Flow Charts. Visual Depiction of Logic Flow

Transcription

1 Flow Charts Visual Depiction of Logic Flow

2 Flow Charts Describe a sequence of events using pictures Often associated with computer programs, but are quite common in other fields General way to depict a process Examples follow...

3 Example 1

4 Example 2

5 Example 3

6 Example 4

7 App Inventor as Flow Chart

8 Flow Chart Symbols: Standard Set

9 One more example

10 Notes Connector symbol used when lines need to be brought together Chart depicts most program statements, but not necessarily all Key idea is depiction of process: should clearly convey algorithm

11 Flow of control In a program, statements execute in a particular order By default, statements are executed sequentially: One after another, from top to bottom One at a time Exactly once This sequential flow of control can be altered through the use of control structures 11

12 Control structures There are two general types of control structures: Selection (aka branching) structures: cause flow of control to take, or not take, a particular direction Repetition (aka iteration, looping) structures: cause a set of statements to execute several times Both of these type of structures depend on the evaluation of logical expressions 12

13 Flow charts for control structures Selection Repetition 13

14 Logical expressions Logical expressions evaluate to true or false Most logical expressions use relational and logical operators In App Inventor, as in most programming languages, the form of a simple logical expression is: Operand1 operator Operand2 The operands can be simple or compound expressions of any type The operator is a relational operator 14

15 Relational operators The relational operators in AI (found in the Math drawer) include: < : is less than <= : is less than or equal to = : equals > : is greater than >= : is greater than or equal to not= : does not equal 15

16 Logical expression examples Suppose you had the following declarations: x = 3, y = 7; Then the following expressions would have the values indicated: x > y // (false) y >= x // (true) x not= y // (true) (x > y) = true // (false) ((y >= x) = (x not= y)) // true 16

17 Logical operators Three operators in AI (found in the Logic drawer) can be used to form compound logical expressions They are: and - logical and or - logical or not logical not 17

18 Logical operators Logical and (&&) combines two expressions; if both sub-expressions are true, then the compound expression is true otherwise, the compound expression is false Logical or also combines two expressions; the compound expression is true if one or both subexpressions is true, false otherwise Logical not reverses the truth value of an expression; if the original expression was true, not makes it false, and vice versa 18

19 Write an expression for each taxrate is over 25% and income is less than $20000 temperature is less than or equal to 75 or humidity is less than 70% age is over 21 and age is less than 60 age is 21 or 22 19

20 Logical expressions & program control Relational expressions can be used to control the flow of logic in a program Depending on the truth value of an expression, a program can be made to perform one task or another (but not both) A control structure that fits this description is called a selection structure 20

21 Selection Blocks in AI Simple if block 21

22 Compound Selection in AI Nested structure 22

23 Compound Selection Example Suppose you need to design the software for a controller that monitors the temperature of a critical piece of equipment If the temperature falls within the normal range (55-80 F), no action is necessary But if the temperature falls below 55 or rises above 80, an alarm should be triggered: 10 degrees in either direction should sound the immediate action alert More than 10 degrees in either direction should trigger an automatic shutoff 23

24 Example continued Variable to test: temp Expressions: Normal range: temp >= 50 and temp <= 80 Action alert: (temp<50 and temp>40) or (temp>80 and temp<90) Automatic shutoff: temp <= 40 or temp >= 90 24

25 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 25

26 Generic loop flowchart 26

27 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: 27

28 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? 28

29 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. 29

30 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. 30

31 MakeList procedure 31

32 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? 32

33 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 Add random numbers to a list 33

34 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 34

35 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 35

36 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 36