Lecture 10: Boolean Expressions

Transcription

1 Lecture 10: Boolean Expressions CS1068+ Introductory Programming in Python Dr Kieran T. Herley Department of Computer Science University College Cork KH (12/10/17) Lecture 10: Boolean Expressions / 1

2 Summary Data type Boolean: variables, values and assignments. Boolean and, or, not operators. KH (12/10/17) Lecture 10: Boolean Expressions / 1

3 True/False values in Python Conditions in if statements have True/False outcomes: if mark >= 40: print( Pass! ) else : print( Fail ) Condition mark >= 40 is an expression with a True/False (Boolean) result KH (12/10/17) Lecture 10: Boolean Expressions / 1

4 George Boole Discovered Boolean Algebra algebraic system based on True/False values and variables AND, Or and NOT operators Important CS concept; foundation for computer circuit theory Self-taught mathematician first Professor of Mathematics UCC (then QCC) ; buried Blackrock George Boole ( ) KH (12/10/17) Lecture 10: Boolean Expressions / 1

5 Data type Boolean Data type Boolean Values: {True, False} Operators: and, or, not (details later) Usage Can use Booleans in assignments and prints etc. x = True y = False z = x and (not x or y) print(x, y, z) KH (12/10/17) Lecture 10: Boolean Expressions / 1

6 Data type Boolean cont d positive = x > 0 negative = x < 0 zero = x == 0 if negative : print( Negative ) else : print( Non negative ) Can use Boolean variables and values in conditions KH (12/10/17) Lecture 10: Boolean Expressions / 1

7 Composite conditions Suppose in order to enter CS student needs: at least 500 points a pass in maths KH (12/10/17) Lecture 10: Boolean Expressions / 1

8 Composite conditions Suppose in order to enter CS student needs: at least 500 points a pass in maths How to express a two-part condition like this? KH (12/10/17) Lecture 10: Boolean Expressions / 1

9 Boolean and operator Boolean operator and can be used to combine Boolean values or expressions X Y X and Y False False False False True False True False False True True True Composite X and Y is True if and only if X is True and Y is True KH (12/10/17) Lecture 10: Boolean Expressions / 1

10 Eligibility example if (maths mark >= 40) and (cao points >= 500 ): print( You're in! ) else : print( Sorry ) KH (12/10/17) Lecture 10: Boolean Expressions / 1

11 Eligibility example if (maths mark >= 40) and (cao points >= 500 ): print( You're in! ) else : print( Sorry ) Or alternatively has maths = maths mark >= 40 has points = cao points >= 500 if has maths and has points: print( You're in! ) else : print( Sorry ) KH (12/10/17) Lecture 10: Boolean Expressions / 1

12 Interval example MIN COMFY TEMP = 12 MAX COMFY TEMP = 25 if (MIN COMFY TEMP <= temp) and (temp <= MAX COMFY TEMP): print ( Comfortable ) else : print ( Uncomfortable ) 1 1 Python allows three-way test. Will see this later. KH (12/10/17) Lecture 10: Boolean Expressions / 1

13 Short circuit principle Notice that if X is False, then X and Y will be False regardless of the value of Y. X Y X and Y False False False False True False True False False True True True For composite conditions using and, Python won t evaluate the second part if the first is False if (num days > 0) and ( total rainfall /num days >= 20): print( It was lashing ) This short circuiting avoids the division in the case when num days is zero KH (12/10/17) Lecture 10: Boolean Expressions / 1

14 Other Boolean operators and X Y X and Y False False False False True False True False False True True True or X Y X or Y False False False False True True True False True True True True Result True if both operands are True Short circuit: Y only evaluated if X is True X False True not X True False not Inverts value of operand Result True if either operands is True (or both) Short circuit: Y only evaluated if X is False KH (12/10/17) Lecture 10: Boolean Expressions / 1

15 Boolean expressions True or False not not False False and (True or False) True and not (False or True) ( False and True) or False (not (True and False)) or ( False or not True) KH (12/10/17) Lecture 10: Boolean Expressions / 1

16 Precedence among operators (partial) ** Exponentiations High *, /, //, % Arithmetic multiplicative operators +, - Addition and subtraction <, <=, >, >=, ==,!= Comparative operators not not and and or or Low KH (12/10/17) Lecture 10: Boolean Expressions / 1

17 Precedence among operators (partial) ** Exponentiations High *, /, //, % Arithmetic multiplicative operators +, - Addition and subtraction <, <=, >, >=, ==,!= Comparative operators not not and and or or Low True or True and False True KH (12/10/17) Lecture 10: Boolean Expressions / 1

18 More examples False and True or False ( False and True) or False not False and True (not False) and True not True and False (not True) and False x < 17 and y!= 3 (x < 17) and (y!= 3) KH (12/10/17) Lecture 10: Boolean Expressions / 1

19 Age example Task Compute age based on today s date and data of birth Assume Dates available in day-month-year format Idea Need to establish whether individual s birthday has occurred yet this year KH (12/10/17) Lecture 10: Boolean Expressions / 1

20 Solution def get today (): # Return today's date in day, month, year format... def get dob(): # Read user date of birth and return result in day, # month, year format... today day, today month, today year = get today() dob day, dob month, dob year = get dob() had birthday = (dob month < today month) \ or (dob month == today month)\ and (dob day <= today day) KH (12/10/17) Lecture 10: Boolean Expressions / 1

21 Solution cont d if had birthday : age = today year dob year else : age = today year dob year 1 print( Age is, age) KH (12/10/17) Lecture 10: Boolean Expressions / 1

22 Leap year example Task Write Python function to determine if a year is a leap year or not Leap years? Leap years are divisible by 4 but not by 100 unless the year is divisible by 400 Examples KH (12/10/17) Lecture 10: Boolean Expressions / 1

23 Leap year example Task Write Python function to determine if a year is a leap year or not Leap years? Leap years are divisible by 4 but not by 100 unless the year is divisible by 400 Examples Leap 2016 Non-Leap 2013, 2014, 2015 KH (12/10/17) Lecture 10: Boolean Expressions / 1

24 Leap year example Task Write Python function to determine if a year is a leap year or not Leap years? Leap years are divisible by 4 but not by 100 unless the year is divisible by 400 Examples Leap 2016, 2000 Non-Leap 2013, 2014, 2015, 1800, 1900, 2100 KH (12/10/17) Lecture 10: Boolean Expressions / 1

25 Solution def is leap (year ): # Return True if specified year is a leap year divby 4 = year % 4 == 0 divby 100 = year % 100 == 0 divby 400 = year % 400 == 0 return divby 4 and (not divby 100 or divby 400) KH (12/10/17) Lecture 10: Boolean Expressions / 1

26 Days in the month Task Write Python function to determine the number of days in a particular month Thirty days hath...? KH (12/10/17) Lecture 10: Boolean Expressions / 1

27 Days in the month Task Write Python function to determine the number of days in a particular month Thirty days hath...? Thirty days have September, April, June, and November. All the rest have 31, Except February alone, And that has 28 days clear, And 29 in a leap year. KH (12/10/17) Lecture 10: Boolean Expressions / 1

28 Solution JAN, FEB, MAR, APR = 1, 2, 3, 4 MAY, JUN, JUL, AUG = 5, 6, 7, 8 SEP, OCT, NOV, DEC = 9, 10, 11, def num days(month, year): # Return number of days in the specified month thirty days = (month == SEP) or (month == APR)\ or (month == JUN) or (month == NOV) thirty one days = not thirty days and not (month == FEB) if thirty days: return 30 elif thirty one days : return 31 elif is leap (year ): return 29 else : return 28 KH (12/10/17) Lecture 10: Boolean Expressions / 1

29 Back Material Notes and Acknowledgements Reading Code Acknowledgements KH (12/10/17) Lecture 10: Boolean Expressions / 1