Tracing Assigment

One of the most important skills that a programmer can develop is the ability to trace code. Tracing code requires walking through your program step by step and examining the content of the variables at each step to determine the final outcome of the program.

For your first assignment, you are to trace the chaos program shown below. You write out the values of each variable at each step of your program in a table. You should also show what the program is printing out at each step. Here's an example program and its trace table. It's only necessary to show the value of a variable when it changes. The ~ (tilde) means that the variable doesn't exist yet (it's out of scope).

An Example Trace

# File:        trace.py
# Written by:  Sue Evans
# Date:        7/23/09
# Section:     All
# Email:       bogar@cs.umbc.edu
# Description: A simple program used to illustrate tracing
#              Inline comments have been intentionally omitted

print "This program illustrates tracing"

a = 5
b = 2

sum = a + b 
difference = a - b
print "sum = ", sum
print "difference = ", difference

a = a + 2
b = b + 1 

sum = a + b 
difference = a - b
print "sum = ", sum
print "difference = ", difference

The Trace Table

This is the simple trace table for the code shown above, as it would appear in a plain text file written with emacs.

Notice that there is one column for each variable and a separate one for the output.
There is one line for each processing step. There should never be more than one column filled on any one line, except the first line where the initial state is shown.

   a    b   sum  difference     output
__________________________________________________________________________

   ~    ~    ~       ~          This program illustrates tracing
--------------------------------------------------------------------------
   5
--------------------------------------------------------------------------
        2
--------------------------------------------------------------------------
             7
--------------------------------------------------------------------------
                     3
--------------------------------------------------------------------------
                                sum = 7
--------------------------------------------------------------------------
                                difference = 3
--------------------------------------------------------------------------
   7
--------------------------------------------------------------------------
        3
--------------------------------------------------------------------------
            10
--------------------------------------------------------------------------
                     4
--------------------------------------------------------------------------
                                sum = 10
--------------------------------------------------------------------------
                                difference = 4
__________________________________________________________________________

The Chaos Program

Even though the chaos program only consists of 5 lines of code excluding the comments, your trace table should contain many more steps than the example trace, since the chaos program contains a looping construct.

Create a trace table for the chaos program shown below. For lines that require user input, just make up a value (not 0 or 1 ).
Show the values of x to 5 decimal places.

# File:        chaos.py
# Written by:  John Zelle
# Date:        ?/?/04
# Section:     2
# Email:       zelle@umbc.edu (not really)
# Description: A simple program executing chaotic behavior
#              Inline comments have been intentionally omitted

print "This program illustrates a chaotic function"

x = input("Enter a number between 0 and 1: ")

for i in range(10):
	x = 3.9 * x * (1-x)
	print x

Working in your account and using submit

• Wait until you've gone to your first lab before starting this homework.
• Log into your account and change directory to your hw1 directory.
• Create a file in your hw1 directory that will contain your trace table for the chaos program. If you already know how to use emacs, xemacs, or vi then use one of these text editors to write your file and name it trace.txt.
• If you don't know how to use a text editor yet, we'll be teaching you to use emacs in next week's lab. If you want to start on this homework now, do it on paper so that you will just have to type it in later.
• Please, do not attempt to submit your homework until you see an announcement on the web page that says submit is ready to use.
• When you've finished your homework, and it's in your hw1 directory, and submit is ready to be used, use the submit command to submit the file.
• You must be logged into your account.
• You must be in the same directory as the file you're trying to submit.
• At the Linux prompt, type

  submit cs201 HW1 trace.txt

The submit command has four parts. The word submit, followed by the class, followed by the name of the assignment (note uppercase HW), followed by the name(s) of the file(s) to be submitted. You must be in the same directory as the files you are submitting for the submit command to work properly. The submit command does produce error messages if the command fails and confirmation if the command works properly.

After entering the submit command shown above, you should get a confirmation that submit worked correctly. Specifically, the confirmation will say:

Submitting trace.txt...OK

If not, check your spelling and that you have included each of the required parts and try again.

If you believe that you have typed everything correctly and the submit command fails again, then send your instructor, Mr. Lupoli (slupoli@cs.umbc.edu) or Dawn Block (dblock@umbc.edu), an email about the problem. In this email, please show exactly what you typed in and the exact message that was displayed. We'll be able to fix submit problems quickly.

You can check to make sure your submission went through by entering
submitls cs201 HW1
You should see the name of the file that you just submitted, in this case, trace.txt.