LoopsTopicsReview: Repetition StructureThe while Repetition StructureExampleGood Programming PracticeAnother while Loop ExampleThe PseudocodeThe CodeVersatile?New PseudocodeNew CodeWhy Bother to Make It Easier?Using a Sentinel ValueThe Priming ReadSlide 16Slide 17Final "Clean" CodeSlide 19Using a while Loop to Check User InputCounter-Controlled Repetition (Definite Repetition)Event-Controlled Repetition (Indefinite Repetition)Event-Controlled RepetitionThe 3 Parts of a LoopThe for Loop Repetition StructureWhen Does a for Loop Initialize, Test and Modify?A for Loop That Counts From 0 to 9We Can Count Backwards, TooWe Can Count By 2’s ... or 7’s … or WhateverThe do-while Repetition StructureSlide 31An Equivalent while LoopAn Equivalent for LoopSo, Which Type of Loop Should I Use?Nested LoopsNested for LoopsThe break StatementExample break in a for LoopThe continue StatementExample continue in a for Loop1Loops2TopicsThe while LoopProgram VersatilitySentinel Values and Priming ReadsChecking User Input Using a while LoopCounter-Controlled (Definite) RepetitionEvent-Controlled (Indefinite) Repetitionfor Loopsdo-while LoopsChoosing an Appropriate LoopBreak and Continue Statements3Review: Repetition StructureA repetition structure allows the programmer to specify that an action is to be repeated while some condition remains true.There are three repetition structures in JavaScript, the while loop, the for loop, and the do-while loop.4The while Repetition Structure while ( condition ) { statement(s) }The braces are not required if the loop body contains only a single statement. However, they are a good idea and are required by the 104 Coding Standards.5Examplewhile ( children > 0 ){ children = children - 1 ; cookies = cookies * 2 ;}6Good Programming PracticeAlways place braces around the body of a while loop.Advantages:Easier to readWill not forget to add the braces if you go back and add a second statement to the loop bodyLess likely to make a semantic errorIndent the body of a while loop 2 to 3 spaces -- be consistent!7Another while Loop ExampleProblem: Write a program that calculates the average exam grade for a class of 10 students.What are the program inputs?the exam gradesWhat are the program outputs?the average exam grade8The Pseudocode<total> = 0<grade_counter> = 1While (<grade_counter> <= 10) Display "Enter a grade: "Read <grade> <total> = <total> + <grade> <grade_counter> = <grade_counter> + 1End_while<average> = <total> / 10Display "Class average is: ", <average>9The Code1. var counter, grade, total, average;2. total = 0;3. counter = 1;4. while (counter <= 10)5. {6. grade = prompt ("Enter a grade : ");7. grade = parseInt(grade);8. total = total + grade;9. counter = counter + 1;10. }11. average = total / 10;12. alert ("Class average is " + average);13.10Versatile?How versatile is this program?It only works with class sizes of 10.We would like it to work with any class size.A better way :Ask the user how many students are in the class. Use that number in the condition of the while loop and when computing the average.11New Pseudocode<total> = 0<grade_counter> = 1Display "Enter the number of students: "Read <num_students>While (<grade_counter> <= <num_students>) Display "Enter a grade: " Read <grade> <total> = <total> + <grade><grade_counter> = <grade_counter> + 1End_while<average> = <total> / <num_students>Display "Class average is: ", <average>12New Code1. var numStudents, counter, grade, total, average;2. total = 0;3. counter = 1; 4. numStudents = prompt("Enter number of students: ");5. numStudents = parseInt(numStudents);6. while (counter <= numStudents) 7. {8. grade = prompt("Enter a grade : ");9. grade = parseInt(grade);10. total = total + grade;11. counter = counter + 1;12. }13. average = total / numStudents;14. alert ("Class average is: " + average);13Why Bother to Make It Easier?Why do we write programs? So the user can perform some taskThe more versatile the program, the more difficult it is to write. BUT it is more useable.The more complex the task, the more difficult it is to write. But that is often what a user needs.Always consider the user first.14Using a Sentinel ValueWe could let the user keep entering grades and when he’s done enter some special value that signals us that he’s done.This special signal value is called a sentinel value.We have to make sure that the value we choose as the sentinel isn’t a legal value. For example, we can’t use 0 as the sentinel in our example as it is a legal value for an exam score.15The Priming ReadWhen we use a sentinel value to control a while loop, we have to get the first value from the user before we encounter the loop so that it will be tested and the loop can be entered.This is known as a priming read.We have to give significant thought to the initialization of variables, the sentinel value, and getting into the loop.16New Pseudocode<total> = 0<grade_counter> = 1Display "Enter a grade: "Read <grade>While ( <grade> != -1 ) <total> = <total> + <grade><grade_counter> = <grade_counter> + 1 Display "Enter another grade: " Read <grade>End_while<average> = <total> / <grade_counter>Display "Class average is: ", <average>17New Code1. var counter, grade, total, average;2. total = 0;3. counter = 1;4. grade = prompt("Enter a grade: ");5. grade = parseInt(grade);6. while (grade != -1) 7. {8. total = total + grade;9. counter = counter + 1;10. grade = prompt("Enter another grade: ");11. grade = parseInt(grade);12. }13. average = total / counter;14. alert ("Class average is: " + average);18Final "Clean" Code1. var counter; /* counts number of grades entered */2. var grade; /* individual grade */3. var total; /* total of all grades */4. var average; /* average grade */5. 6. /* Initializations */7. total = 0;8. counter = 1;9. 10. /* Priming read to get initial grade from user */11. grade = prompt("Enter a grade: ");12. grade = parseInt(grade);13. (continued)19Final "Clean" Code17. /* Get grades until user enters -1. Compute 18. grade total and grade count */ 19. while (grade != -1) 20. {21. total = total + grade;22. counter = counter + 1;23. grade = prompt("Enter another grade: ");24. grade = parseInt(grade);25. }26. 27. /*