Python Activity 5: Boolean Expressions and Selection Statements

Transcription

1 Python Activity 5: Boolean Expressions and Selection Statements "True or False and making choices" Learning Objectives Students will be able to: Content: Explain the three types of programming structures Explain how conditional operators and logical operators are used in programming Use conditional operators with strings and numeric values Implement the Python syntax of an if/elif/else statement Determine good test data for programs that include if/elif/else statements Process: Write code that includes if statements and if/elif/else statements Write correct Boolean expressions and compound expressions Prior Knowledge Python concepts from Activities 1-4 Understanding of flowchart input symbols Further Reading Programmer%27s_Tutorial_for_Python_2.6/Boolean_Expressions Model 1: Conditional Operators Conditional operators, also known as relational operators, are used to compare the relationship between two operands. Expressions that can only result in one of two answers are known as Boolean expression. Critical Thinking Questions: 1. Determine the meaning of each of the following conditional operators. If you are not sure of the meaning of any symbol, create some example expressions, type them into the python shell of your Thonny IDE (see figure to the right) and examine the results. In[1]: 34 < 56 Out[1]: True a. < b. > c. <= d. >= e.!= f. ==

2 2. What is the result of each of the following expressions given the following variable assignments? x = 4, y = 5, z = 4 Expression x > y x < y x == y x!= y x >= z x <= z x + y > 2 * x y * x z!= 4 % pow(x,2) == abs(-16) Boolean expression 3. What is the result of the following expressions given the following variable assignments? word1 = "hello", word2 = "good-bye" Expression Boolean expression word1 = = word2 word1!= word2 word1 < word2 word1 >= word2 4. Explain how the conditional operators are used when the operands are strings. 5. What are the only two possible answers to all of the expressions in questions 2 and 3? Model 2: Programming Structures Sequence Structure Decision or Branching Structure Looping Structure

3 6. Each of the flowchart symbols in the chart above represents lines or segments of code. After examining the chart, write a description of: a. sequence structure b. decision or branching structure c. looping structure 7. Which structure best describes the types of Python programs you have written so far? Do you think this structure is the best structure to use for all programs? Why or why not? 8. Which structure allows the programmer to create code that decides what code is executed? Model 3: IF/ELSE Statement Flowchart Python Program 1 # Programmer: Monty Python # Date: Some time in the past # Program showing IF statement examscore = 95 if examscore >= 93: print("excellent!") 9. Consider Model 3 above: a. What is the Python code equivalent of the diamond flowchart symbol in the program above? b. What is the output of the program? c. What would the output of the program be if the value of the variable examscore was set to 85?

4 10. Execute the two following python programs by copying and pasting into your editor window of Thonny IDE. Python Program 2 Python Program 3 # Programmer: Monty Python # Date: Some time in the past # Program showing IF statement # Programmer: Monty Python # Date: Some time in the past # Program showing IF statement examscore = 85 if examscore >= 93: print("excellent! ") print("done! ") examscore = 85 if examscore >= 93: print("excellent!") print("done! ") a. What was the output for each program when examscore variable is set to 85? b. Change the examscore values to 95 in each program and run again. What is the output for each program now? c. What can you say about the importance of the indentation in a python program using an if statement in Python? Python Program 4 # Programmer: Monty Python # Date: 1/2/2018 # Program showing IF/Else statement temperature= float(input("enter a water temperature in degrees Celsius: ")) if temperature >= 100: print("the water is boiling.") else: print("the water is not boiling.") 10. Enter and execute the Python program above. a. Test the program at least three times. List the three test data points you used and the corresponding output. Be sure you test each part of the condition. Explain why the data you chose were the best data to use to thoroughly test for the program. Input 1= Output 1= Input 2= Output 2= Input 3= Output 3=

5 b. Now add print statements to the Python program above so that it prints "It s turning to steam!" when the water is 100 degrees or hotter, and says "Done!" when the program finishes regardless of what temperature the water is. Rewrite the if/else below with this statement included. 11. Consider the following program: a. What new keyword is used in this program? b. What would the output be if: The water temperature was 100 C The water temperature was 0 C The water temperature was 50 C The water temperature was c. You can use the elif as many times as you need. Suppose you wanted to add an option that indicated that the water is 37 C and is human body temperature. Where would you add it? What would that code look like? d. Does the placement of an addition elif clause matter? e. When is the code associated with the else statement executed?

6 f. Is the use of the else statement mandatory when creating an if/elif statement? Explain your answer. FYI: We can use logical operators to determine logic between conditions (relational expressions). 12. Sometimes you want to include more than one condition to determine which code segment is executed. You can use the following logical operators to create compound conditions. Examine each operator and a sample of its use and provide an explanation of how the operator works. Operator Example Explanation and (age >= 17) and (haslicense = = true) or (cost < 20.00) or (shipping = = 0.00) not not (credits> 120) 13. Assume the value of the variable numbooks is 40. State the values of each of the Boolean expressions that include a compound condition. Expression (numbooks > 5) and (numbooks < 100) (numbooks < 5) or (numbooks > 100) not(numbooks * 10 == 100) Boolean Value 14. Suppose you want to determine if a student is ready to graduate. The 3 criteria for graduation are that the student has earned at least 120 credits, their major GPA is at least 2.0 and their general GPA is also at least 2.0. Which Boolean expression (next page) would be the correct test for the Python code? Missing Boolean expression

7 The Missing Boolean expression is a. numcredits >= 120 or majorgpa >= 2.0 or overallgpa >= 2.0 b. numcredits > 120 and majorgpa > 2.0 or overallgpa > 2.0 c. numcredits > 119 and majorgpa >= 2.0 and overallgpa >= 2.0 d. numcredits >= 120 and majorgpa >= 2.0 and overallgpa >= Enter and execute the program in #17. Include your choice for the correct Boolean expression and create several sets of data to test all possibilities for the Boolean expression. List the data you used to test all possibilities for the expression and explain your choices. Data Set numcredits majorgpa overallgpa Expression Result (True or False) Group Application Questions: Use the Thonny IDE to check your work 1. Write a Boolean expression that tests if the value stored in the variable num1 is equal to the value stored in the variable num2. 2. Write a python program that sets a variable called isvalid to a Boolean value. Then create an if statement that prints a random number between one and six if the value stored in the variable isvalid equals the Boolean value true. 3. Write the code for an if statement that adds 5 to the variable num1 if the value stored in the variable testa equals 25. Otherwise subtract 5 from num1. 4. Write a Python program that prompts the user for a word. If the word comes between the words apple and pear alphabetically, print a message that tells the user that the word is valid, otherwise, tell the user the word is out or range. 6. Write a Python program that prompts the user for a multiple of 5 between 1 and 100. Print a message telling the user whether the number they entered is valid.

8 Individual Homework Activity: 1. Write a Python program that prompts the user for the cost of two items to be purchased. Then prompt the user for their payment. If they enter an amount that is less than the total cost of the two items, print a message that tells them how much they still owe. Otherwise, print a message that thanks them for their payment and tells them how much change they will receive. Thoroughly test your code for all possible input. Your program must contain documentation lines that include your name, the date, a line that states "PA5 Individual Homework problem 1" and a description line that indicates what the program is supposed to do. Take a screen shot of the program and the output and attach the files to homework python activity 5 in Moodle. You must also attach the file that ends in.py to the assignment. (10 points) 2. Create a Python program that prompts the user for four test grades. It will be used to determine if a student has earned an A, B, C, D or F in the class, using the following grade cut-offs A >= 90% B % C % D % F <60.00 A student can drop the lowest exam score of the four exam scores. Therefore, the program should find the average of the three highest grades and print this average. It should also print a message stating the letter grade they earned in the course. Thoroughly test your program with data that calculates a grade for each letter grade. Also be sure that the lowest grade is not included in the calculation independent of whether it is the first, second, third or fourth test score entered by the user. Hint: look up the python min() predefined function to help you. The link is in the reading for this assignment. Sample output: