Conditional Expressions and Decision Statements

Transcription

1 Conditional Expressions and Decision Statements June 1, 2015 Brian A. Malloy Slide 1 of 23

2 1. We have introduced 5 operators for addition, subtraction, multiplication, division, and exponentiation: +, -, *, /, ** We now introduce a very useful operator called modulus, which returns the remainder after division. Use operator for conditional expressions The modulus operator is % For example, 10%3=1, 13%2=1, 1%2=1 Slide 2 of 23

3 1.1. What good is modulus? Useful for lots of conditions: Determine if a number is even Limit the range of a number Wrapping around Finding the value of a digit... How to determine even/odd: Assume number is stored in x if x % 2 is 0, then x is even, otherwise x is odd. Example: 5 % 2 == 1 5 is odd Example: 6 % 2 == 0 6 is even Slide 3 of 23

4 2. We have described three data types: integer, float, string. We now add a fourth data type, bool, which evaluates to True or False. Named after mathematician, George Boole A boolean expression evaluates to either True or False. Slide 4 of 23

5 3. There are 6 boolean operators: ==,!=, <, >,, These operators are interpreted as follows: (a) == compares two values; returns True if operands are equal, False otherwise. (b)!= compares two values; returns True if they are not equal, False otherwise (c) etc. When used in a Python program, must be written <=, and must be written >= Examples: (a) 3 < 3 is False, but (b) 3 <= 3 is True Slide 5 of 23

6 3.1. in Python Can print the type of a Boolean expression: print type(true) <type bool > Can print the value of Boolean expression: $ python Python (default, Mar , 22:59:56) [GCC 4.8.2] on linux2 >>> x = 5 >>> x!= 7 True >>> x >= 12 False >>> x > 5 False Slide 6 of 23

7 3.2. Determine if a number is even Python (default, Mar , 22:59:56) [GCC 4.8.2] on linux2 Type "help", "copyright", "credits" >>> x = input("type a number: ") Type a number: 7 >>> x % 2 1 >>> Since x % 2 is 1, this means x is odd Explanation: for the above program: if the value of x % 2 == 0 is True then the number is even, otherwise the number is odd. Slide 7 of 23

8 4. Much of the power of computer programming, and computers in general, derives from the ability to make decisions and loop. Decision structures in Python, and other languages, take many forms: 1. Conditional execution 2. Alternative execution 3. Chained conditionals 4. Nested Conditionals We ll describe each decision structure next We ll save loops for future presentations Slide 8 of 23

9 4.1. Conditional Execution Means that a sequence of 1 or more statements will execute only if a certain condition is satisfied. Provides the ability to change the bahavior of a program for each execution. Format: Slide 9 of 23

10 Conditional Execution: input Example, the following program: has output if x is greater than zero has no output if x is zero or negative Notice use of colon : x = input("type a number: ") if x > 0: print x, "is positive" $ python if.py Type a number: is positive $ python if.py Type a number: -4 $ python if.py Type a number: 0 Slide 10 of 23

11 Conditional Execution: spaces In the following, if x > 0, then 2 statements are executed, numbered (3) & (4) Spaces must be the same on, (3) & (4) This one works: (1) x = input("type a number: ") (2) if x > 0: (3) print x, "is positive" (4) print x, "is not zero or negative" This one doesn t work: (1) x = input("type a number: ") (2) if x > 0: (3) print x, "is positive" (4) print x, "is not zero or negative" Slide 11 of 23

12 Conditional Execution: Flowchart The flowchart on the title page of these slides aptly describes conditional execution. That is, statement(s) are executed only if condition is true: Slide 12 of 23 If condition is False, statement(s) are skipped, and execution goes straight to rest of program

13 4.2. Alternative Execution The problem with Conditional Execution is that statement(s) are only executed if the condition is True. We may want output in either case: statement(s) if condition is True. statement(s) if condition is False. Slide 13 of 23

14 Alternative Execution: input Now we get output in either case: x = input("type a number: ") if x % 2 == 0: print x, "is even" else: print x, "is odd" $ python ifelse.py Type a number: 4 4 is even $ python ifelse.py Type a number: 7 7 is odd $ python ifelse.py Type a number: 0 0 is even Slide 14 of 23

15 4.3. Chained Conditionals Use this if more than 2 possible outcomes Notice keywords elif and else Consider a trichotomy example: 1 x = input("type a number: ") 2 if x > 0: 3 print x, "is greater than zero" 4 elif x == 0: 5 print x, "is zero" 6 else: 7 print x, "is less than zero" Slide 15 of 23

16 4.4. Nested Conditionals Can nest conditional within a conditional spacing is intended to make it clear what gets executed But Nested Conditionals can be confusing Consider an alternative to trichotomy; statements 5 8 happen only if x == 0 is false. 1 x = input("type a number: ") 2 if x == 0: 3 print x, "is zero" 4 else: 5 if x > 0: 6 print x, "is greater than zero" 7 else: 8 print x, "is less than zero" Slide 16 of 23

17 5. These include and, or, and not They can simplify conditional execution The following examples do the same thing; Which is easier to understand: if x > 0: if x < 10: print x, "is a single digit number" if x > 0 and x < 10: print x, "is a single digit number" Slide 17 of 23

18 6. It s possible, sometimes necessary, to return from function before last statement. Remember, division by 0 is undefined 1 def findaverage(sum, n): 2 if n <= 0: 3 return 0 4 return sum/n 5 6 print findaverage(25, 7) 7 print findaverage(25, 0) Slide 18 of

19 6.1. If a function contains no, there is always one implied at the end of a function The following functions are the same: 1 def display(n): 2 print "n is:", n 1 def display(n): 2 print "n is:", n 3 return Slide 19 of 23

20 7. Need to write programs Use conditionals/decisions to solve problems Slide 20 of 23

21 7.1. Evaluate Try to evaluate the following expressions in your head, then check your answer with Python interpreter: 1. >>> 5 % 2 2. >>> 9 % 5 3. >>> 15 % >>> 12 % >>> 6 % 6 6. >>> 0 % 7 7. >>> 7 % 0 What happened with the last example? Explore modulus until you re convinced you understand how it works Slide 21 of 23

22 7.2. True/False 1/0 What s the output? Why? 1 if A : 2 print yes 3 else: 4 print no What s the output? Why? 1 if 1: 2 print yes 3 else: 4 print no 0 False 1 True, 2 True, 3 True,... Slide 22 of 23

23 7.3. Problems Are negative numbers True or False Write code to determine if: 1. A number is even 2. If a number is a multiple of 3? 3. If a number is a multiple of 17? 4. If user input is a digit? 5. If user input is a letter? 6. If x is off a screen that s 640x480? 7. If user input is a vowel? Slide 23 of 23