Box-and-Pointer Diagrams - Rules for Drawing and Reading - CSSE 120

Oct 25, 2026

Upload by James

Box-and-pointer diagrams provide a visual snapshot of variables, references, and object values at a given program point. The material explains how variables point to objects, how containers hold references to other objects, and how assignment and function calls create new references. It gives step-by-step drawing rules for non-container objects, variables, and container objects, then shows how re-assignment changes arrows while mutation changes the contents of containers without changing their referenced objects. It also warns about mutable objects’ risks.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages5

Reading Time13 min

AccessFree to download

Introduction to Box-and-Pointer Diagrams

Objects, Variables and References

How to Draw a Box-and-Pointer Diagram

Re-assigning a variable

Mutable objects

CSSE 120 – Introduction to Software Development Box and Pointer Diagrams – Page 1 of 5 Introduction to Box-and-Pointer Diagrams A Box-and-Pointer Diagram displays a snapshot of the variables and their values at a given point of execution of a program. For example, after the code below executes: x = 48 z = x p1 = zg.Point(100, 150) the corresponding box-and-pointer diagram is as shown to the right. (We will work through the details of this and other examples in this exercise.) | Box and Pointer diagram: | | | --- | --- | | x
z
p1 | 48

x y ... | In this reading and the exercise that accompanies it, you will learn how to draw and read box-and-pointer diagrams. Additionally, you will see how they clarify the following key ideas (all of which you’ll examine in this exercise): • Variables are REFERENCES to objects. • Assignment (e.g. x = 100) causes a variable to refer to an object. • Function calls ([e.g.](e.g. foo)[ foo](e.g. foo)(54, x)) also cause variables to refer to objects. • Some types of objects are mutable, which means that the “insides” of such an object can be re-assigned. (We’ll be more explicit shortly.) Other types of objects are immutable. Objects, Variables and References Recall that in Python, every piece of data is an object. Objects have a type (which determines what the object can do and the types of data that the object contains/knows) and a value. Variables are REFERENCES to objects – we say that the variable refers to an object, or points to an object, or is the name for an object (several ways to say the same thing, in this context) . Some objects contain references to other objects; these objects are called containers. Azg. Point is a container (as is any object of a user-defined class) . Lists, strings and tuples are containers. Numbers are not containers. We will see that box-and-pointer diagrams help clarify what we mean by references. CSSE 120 – Introduction to Software Development Box and Pointer Diagrams – Page 2 of 5 How to Draw a Box-and-Pointer Diagram Rule 1 : Draw a NON-CONTAINER OBJECT by putting its value inside a box, as in these examples: 48 10.13 Rule 2 : Draw a VARIABLE using a box labeled with the variable’s name and with arrows from the box to the object to which the variable currently refers. For example, after this code executes: x = 48 | Box and Pointer diagram: | | | --- | --- | | x | 48 | the corresponding box-and-pointer diagram is as shown above and to the right. Rule 3 : Draw a CONTAINER OBJECT by making a box for it, and then creating sub-boxes that are drawn as if they were variables, but with field names for fields of an object and indices for items of a sequence. For example, after this code executes: x = 48 p = zg.Point(100, 150) numbers = [4, 30.2, 10] the corresponding box-and-pointer diagram is as shown to the right. The ellipsis (…) for the zg.Point object reflects the fact that zg.Points have an outline color, etc. | Box and Pointer diagram: | | | | --- | --- | --- | | x | 48 | | | p | | | | x y ...
4 30.2 10
numbers
0 1 2
| | | CSSE 120 – Introduction to Software Development Box and Pointer Diagrams – Page 3 of 5 Re-assigning a variable Recall that variables are REFERENCES to objects – we say that the variable refers to an object, or points to an object, or is the name for an object (several ways to say the same thing, in this context) . Box-and-pointer diagrams help clarify what we mean by references, as in the following example: The box-and-pointer diagram on the right shows the situation after the code on the left runs. x = 48 | Box and Pointer diagram: | | | --- | --- | | x | 48 | If we now RE-assign the variable x, as shown in the example to the right, the arrow out of the box for x changes – it points to the new value for x. Note that we put an through the old arrow to indicate that the old arrow no longer exists. x = x + 3 W

FAQs

What does a box-and-pointer diagram show at a given point in a program?

It shows variables and the objects they refer to, including how container objects hold references and how values are represented in boxes.

How do assignments and function calls affect variables in these diagrams?

They cause variables to refer to objects, so the arrows from the variable boxes point to the newly referenced objects after the code runs.

What is the difference between re-assigning a variable and mutating a container object?

Re-assigning changes the arrow out of the variable box to point to a new object, while mutation changes the contents “inside” the container object without changing which object the container variable refers to.

Oct 25, 2026

Upload by James