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Scope

 

Static Scope

 

 

 

 

Dynamic Scope

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Scope in Programming Languages

 

Amruth N. Kumar

Eric Fernandes, Steve Obringer

Ramapo College of New Jersey

amruth@ramapo.edu

 

Introduction

 

Scope is an important concept in programming languages – one cannot read or write large programs without properly understanding the concept of scope. The scope of a variable in a program is the lines of code in the program where the variable can be accessed.

 

Historically, two different types of scope have been used in programming languages: static scope and dynamic scope. Most modern programming languages such as Pascal, C++ and Java use static scope. Some languages such as APL, Snobol4 and early versions of LISP use dynamic scope. As students of Programming Languages, we will study both these types.

 

The converse of scope is referencing environment. The referencing environment of a procedure is the set of all the variables whose scope extends to the procedure. It may be divided into local and non-local environments. The local referencing environment of a procedure consists of all the variables declared within the procedure, and includes any formal parameters of the procedure. The non-local referencing environment consists of all the variables not declared in the procedure, but whose scope extends to the procedure.

 

The concept of scope applies not only to variable names but also to the names of procedures. The scope of the name of a procedure determines all the procedures/functions in a program that can call the procedure.

 

Scope Courseware

 

This courseware addresses all these aspects of scope. It comprises of five modules addressing the following topics:

1.  Static Scope of variables

2.  Static Referencing Environment of a procedure

3.  Dynamic Scope of variables

4.  Dynamic Referencing Environment of a procedure

5.  Callability of procedures - Scope of procedure names

 

Pascal is popularly used to illustrate the intricacies of static scope since it allows procedure definitions to be nested. In order to help you compare and contrast static scope with dynamic scope, and to reduce the burden of having to learn new syntax, this courseware will use Pascal-like programs to illustrate dynamic scope also. 

 

Contents of Each Module

 

Each of the five modules contains a tutorial and an applet. The applet is hyperlinked from the tutorial. The information in each tutorial is organized under the following titles:

 

Definition: In this section, the definition of the concept is presented. Where applicable, the definition is clarified in the context of Pascal.

 

Example: This section includes the following items:

1.  A sample program, and a problem based on the program;

2.  Answer to the problem, along with explanation.

 

Pragmatics: In this section, additional questions are presented based on the earlier example. These questions are designed to help you consider other aspects of the problem, and extend what you have learned to other concepts.

 

Algorithms to determine “Scope”: Whereas the definition section describes the relevant “theory”, this section lists the algorithms that you may want to use to apply the theory to solve problems.

 

Test your understanding - Instructions: This section lists the following items:

1.  Your options for solving problems using the applet;

2.  The steps you must follow to solve each problem;

3.  A screen shot illustrating these steps;

4.  A hyperlink to the applet itself. Note that when you click on the hyperlink, the following two windows will be launched:

a.  A web page to record how many problems you have attempted and how many problems you have solved correctly.

b.  The applet window that you will use to solve the problems. You may solve as many problems as you wish without exiting the applet window. Please pay particular attention to the feedback provided by the applet for each problem.

Once you exit the applet, you can re-launch it by clicking on the “Create New Problem” button on the accompanying web page.

 

Acknowledgements

 

Partial support for this work was provided by the National Science Foundation’s Course, Curriculum and Laboratory Improvement Program under grant DUE-0088864, January 2001-December 2002.