Lecture 18 - Logical Operators and Logic

As usual, create two directories for today's class. Create a directory called lecture18 under activities, and a directory called lab18 under labs.

Logical Operators and Truth Tables

Most of you were trying to use the logical operators before. However, I forced you to not use them last week. The reason is simply because last week, you should have been able to write everything without the logical operators. Today, we will explore how the logical operators work, and how we can leverage them to our benefit.

In-class Activity 1

Truth Tables

Create a file (using Emacs) called truth_tables.txt in your lecture18 directory. In this file, write the truth tables for the following expressions:

• not p and q
• p or not q
• (p or q) and not p
• (p or q) and not (p and q)

If you put your file in the "wrong" location:

  cd ~/cs120/activities/lecture18
  cp ../lecture16/truth_tables.txt .

In-class Activity 2

Date Validation Redux

Now that you know the logical operators, you can vastly simplify your code for date validation. First, copy date_validation.py from your lab16 directory:

cd ~/cs120/activities/lecture18
cp ~/cs120/labs/lab16/date_validation.py .

You should have three functions: is_leap_year, days_in_a_month, and validate_date. Simplify the conditional expressions in each function using your logical operators. Don't forget to check the month in days_in_a_month, to make sure it is in the specified range.

Challenge: You can completely eliminate conditionals from is_leap_year. You can also reduce days_in_a_month to 5 total conditionals.

Lab Assignment 18

T-9 Word

Back in the dark ages, before smart phones existed, whenever you went to write a text message to your friend you had to type in a letter by hitting the corresponding number on the number pad. Each number (Except for 1 and 0) represented 3 different letters:

2

ABC

3

DEF

4

GHI

5

JKL

6

MNO

7

PRS

8

TUV

9

WXY

Notice that there are no numbers to represent Q or Z. Many phones added upgraded software which allowed Q and Z to be typed, but older phones couldn't input messages with Q's and Z's. So much for texting your friends the word "Quetzalcoatl."

Create a file called old_school_texting.py in your lab18 directory. In this file, write a function t9_word, which takes a single letter as a parameter. Using your logical statements, your function should return the numeric digit that one could press to enter that letter. Your function should return the string "Error" if the user types Q or Z. You should only have one return statement in this function. Look at lecture 17's test_me.py file, to see how you could accomplish this.

At the bottom of your file, provide test cases for your t9_word function. You should provide (in comments!) and explanation on why you chose your test cases, and an argument as to why they are sufficient.

Challenge: You can actually accomplish this by using less than 9 conditionals. Do it!

Submission

When you have finished, create a tar file of your lab18 directory. To create a tar file, execute the following commands:

cd ~/cs120/labs
tar czvf lab18.tgz lab18/

To submit your activity, go to cseval.roanoke.edu. You should see an available assignment called Lab Assignment 18. Only one of your pair should submit your activity. Make sure both partners are listed in the header of your file.

Do not forget to email your partner today's files!