No grades were entered", 62 "Class Average", JOptionPane.INFORMATION_MESSAGE ); 63 64 System.exit( 0 ); // terminate application 65 66 } // end main 67 68 } // end class Average2

Case Study: Nested Control Structures

Nested control structures Problem –A college has a list of test results ( 1 = pass, 2 = fail) for 10 students –Write a program that analyzes the results If more than 8 students pass, print "Raise Tuition" Notice that –The program must process 10 test results Counter-controlled loop will be used –Two counters can be used One for number of passes, one for number of fails –Each test result is a number—either a 1 or a 2 If the number is not a 1, we assume that it is a 2  2003 Prentice Hall, Inc. All rights reserved.

Nested control structures Top level outline Analyze exam results and decide if tuition should be raised First Refinement Initialize variables Input the ten quiz grades and count passes and failures Print a summary of the exam results and decide if tuition should be raised Refine Initialize variables to Initialize passes to zero Initialize failures to zero Initialize student counter to one  2003 Prentice Hall, Inc. All rights reserved.

Nested control structures Refine Input the ten quiz grades and count passes and failures to While student counter is less than or equal to ten Input the next exam result If the student passed Add one to passes else Add one to failures Add one to student counter Refine Print a summary of the exam results and decide if tuition should be raised to Print the number of passes Print the number of failures If more than eight students passed Print “Raise tuition”  2003 Prentice Hall, Inc. All rights reserved.

25 Initialize passes to zero Initialize failures to zero Initialize student to one While student counter is less than or equal to ten Input the next exam result If the student passed Add one to passes else Add one to failures Add one to student counter Print the number of passes Print the number of failures If more than eight students passed Print “Raise tuition” Fig 4.10 Pseudocode for examination-results problem.  2003 Prentice Hall, Inc. All rights reserved.

Outline 26 Analysis.java Line 19 Line 29 1 // Fig. 4.11: Analysis.java 2 // Analysis of examination results. 3 import javax.swing.JOptionPane; 4 5 public class Analysis { 6 7 public static void main( String args[] ) 8 { 9 // initializing variables in declarations 10 int passes = 0; // number of passes 11 int failures = 0; // number of failures 12 int studentCounter = 1; // student counter 13 int result; // one exam result 14 15 String input; // user-entered value 16 String output; // output string 17 18 // process 10 students using counter-controlled loop 19 while ( studentCounter <= 10 ) { 20 21 // prompt user for input and obtain value from user 22 input = JOptionPane.showInputDialog( 23 "Enter result (1 = pass, 2 = fail)" ); 24 25 // convert result to int 26 result = Integer.parseInt( input ); 27 28 // if result 1, increment passes; if...else nested in while 29 if ( result == 1 ) 30 passes = passes + 1; Loop until student counter is greater than 10 Nested control structure

 2003 Prentice Hall, Inc. All rights reserved. Outline 27 Analysis.java 31 32 else // if result not 1, increment failures 33 failures = failures + 1; 34 35 // increment studentCounter so loop eventually terminates 36 studentCounter = studentCounter + 1; 37 38 } // end while 39 40 // termination phase; prepare and display results 41 output = "Passed: " + passes + "\nFailed: " + failures; 42 43 // determine whether more than 8 students passed 44 if ( passes > 8 ) 45 output = output + "\nRaise Tuition"; 46 47 JOptionPane.showMessageDialog( null, output, 48 "Analysis of Examination Results", 49 JOptionPane.INFORMATION_MESSAGE ); 50 51 System.exit( 0 ); // terminate application 52 53 } // end main 54 55 } // end class Analysis

do/while Loop

The do / while Repetition Structure The do / while repetition structure –Similar to the while structure –Condition for repetition tested after the body of the loop is performed All actions are performed at least once –Format: do { statement(s); } while ( condition );

4.8The do / while Repetition Structure Flowchart of the do / while repetition structure true false action(s) condition

31 public class DoWhileTest { public static void main(String args[]) { int counter; counter = 1; do { System.out.println ("counter: "+ counter); counter ++; } while (counter <= 10); }

Summary of Looping

Two broad types of loops: –Counter-controlled repetition A counter controls the number of repetitions. Also known as a definite repetition, because we know in advance how many times the loop will be executed. –Sentinel-controlled repetition A sentinel controls the number of repetitions Also known as indefinite repetition, because we do not know in advance how many times the loop will be executed. In either case, watch out for infinite loops! If your program requires some kind of loop, first determine which kind of loop you want.

Summary of Looping Once you know which kind of loop you want, determine which while loop you want: –While loops condition is tested first; then action occurs. While loops are much more common than do/while loops. –Do/while loops action is run first; then, condition is tested. Use this if you want to make sure that your loop is guaranteed to be run at least once.

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