CSCI 1101B. Boolean Expressions

Transcription

1 CSCI 1101B Boolean Expressions

2 Announcements Please respect the TAs Be respectful of their boundaries and needs Go to TA hours instead of knocking on their door Meet with TAs early (instead of the night before) Reminder: Collaboration Policy You should be able to explain any code you submit You can always refer to the Python docs or ask the teaching staff You should not blindly copy code or look at your neighbor s work (We can tell when you do this. Please don t do this.)

3 Today s Outline There s a Problem. Now What? Understand, Plan, Execute, Review Debugging Strategies Boolean Expressions Conditional Expressions

4 Part I There s a problem! Now what?

5 General Problem-Solving Approach 1. Understand the problem. 2. Make a plan. 3. Carry out the plan. 4. Review your solution. Does it work for all test cases? Can you make it better?

6 Understand the Problem Take notes, highlight key parts, talk out loud, draw/write your own understanding, ask questions. Write a procedure that asks a user for their age in years, and then prints their age in days.

7 Understand the Problem Take notes, highlight key parts, talk out loud, draw/write your own understanding, ask questions. Write a procedure that asks a user for their age in years, and then prints their age in days.

8 Make a Plan Write in pseudocode or comments (top-down). # Inside a function # TODO: Get the user s age with input() # TODO: Convert the age to be in days # TODO: Print out the new age

9 Carry Out the Plan Get more specific, solve and test sub-tasks. def print_days(): # TODO: Get the user s age with input() # TODO: Convert the age to be in days # TODO: Print out the new age

10 Carry Out the Plan Get more specific, solve and test sub-tasks. def print_days(): # TODO: Get the user s age with input() # TODO: Convert the age to be in days # > Cast to int, * 365, cast to str # TODO: Print out the new age

11 Carry Out the Plan Get more specific, solve and test sub-tasks. def print_days(): # TODO: Get the user s age with input() age = str(int(age) * 365) # TODO: Print out the new age

12 Carry Out the Plan Get more specific, solve and test sub-tasks. def print_days(): # TODO: age = get input() from user age = str(int(age) * 365) # TODO: Print out the new age

13 Carry Out the Plan Get more specific, solve and test sub-tasks. def print_days(): age = input( How old are you? ) age = str(int(age) * 365) # TODO: Print out the new age

14 Carry Out the Plan Get more specific, solve and test sub-tasks. def print_days(): age = input( How old are you? ) age = str(int(age) * 365) print( That s + age + days old. )

15 Review the Solution Does it work? Do all tests pass? Did you document your work with docstrings/comments? Good style? # TODO: what about if the user entered a negative (or zero) age? # TODO: what if the age wasn t an int?

16 Debugging Strategies Try small tests in the interactive shell Check your types, and what they re doing Check syntax - e.g., be sure to close parentheses Use print statements to show you the value of variables or expressions along the way Isolate the bug - comment out sections until you find it

17 Part II Booleans

18 The Boolean Data Type ( bool ) Sometimes we have data that isn t a number or a string. Instead, it is either True or False. This is called a bool. at_bowdoin = True hungry = False

19 Boolean Operators >>> hungry = False >>> hungry False

20 Boolean Operators: NOT >>> hungry = False >>> hungry False >>> not hungry True

21 Boolean Operators: AND x AND y will return True only if x and y are both true >>> happy = True >>> healthy = True >>> happy and healthy True

22 Boolean Operators: OR x OR y will return True if either x or y are true >>> wearing_hat = True >>> has_sandwich = False >>> wearing_hat or has_sandwich True

23 Comparisons and Equality Comparing numbers is similar to the math we know. 10 > 5 => True 10 <= 5 => False

24 Comparisons and Equality Comparing numbers is similar to the math we know. 10 > 5 => True 10 <= 5 => False 2 == 2 => True

25 Comparisons and Equality Comparing numbers is similar to the math we know. 10 > 5 => True 10 <= 5 => False 2 == 2 => True 3!= 2 => True

26 Comparisons and Equality Comparing strings is character by character and alphabetical. tiger == tiger => True (these values are identical) a < apples => True (alphabetical order) ABC!= abc => True (capitalization matters) Alan < alan => True (uppercase before lowercase)

27 Chained Comparisons 5 < 10 == False =>? 5 < 10 == True =>?

28 Chained Comparisons 5 < 10 == False => False 5 < 10 == True => False

29 Chained Comparisons 5 < 10 == False => False (5 < 10) and (10 == False) 5 < 10 == True => False (5 < 10) and (10 == True)

30 Membership ( in and not in ) in: True if it finds a variable in a given sequence. >>> good in a great example False >>> good not in a great example True

31 Credit: chatbot.com

32 Operator Precedence - Revisited Operator (all other operators we ve seen) <, >, <=, >=, ==,!=, in, not in not and or Description Arithmetic Comparisons, Equality, Membership NOT (Boolean Logic) AND (Boolean Logic) OR (Boolean Logic)

33 Part III Conditionals

34 Conditional Statements if <<condition>>: <<block>>

35 Conditional Statements if mood == happy : print( Yay! )

36 Conditional Statements if mood == happy : print( Yay! ) else: print( Sigh. )

37 Conditional Statements if mood == happy : print( Yay! ) elif mood == angry : print( Grr! ) else: print( Sigh. )

38 Conditional Statements my_money = $100 ticket_cost = $5 if my_money > ticket_cost: print( I can buy a ticket! )

39 Keywords - Overwatch Credit: Blizzard

40 Keywords - Overwatch GG EZ Credit: Blizzard

41 Keywords - Overwatch Credit: GameAddik, Blizzard

42 Keywords - Overwatch user_input = input( [Match]: )

43 Keywords - Overwatch user_input = input( [Match]: ) if gg ez in user_input:

44 Keywords - Overwatch user_input = input( [Match]: ) if gg ez in user_input: user_input = Wishing you all the best

45 Keywords - Overwatch user_input = input( [Match]: ) if gg ez in user_input: user_input = Wishing you all the best print(user_input)

46 Keywords - Overwatch user_input = input( [Match]: ) if gg ez in user_input: user_input = Wishing you all the best print(user_input)

47 Nested If Statements Conditional blocks can be nested. (You can place an if statement inside another if statement.)

48 Nested If Statements if some_condition: # do something

49 Nested If Statements if some_condition: # do something else: # do something else

50 Nested If Statements if some_condition: else: # do something else

51 Nested If Statements if some_condition: if some_other_condition: # do something else: # do something else else: # do something else

52 Nested If Statements Example: Character interactions in a video game (like voice lines) Credit: Dragonrage on Stack Exchange

53 Nested If Statements Veigar is a tiny master of evil. Tristana likes to taunt Veigar. Credit: Riot Games

54 Nested If Statements if near_veigar Credit: GiantBomb

55 Nested If Statements if near_veigar and taunt_pressed:

56 Nested If Statements if near_veigar and taunt_pressed: if champion == Tristana :

57 Nested If Statements if near_veigar and taunt_pressed: if champion == Tristana : print("aw, Veigar! You're cute when you're angry.")

58 Nested If Statements if near_veigar and taunt_pressed: if champion == Tristana : print("aw, Veigar! You're cute when you're angry.") else:

59 Nested If Statements if near_veigar and taunt_pressed: if champion == Tristana : print("aw, Veigar! You're cute when you're angry.") else: print( Get out of my way! )

60 Nested If Statements if near_veigar and taunt_pressed: if champion == Tristana : print("aw, Veigar! You're cute when you're angry.") else: print( Get out of my way! )

61 A New Debugging Strategy Define one or more functions that will test your code def test(): if get_last_name( Jane Smith ) is Smith : print("get_last_name works for Jane Smith ) else: print("get_last_name fails for Jane Smith ) #(etc.)

62 Lazy Evaluation Python doesn t evaluate more than it needs to

63 Lazy Evaluation Python doesn t evaluate more than it needs to hungry = True thirsty = False

64 Lazy Evaluation Python doesn t evaluate more than it needs to hungry = True thirsty = False if thirsty and hungry... # never evaluates hungry

65 Lazy Evaluation Python doesn t evaluate more than it needs to hungry = True thirsty = False if thirsty and hungry... # never evaluates hungry if hungry or thirsty... # never evaluates thirsty

66 Summary & Next Steps Remember to: Understand, Plan, Execute, Review. The bool data type is associated with True/False values. Boolean operators (and, or, not) help us specify logical conditions. Comparisons also help us make decisions. Conditional statements let us specify what to do (e.g.) if x happens, else if y happens, else z. Keep working on Lab 2 and Project 1