Due to CMS by Sunday, September 28th at 11:59 pm.
This is a simple written assignment where you are going to diagram a few call frames.
This assignment should (hopefully) only take you an hour or two. So even if you are
busy revising assignment 1, you should be able to do this assignment as well. With
that said, keep track of the actual time that you spent on this assignment for the
“Diagramming” is what we do in the lecture notes where we draw boxes
to represent function calls and folders to represent the objects. These are a way of
giving us a visual representation of what is going on in the computer when it runs
Python. The purpose of this assignment is to solidify your understanding of how
function calls work.
This assignment is design to help you understand the following concepts.
How to use call frames to model function execution.
The difference between a call frame and global space.
How folders represent Python's handling of objects.
Table of Contents
Authors: W. White, L. Lee, S. Marschner
Academic Integrity and Collaboration
This assignment is a slightly modified version of an assignment given in a
previous semester. Please do this assignment without consulting (or seeking)
previous solutions. Consulting any prior solution is a violation of
You may do this assignment with one other person. If you are going to work together, then
form your group on CMS as soon as possible. This must be completed before you
submit the assignment. Both people must do something to form the group. The first
person proposes, and then the other accepts. You do not need to have the same
partner as last time; you can change your partner with each assignment.
If you do this assignment with another person, you must work together. It is against the
rules for one person to to fill out this assignment and just have the other person put
their name at it. You should actively collaborate on this assignment. You may not collaborate
with anyy other than your CMS partner.
Using the Online Python Tutor
We have demonstrated the Online Python Tutor
several times in class. As you may realize, using the Tutor makes (parts of) this assignment
really easy. We are perfectly okay with this. It is a good way to help you understand the
formalism if you are particularly lost.
If you used the Python Tutor to help you with part of the problem, please cite this
(for each part that you used it) in your submission.
We will not take off any points if you use the Tutor and cite it. We only
require a citation because it is part of our
Academic Integrity Policy.
Even though this is just a written assignment, some of you may still have some
difficulty with it. You might not completely understand how function calls work
or the purpose of folders. If you are lost, please see someone in the
This can be the course instructor, a TA, or a consultant. Furthermore, do not
wait until the last minute, as the course staff is generally overwhelmed
just before an assignment deadline.
Most of our diagramming conventions follow the lecture notes. However, just to be safe,
we will make our conventions explicit here.
A diagram for a variable should include the variable name and a box for the value. We
we reassign a variable, old values are crossed out when they are replaced by new ones.
For example, here is a diagram for the variable b, currently holding the integer 2:
When writing the value, the only rule is to make sure it is unambiguous what the value
is, and what type it is. Write floats with a decimal point or in scientific notation,
strings with quotes around them, and so forth.
All values in Python are objects, but we only bother to draw an object separately
from variables referring to it if the object is mutable (e.g. the contents
of the folder can be changed). All of the base types —
— are immutable. Therefore we draw all of these values inside the
variable referring to them.
So far the only mutable objects we have seen are those with named attributes, like
those of type Point.
You will not be responsible for figuring out how to diagram an arbitrary object
in this assignment. For any mutable object (which requires a folder), we will
show you how to diagram the object. When you diagram an object you should
obey the following conventions:
The folders identifier (ID, or true name) should be unique and go on the folder tab.
The type of the object should go at the top right of the folder.
The attributes should be listed inside the folder and diagrammed just like variables.
For example, a folder for an object of the type
might look like this:
Diagramming Function Calls
Call frames are the way we diagram the temporary workspace where the variables used during
a call to a function are stored. Diagrams of frames should obey the following conventions:
The top left corner is a box with the function name.
The top right counter is the program counter. This is
next line of code to execute in the function.
Unless otherwise stated, the first line of code after the function header
corresponds to a counter value of 1.
Local variables and parameters are diagrammed in the frame body;
we do not distinguish between them.
Local variables are not added until the instruction counter reaches
the line where they are first defined.
See the lecture notes for more details on how to create a call frame. In order
to help you with the instruction counter, we will always number the lines
of code for you. For example, suppose we have the procedure
def print_name(last, first):
1 greet = 'Hello '+first+' '+last+'!'
2 print greet
If we execute the call
print_name("White","Walker"), then the call
frame starts out as follows:
The Evolution of a Call Frame
A call frame is not static. As Python executes the body of the function, it alters
the contents of the call frame. For example, after we have executed line 1, the
call frame now looks as follows:
This is not a new call frame; this is the same call frame shown at a different
point in time. The call frame does not go away until the function call is
completed. When we diagram a call frame, we are actually diagramming a
sequence of boxes to show how it changes over time.
As a general rule, we should draw a separate frame diagram for each value of the
program counter, corresponding to a single line of Python code. The diagram
above refers to the call frame after we finish line 1. Once we finish line 2,
the diagram for the call frame is as shown below.
Note the instruction counter is blank. That is because there are no more
instructions to execute. However, the call frame is not yet deleted. To
represent the deletion of the call frame, we draw one more diagram, crossing
out the frame.
Frames and Return Values
Remember from class that fruitful functions have a return value. For
example, consider the following variation on a function from class.
1 tmp = 5*(x-32)
2 return tmp/9.0
When you execute a return statement, it makes a special variable called
RETURN which stores the result of the expression. For example,
suppose we have the function call to_centigrade(212). After executing
line 1 of the code, above, the call frame is as follows:
When you execute the next line, the instruction counter becomes blank (because
there is nowhere else to go), but you add the RETURN variable.
If you play with the
Online Python Tutor,
you will note that it always adds a RETURN variable, even
for procedures (i.e. functions with no return statement). In that case it stores
the value None into the RETURN variable. For example, when
processing the call
print_name("White","Walker"), this is what
the Python Tutor says that the frame looks like just before it is erased.
We are not following this convention, and you are not expected to do this.
You should only add a RETURN variable for fruitful functions.
Frames and Global Space
Remember that you can use a function call as an expression. The call evaluates to
the result of the return statement. If this call is used in an assignment statement,
the is a way to copy from the RETURN variable to another place in memory, such
as global space. For example, the call
y = to_centigrade(212.0)
will copy the final result (100.0) into the global variable y.
If function call is used in an assignment statement, then the value in the box is not
changed until the step where you erase frame. In fact, if this is the first time that you
are assigning the variable, you do not even create the box for the variable until the
frame is erased.
You should denote changes to global space right next to the diagram for the frame. In
our example above, that your diagram before erasing the frame is as follows:
When you do erase the frame, the global space has changed to
Modules and Functions in Global Space
As we saw in class, global space also has variables for the function and module names.
For example, when you define a function, it creates a folder for the function body
and stores the name of that folder in a variable with the same name of the function.
We do not want you to draw these variables in global space. You can ignore
what happens to function definitions (and module importing) for now.
Remember that there are three memory locations that we talked about in class: global space,
call frames, and heap space. Every time that you execute a line of code, these can all
change. We want you to draw pictures to show how they change over time.
For each problem, you will be given a function call. We want you to diagram the execution
of four different function calls (one each in Part A, C, B, and D). You will draw a separate diagram for each line of Python
that is executed (and only those lines that are executed). We also want you to
draw one last diagram crossing the frame off at the end.
Some of the problems below will have associated values in both global space and heap space.
For those problems, you should also diagram the evolution of both global space and heap
space. For each line of code, next to the diagram for the call frame, you should redraw
both global space and heap space.
Important: You must pay close attention to types in this assignment. The value
3 is an integer while the value 3.0 is a float. Hence they are different
values. If you answer with an integer like 3 when the correct answer is a float
like 3.0, we will deduct points.
min is built into Python. However, if we were to design
it ourselves, it might look like this:
"""Returns: min of a and b"""
1 minimum = a
2 if (b < a):
3 minimum = b
4 return minimum
Part A: Diagram the Execution of
c = min(1,2)
Draw the execution of the statement
c = min(1,2). You
will need to draw the evolution of the call frame over time. This will
require several diagrams. You will need a diagram when the frame is created,
and another when it is erased. In between, you will need a diagram for
each line of code that is executed (and only the lines which
are executed). Remember, this is one call frame, but several diagrams
showing how the call frame changes over time.
When an assignment changes the value of an existing variable in the frame,
we would like you write the old value in the box, cross it out, and write
the new value in the box.
In addition to the diagrams for the (evolution of the) call frame, you
also need to diagram the evolution of the global variable
particular, global space starts out empty (we are ignoring the variable for
the function min); we want to know when the variable c appears
in global space. You do not need to draw the evolution of heap space, as there
is nothing interesting in heap space for this problem.
Part B: Diagram the Execution of
d = min(b,a)
For this next part, suppose that you have the following (global) variables:
As we said in class, these variables are global variables and are not
the same as the parameters of
min, which are local to that function.
Repeat the previous exercise for the statement
d = min(b,a). Once
again, you need to draw the evolution of global space right next to each individual
frame diagram. The difference is that this time global space does not start out
empty. In addition, the new global variable is now named
Functions on Point Objects
The next two problems have to do with the Point class introduced in lecture.
Objects of the Point class have three attributes: x, y, and
z. These objects are created by the constructor call Point(x,y,z).
When dealing with objects, you should remember that a variable does not contain
an object; it can only hold the identifier of the object. Therefore, the
p = Point(1,2,3)
q = Point(4,5,6)
would produce the following contents in both global and heap space
In this case we have folders representing two Point objects, and two global
variables storing these object identifiers. We made up the folder identifier numbers,
as their value does not matter. If we ask you to make a Point object, you should
feel free to make up the identifier number, but make sure it is different from the ones
currently in p and q.
Part C: Diagram the Execution of
rot_x(q) function is a variation on the
cycle_left(q) function from the optional exercises in
It takes a point as input and swaps the y and z attributes; the x attribute is
left unaltered. This is a rotation around the x axis. We define this
function as follows:
"""Swap the y and z attributes of q
Example: The point (1,2,3) becomes (1,3,2)
Precondition: q is (refers to, contains the ID of) a point object"""
1 tmp = q.y
2 q.y = q.z
3 q.z = tmp
The function rot_x is a procedure (it has no return statement) and
so it does not need to be used in an assignment statement. Hence the call
rot_x(p) is a statement by itself.
Diagram the call
rot_x(p) given the global variables and heap space
shown above. Your answer will in include a sequence of diagrams for the
call frame for
rot_x(p). Again, you should have a diagram when
the frame is created, a diagram for each line executed, and a diagram for
deleting the frame.
Your diagrams should also include the evolution of the global variables
q, and the corresponding objects in heap space.
You should draw the global space and heap space next to every single diagram
in your evolution of the frame. If the contents of any global variable change,
or the contents of the object folders are altered, you should cross out the old
value and write in the new value.
Part D: Diagram the Execution of
p = rot2_x(q)
rot2_x(q) is similar to
that it is a fruitful function instead of a procedure. We define the function as
"""Returns: A copy of q with the y and z attributes swapped
Example: rot2_x with argument (1,2,3) creates the point (1,3,2)
Precondition: q is (refers to, contains the ID of) a point object"""
1 r = Point(0.0,0.0,0.0)
2 r.x = q.x
3 r.y = q.z
4 r.z = q.y
5 return r
Diagram the execution of the statement
p = rot2_x(q) given the global
variables and heap space shown above. Assume that you are starting over with
p and q; do not include any changes from Part C.
As always, you should have a diagram when the frame is created, a diagram for each
line executed, and a diagram for deleting the frame. You should draw the global space
and heap space next to every single diagram in your evolution of the frame. If the
contents of any global variable change, or the contents of the object folders are
altered, you should cross out the old value and write in the new value.
Finishing the Assignment
When you finish the assignment, put your name and netid at the top of
the page. Otherwise, we will not know that the assignment is yours, and we
cannot give you credit for your work. If you are working with a partner,
then the partner's name must be on the assignment as well. In addition,
to help us out with organization, please underline your last name(s).
Creating a PDF for Submission
Because of the size of this class, we want you to submit this assignment
online through CMS. To do this, you must submit your file as a PDF. If you
created your assignment on a computer, this should be relatively easy.
Otherwise, you will need to scan your paper as a PDF. There are
Olin and Uris Library to help you with this.
Your solution must be a single PDF file. You cannot submit multiple PDF files.
If your scanner makes more than one file, you must combine them together. On Windows,
the program PDFtk
can merge multiple PDF files into one file. In OS X, the built-in application
can be used to merge PDF files.
Your final file must be less than 100MB in size; ideally it should be less than 10MB.
This should not be a problem as long as the resolution is reasonable. Do not scan at a
higher resolution than 300 dpi. This is not an art class; we do not need that level of
detail. If your file is too large, use the web page
SmallPDF to reduce your file size.
Your submission must be uploaded to CMS by Sunday, September 28th at 11:59 pm. If you
are planning to scan your submission, we recommend that you finish
long enough ahead of time to give you time to scan.
Once again, this assignment comes with a survey on CMS. This survey asks
how long you spent on the assignment, your impression of the difficulty,
and what could be done to improve it.
Please try to complete the survey within a day of turning in this assignment.
Remember that participation in surveys compromise 1% of your final grade.
We also ask that you be honest in your answers.