Introduction to Programming Decision Statements

1. Introduction to Programming Decision Statements

2. Q: What is a decision? • Something that represents a branching point in a solution • Outcomes are often dependent on initial conditions

3. Decisions in Programs • Without decision statements (or other dynamic control structures), programs are static • Static programs do exactly the same things each time they are executed • Dynamic programs do not

4. Boolean Algebra • Based on values that are either True or False • True and False values are often represented by 1’s and 0’s, respectively

5. Logical Operations: And • AB • Expression is True iffA and B are both true

6. Logical Operations: Or • AB • Expression is True if either A or B are True • Note: Also True when A and B are both True

7. Logical Operations: Exercises A = True, B = True, C = False 1. AB 2. AC 3. ABC 4. (AB)  (AC)

8. Relational Operations • A < B “A less than B” • A > B “A greater than B” • A = B “A equal to B” • A  B “A less than or equal to B” “A not greater than B” • A  B “A greater than or equal to B” “A not less than B” • A  B “A not equal to B” “A less than or greater than B”

9. Relational Operations: Exercises A = 5, B = 3, C = -7 1. A < B 2. AC 3. (A < C)  (B < C)

10. Boolean Operations: Java • A && B • A | | B • A < B • A > B • A == B • A >= B • A <= B • A !=B • A  B • A B • A < B • A > B • A = B • A  B • A  B • A  B

11. Try this! Problem: • You’d like to go see a movie. • The movie costs $8.00, a soda costs $2.50 and a large popcorn costs $4.50. • Based on the amount of money in your pocket, determine whether you could...(a) See the movie and buy a soda, (b) See the movie, and buy soda and popcorn, or(c) Stay home

12. Know? • Movie costs $8.00 • Soda costs $2.50 • Popcorn costs $4.50 • How much money I have in my pocket

13. Need? • Cost of movie and soda • Cost of movie, soda and popcorn • Way to select one of the three options(that is, make a decision!)

14. Do? • Option (a) costs $10.50 • Option (b) costs $15.00 • Option (c) costs nothing • What next?

15. How about a diagram? • This is called a flowchart Money < $10.50 Money < $15.00 Stay home Movie, soda & popcorn Movie & soda

16. How about a diagram? • Boxes represent actions Money < $10.50 Money < $15.00 Stay home Movie, soda & popcorn Movie & soda

17. How about a diagram? • Diamonds represent decision points Money < $10.50 Money < $15.00 Stay home Movie, soda & popcorn Movie & soda

18. How about a diagram? • Arrows show flow Money < $10.50 Money < $15.00 Stay home Movie, soda & popcorn Movie & soda

19. How about a diagram? • The arrow at the top tells us there were previous steps • The arrow at the bottom tells us there are subsequent steps Money < $10.50 Money < $15.00 Stay home Movie, soda & popcorn Movie & soda

20. How would I write this? • Using Pseudocode • Wait! • Do you remember what the CENSORED Pseudocode is ?

21. Pseudocode • Looks like a programming language • Has all the structure of a programming language • Has a verrrrrry loose syntax

22. Pseudocode • Example: get x result <- x2 + 5x + 7 print result • That’s it! • Sloppy, ain’t it?

23. One more time! • Pseudocode...If (Money < $10.50) then Stay home else If (Money < $15.00) then Movie, soda else Movie, soda, popcorn

24. How would I write this? • First, we need to decide how to organize our solution • Should we “hard code” the costs of the movie, soda and popcorn into the algorithm? • Should we input these values? • Let’s take another look at that problem!

25. How would I write this? • The problem statement tells us the individual costs • So, let’s assume they’re fixed or constant • No need to ask the user for them • Problem: • You’d like to go see a movie. • The movie costs $8.00, a soda costs $2.50 and a large popcorn costs $4.50. • Based on the amount of money in your pocket, determine whether you could...(a) See the movie and buy a soda,(b) See the movie, and buy soda and popcorn, or(c) Stay home

26. How would I write this? • Another question: Should we pre-compute the cost of each option? • Or, should we let the program do this? • Since we’ve already stated that the item costs are fixed, it would seem logical to pre-compute the cost of each option • Movie: $8.00 • Movie & soda: $10.50 • All three: $15.00

27. How would I write this? • Next, we need to make sure we have a complete algorithm Input MoneyIf (Money < $10.50) then Display “Stay home.” else If (Money < $15.00) then Display “Go to a movie;buy a soda.” else Display “Go to a movie; buy a soda and popcorn.” • Almost done!

28. How would I write this? • Determine how we wish to organize our program • Do we want one method? • Or, should we create a few methods? • Let’s two functions: One to input Money from the user • And a second to determine the outcome

29. How would I write this? • Here’s the prototypes for the methods int getMoney()void showResults(int myMoney)

30. Program • Okay, now we get to use our algorithm and program design to create a program • Well, what are you waiting for? Do It!!!

31. Multiway Branching • If statements can be used for multiway branching • That is, choosing one of n mutually exclusive outcomes • But what about n outcomes that are not totally unique?

32. Multiway Branching • Consider the following problem:Each year, a local middle school requires students to purchase supplies based on their grade level. 6th graders need pencils and five notebooks. 7th graders also need a calculator. 8th graders add to this a 3-ring binder with loose leaf paper.

33. Multiway Branching • We could use a nested If statement to handle this, but there is an alternative • Whenever we need to represent a decision step, with n possible outcomes, where • the outcomes form subsets of each other, and/or • the outcomes are chosen based upon unique scalar values for a control expression, • we can use a Case (switch) structure

34. Multiway Branching • Case When Grade = 8th 3-ring binder loose leaf paper When Grade = 7th calculator When Grade = 6th pencils 5 notebooks

35. Multiway Branching • In Java... switch (grade){ case 8: System.out.print(“3-ring binder, loose leaf, “); case 7: System.out.print(“ calculator, “); case 6: System.out.println(“ 5 notebooks, & pencils.”); } • When the switch is encountered, control jumps to the matching case statement and continues until either a break is found or the end of the switch

36. Multiway Branching • Here’s an example with a few break’s System.out.println(“ Your lunch period comes “; switch (grade) { case 8: System.out.println(“ first.”); break; case 7: System.out.println(“ second.”); break; case 6: System.out.println(“ third.”); } No final break

37. Exercise: • Create a program that will inform the user which advisor they should go to based on their major code number Well? Get started!