CS 105 Lab As a review of what we did last week a. What are two ways in which the Python shell is useful to us?

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Julie Eaton
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1 1 CS 105 Lab 3 The purpose of this lab is to practice the techniques of making choices and looping. Before you begin, please be sure that you understand the following concepts that we went over in class: How to use: if, elif, else The difference between a while loop and a for loop Error checking of input Examples we did in class such as the prime number program, and getting the computer to count. If you are unsure about any of the above, please don t hesitate to ask for help. On your USB drive or computer account, please create a new folder called lab03. All the files you create will go into this folder. Start the Python system like you did in the previous lab. You will need two windows: the shell, and the editor. 1. As a review of what we did last week a. What are two ways in which the Python shell is useful to us? b. Why does a program need comments? c. Give an example Python statement that will read a real number as input from the user. Hint: you need to use two built-in functions to accomplish this. Part 1: Fahrenheit Celsius converter Let s write a program that allows a user to convert between Fahrenheit and Celsius temperatures. This program will have two inputs. First, we want the program to let the user choose which way to convert, either Fahrenheit-to-Celsius or Celsius-to-Fahrenheit. The user will specify this choice by entering either the letter F or C, respectively. The second input will be the temperature. Then, the program can perform the appropriate conversion and output the result. 2. What are the types of these two inputs? Specify which one is which.

2 2 Here is the code for the program. Please type it in, and save the program as convert.py. Please note that there are some details that you need to fill in, such as the appropriate condition to use with the ifstatement, and the two conversion formulas. In your if-statement, you will need to use quotes around the letter C or F so that Python does not think that you are using this letter as a variable. Another hint: F = (9/5) C You can solve for C to determine the other formula. # convert.py Convert between Fahrenheit and Celsius # Purpose: to practice the if-else statement. # input print("would you like to convert from F to C or from C to F?") print("enter F if you want to convert a Fahrenheit temperature, or") print("enter C to enter a Celsius temperature.") choice = input("enter your choice (F/C): ") # Perform calculation based on whether F or C was entered. if : f = input("enter the Fahrenheit temperature: ") # insert Celsius formula... c = print("the equivalent Celsius temperature is ", c) else: c = input("enter the Celsius temperature: ") # insert Fahrenheit formula f = print("the equivalent Fahrenheit temperature is ", f) print("thank you for using my temperature conversion program.") 3. Oops! There was an error in the program I gave you. How did you fix it? 4. Run the program both ways, to check that both formulas you entered work correctly. Ask your program for these conversions: 14 Fahrenheit = in Celsius 25 Celsius = in Fahrenheit

3 3 5. How would the program behave differently if the last statement in the program were indented? In other words, when running the program, how could we tell that something is abnormal? Part 2: Adding error checking As we saw in class, there is a straightforward way to get a program to check the validity of the user s input. If the input is invalid, we can print an appropriate error message and allow the user to try again without having to quit the program and start over. For example, here is how we could check to make sure that the user enters a positive number: needinput = True while needinput: n = int(input("enter a positive integer: ")) if n > 0: needinput = False else: print("sorry, input is invalid. Please try again.") 6. What data type is the value contained in needinput? Modify convert.py so that it performs error checking on the first input. It must be the capital letter F or the capital letter C. If it is any other character, this would be considered invalid. Be sure to use quotes around the F and the C, so that the computer does not think that these are variable names. Also note that some of the code that you wrote earlier now needs to be indented. 7. Write down the if-statement s condition that helps you perform the error checking.

4 4 Part 3: Practice with multiple-choice if-statement Let s write a simple program that works with days of the week. Create a new source file called monday.py. The purpose of this program is to ask the user for a positive integer n, and then output what day of the week it is n days after a Monday. For example, if the user enters 27, the answer should be Sunday. The general format of the program is as follows. I ll describe the structure in English: First, we ask the user to enter a positive integer, and put this answer in a variable called n. Second, we perform the calculation n % 7 and put the result back into n. 8. What are the smallest and largest possible values of n after we perform this calculation? Third, we create a new variable called day, which will contain a string. This string will be the name of the day of the week that is n days after Monday. Finally, we print a sentence telling the user what the value of day is. To start you off, you may copy the following code into your monday.py file. There are two things you need to add to this program. First, you need to make the appropriate assignment statement that modifies n. And second, you need to add the additional cases in the if-statement to handle all possible days of the week. # monday.py What day of the week is it n days after a Monday? # input n = int(input("enter a positive integer: ")) # mod n by 7 and make this the new value of n # You need to enter this assignment statement here: # Create a new variable called day, and set it to a string # such as "Monday", "Tuesday", etc. depending on the value of n. # Because there are several choices, we will make use of the # Python words if, elif, and else. if n == 0: day = "Monday" # Insert the remaining cases here... # Ready for output: print(n, " days after Monday will be ", day)

5 5 9. When you are ready to run your program, test it a few times with various input. Try these: 12 days after a Monday is 100 days after a Monday is 365 days after a Monday is One billion days after a Monday is 10. How can you tell if your output (e.g. that you just wrote down above) is correct? 11. If the user entered the number 1 as input, our output will begin as follows 1 days after Monday This is not grammatically correct. Show how you would use an if-statement to handle this special case of n equals 1. In this case we want to print the word day instead of days. 12. When you run the program with n equals 12, the program responds by saying 5 days after Monday The reason why the 12 became 5 is that we performed a mod operation on n. Let s modify the program so that it also retains the original value of n. Explain how you would do this: Make the necessary modification to your program, and show it to the instructor or lab aide.

6 6 Part 4: Practice with a while loop Let s use a while loop to help us add a series of numbers of this form: n The format of this program will be simple. First, the program will ask the user for a positive integer n. Next, the program will have a while loop that counts from 1 to n. This should sound familiar because we did something similar to this in class. However, now, instead of simply counting, we want to perform a cumulative addition. So, we are going to need another variable called sum or total. Create a new source file called sum.py. You may use the following code to start you off. Some details have been left out, such as the initial values of count and sum, as well as the assignment statement for sum inside the while loop. # sum.py Find the sum of the numbers from 1 to n, where n # is specified by the user. # input n = int(input("please enter a positive integer: ")) count = sum = while count <= n: # You need to insert an assignment statement to modify sum. # output print("the sum from 1 to ", n, " is ", sum) 13. Once you are finished entering in your program, run it several times on various cases. Write down these results. If n = 3, the sum is If n = 10, the sum is If n = 1414, the sum is If n = 5, the sum is

7 7 Part 5: Practice with a for loop It turns out that Python has two kinds of loops: while loops and for loops. Copy your sum.py program into a new version called sum2.py. We will make some changes to this program. First, replace the while loop with a for loop. In order to accomplish this, you will need to use the range function. 14. Let s review. If we call the range function with the parameters 1 and 5 like this: range (1, 5), what is the resulting list of numbers? Next, we will perform one more modification to sum2.py. Rather than summing just the positive integers from 1 to n, let s sum the squares of these numbers. In other words, your program should calculate: n 2. To accomplish this change, you need to modify the assignment to sum inside the for-loop. Also, please change the output statement so that it reads The sum of the squares from 1 to 15. When you are ready, run your program on several different input values for n. And write down your results of these tests: If n = 3, the sum is If n = 175, the sum is If n = 5, the sum is 16. Write down the while loop you have in sum.py, and beside it, write down the for-loop you have in sum2.py. Which type of loop do you prefer, and why? 17. Explain how you would modify the program so that it prints the value of sum each time we add the next perfect square. Show the instructor or lab aide your program s output.

8 8 Part 6: The break statement The purpose of the break statement is to allow a program to abandon the remaining iterations of a loop. Let s use the break statement in one of the earlier example programs we saw in class. Specifically, it will be the program that determined if a number is prime. Create a new Python source file called prime.py. From your class notes, copy the prime number program into prime.py. 18. Run the program to make sure that it works correctly. What input value did you test to see that your program could tell if a number is a. Prime? b. Not prime? The idea behind this program is that we are counting divisors. If the number of divisors is 2, then the input number is prime. But in general, this can be inefficient. During the loop, if we have already encountered 3 or more divisors, what is the point in continuing? Modify your program so that it will break from the loop if the number of divisors found is 3 or more. 19. Have the instructor or lab aide check your work for this part. 20. What is an example of an input value for which your program will now run much faster thanks to having a break statement? What made you choose this number? 21. Suggest another way that this program can be improved or enhanced.

LOOPS. Repetition using the while statement

LOOPS. Repetition using the while statement 1 LOOPS Loops are an extremely useful feature in any programming language. They allow you to direct the computer to execute certain statements more than once. In Python, there are two kinds of loops: while