In Class: Week 6, Tuesday, Thursday

Run update21 to create a w06-functions subdirectory and copy example programs

    $ update21
    $ cd cs21/inclass/w06-functions
    $ pwd
      /home/your_user_name/cs21/inclass/w06-functions
	

Topics

• Using function call stack diagrams to trace program execution
• More FUNctions!
• Mutable vs immutable parameters

1. We will see how to use stack diagrams to trace through a program. First, open oops_squareFunc.py. Read the program and run it. Why did the program not work?

A useful website for visualizing code is Python Tutor. Use this tool to see how we represent the execution of a program as a stack diagram. Use this link to load oops_squareFunc.py into the python tutor. Step through the function.

2. Let us do a trace of a program using stack diagrams. Work through this program on pen and paper and attempt to maintain a stack diagram. The dir() function returns a list of variables in scope

     1   def main():
     2      a = 10
     3      b = 55 
     4      print("in function...dir() = "+str(dir()))
     5      result = absval(a,b)
     6      print("The absolute value of %d - %d is %d" % (a,b, result) )
     7      
     8   def absval(x,y): 
     9      print("in function... dir() = "+str(dir()))
     10     if x > y:
     11       z = x - y
     12     else:
     13       z = y - x
     14     #
     15     #Pause program and show full stack diagram
     16     #
     17     return z
     18     
     19  main()
   

   
   What is the final output?
3. Run squareList.py through Python Tutor. Why do we get different behavior with this program than with oops_squareFunc.py?
4. Draw a stack diagram in the following program. Note that there may be an error that prevents this program from running. Can you find it?:

   def main():
   
     list_x = [7, 10]
     size = len(list_x)
     
     print(list_x[0])
   
     size = some_function2(list_x)
   
     print(size)
     print(list_x[0])
   
   def some_function2(list_nums)
     
     list_nums.append(15)
     list_nums[0] = 23
     print(list_x)
     print(size)
   	
     #Draw stack
     
     return len(list_nums)
   main()
   

5. There were a few examples from last week that we did not get to cover. However, they each demonstrate different ways to use functions. First, open w05-graphics/animate.py to see how we can create animated objects. We will learn about the sleep(x) function, which causes the program to pause for x seconds.
6. Next trace through w05-graphics/duplicate.py from last week. Why does this program not do what we desire? How can we fix it?
7. Open w05-graphics/biggest_circle_soln.py and read through the solution. We will trace through this program to see how a stack diagram helps illustrate understand our solution.
8. In w05-graphics/lights.py, we will see another use of functions with graphics. We will also seen an example of the None value and the use of checkKey() to detect user feedback
9. Lastly, in w06-functions/guess.py, we will practice more defense programming against incorrect user input. In particular, we will learn about about exception handling.