Loops. Repeat after me

Transcription

1 Loops Repeat after me 1

2 Loops A loop is a control structure in which a statement or set of statements execute repeatedly How many times the statements repeat is determined by the value of a control variable, which is tested at the beginning of each loop iteration 2

3 Generic loop flowchart 3

4 Two Types of Loops count controlled loops repeat a specified number of times; control variable is an integer that is automatically incremented (or decremented) event-controlled loops some condition within the loop body changes and this causes the repeating to stop; control variable changed via input 4

5 Loops often used to count all data values iteration counter - increment every time loop executes count special data values event count - increment only when specific trigger event occurs sum data values blood pressure program was example keep track of previous and current values 5

6 Loop examples in App Inventor Copyright note: this slide and the next several slides are adapted from tutorials written by Ralph Morelli for his course at Trinity College; link to the source material is below: URL: 6

7 Loops in App Inventor Suppose you wanted a write a simple game that: displayed, momentarily, an increasingly long lists of numbers and then invited the user to repeat the list from memory. How long a list can users remember? 7

8 User interface Each time the New List Button is clicked, it will generate a list of random digits between 0 and 9 and display it in the text box. Each time the Reset Button is clicked, it will re-initialize the list to the empty list. 8

9 Basic logic size: number of digits thelist: actual digits Each time ButtonNewList is clicked we will make a list, display it, and add 1 to the size variable for the next play. When the ResetButton is clicked, we will simply reset the variables and the text box to their original states. 9

10 MakeList procedure 10

11 Loop Design - considerations What process is repeated? How is process initialized & updated? What condition ends loop? How is condition initialized? How is condition updated? What is state of program upon loop exit? 11

12 Designing loop process Decide what a single iteration will do May need to initialize variables before loop body and update their values within loop body Examples accumulating a sum keeping a running tally 12

13 Loop ending condition -- designing flow of control Can usually determine condition by looking at the problem statement Initialization: assignment (e.g. for count control) primer read (for event) Update autoincrement vs. input 13

14 Program state on loop exit All variables involved in the loop will have values at exit Need to initialize variables with care to avoid off-by-one errors 14

15 Infinite loop Loop that never stops iterating Happens when terminating condition can t be reached Need to ensure that the control variable changes during body of loop 15

16 Do-while loops Slight variation of while loops Instead of testing condition, then performing loop body, the loop body is performed first, then condition is tested A do-while loop is guaranteed to perform one iteration, because the validity of the condition is not known until after the iteration is complete 16

17 Applications for do-while loops Often used to display an initial menu of choices, one of which is quit program you want user to see this at least once, even if the option they pick is to quit Can work well for file input Useful for checking validity of input value eliminates having to prompt twice for same data (see examples, next two slides) 17

18 Do-While Loop vs. While Loop POST-TEST loop (exitcondition) The looping condition is tested after executing the loop body. Loop body is always executed at least once. PRE-TEST loop (entrycondition) The looping condition is tested before executing the loop body. Loop body may not be executed at all. 18

19 For loops Count-controlled loops are so common in programming that most languages have a special set of syntax designed specifically for this construct In C, the special syntax for a count-controlled loop is called a for loop In AI, the specialized version is a forrange loop There is also a foreach loop, but its application is limited to use with lists 19

20 Count-controlled loops Count-controlled while loop: initialize counter before loop starts test counter in loop control expression update counter inside loop body A for loop is a stylized version of a countcontrolled while loop; all of the elements above appear at the top of the loop 20

21 Example Problem Write and test a function named getnumbersequence that uses a while loop to generate the numbers 1 to n where n is an argument. For example, if size is 5, then your function should generate the list ( ). The function takes a single argument, n1, which specifies both the length of the list and the value of the last item in the sequence 1, 2, 3,..., n1 21

22 App Inventor: while loop vs. forrange loop 22

23 Nested loops Just as a selection structure can be nested within another selection structure (or within a loop), a loop can also be nested When one loop is nested within another, each iteration of the outer loop contains several iterations of the inner loop 23

24 Pattern of a Nested Loop initialize outer loop while ( outer loop condition ) {... } initialize inner loop while ( inner loop condition ) { inner loop processing and update }... 24