In Class: Week 3 Tuesday

Run update21 to Create a week03 subdirectory in your cs21/class directory:

      % update21
      % cd 
      % cd cs21/class
      % cd week03        
      % pwd
        /home/your_user_name/cs21/class/week03
      % ls
	

Topics

• Accumulation pattern with strings
• bool type and values True and False
• Conditional Statements
• Simple decisions with if statements
• Two-way decisions with if-else statements
• Multi-way decsions with if-elif-else statements
• Relational operators ==, !=, <, <e, >, >e
• Logical operators and, or, not
• Accumulation pattern with booleans using flag variables
• Random library

We are going to do some of the following:

Accumulating Strings

1. open drop_letter.py in vim: together we will write a program that takes an input string and creates a new string that consists of every other character dropped from the first string. This is an example of using the accumulator pattern to accumulate a string value.

Conditional statements

2. Open is_odd.py. Together, we'll use an if-statement to determine if a number is odd or not.
3. Now, open up grade.py and try using a multi-way decision statement (a series of if-elif statements) to convert a student's numeric test score into a letter grade assuming a standard curve
4. Next, we will combine accumulation patterns with boolean variables in the program contains_a.py. We will write an algorithm that checks to see if a work contains the letter 'a'. We will use a boolean variable to assume there are no a's, and only change the value to True if an 'a' is found. containsA is a flag variable because it's value only changes to indicate a condition has been met. Once its value has changed, it cannot go back. Here, once we find an 'a', containA should never go back to being false.
5. If you finish contains_a.py, try changing the algorithm so that it takes if there is an 'a' OR an 'A'. Once that is done, can we modify our algorithm to search if there is a second 'a'? This requires us to store the index of the first 'a' and begin the search anew but starting the for loop from the stored index.
6. Together, we will implement noWhammy.py. We'll ask the user to enter numbers and check to see if they enter the whammy value (e.g., 0). First, we'll use a boolean flag variable to see if they've entered the value whammy value. Notice that the game continues even after hitting a whammy since the for loop isn't finished. We'll add a break statement to immediately stop the loop.

   Random library

7. Open up coinFlip.py. Write a program that flips a virtual coin by using the choice() method from the random library. Flip a coin 10 times and use an accumulator pattern to count how many times the coin came up heads. Once that is done, add a second accumulator pattern to store the sequence of coin flips. We'll represent a heads as "H" and tails as "T"

Pattern of stars

8. In stars.py we'll closely inspect the relationship between the loop variable and the inner-loop body by printing out several different patterns of stars and other characters