Sigrid Ewert

A shrinking lemma for random forbidding context languages

Random context grammars belong to the class of context-free grammars with regulated rewriting. Their productions depend on context that may be randomly distributed in a sentential form. Context is classified as either permitting or forbidding, where permitting context enables the application of a production and forbidding context inhibits it. We concentrate on non-erasing grammars that use forbidding context only. We show that they are strictly weaker than the non-erasing random context grammars and prove a shrinking lemma for their languages.

A pumping lemma for random permitting context languages

Random context grammars belong to the class of context-free grammars with regulated rewriting. Their productions depend on context that may be randomly distributed in a sentential form. Context is classified as either permitting or forbidding, where permitting context enables the application of a production and forbidding context inhibits it. For random context languages of finite index a generalization of the well-known pumping lemma for context-free languages has been proven. We drop the finite index restriction and concentrate on non-erasing grammars that use permitting context only. We prove a pumping lemma for their languages that generalizes and refines the existing one, and show that these grammars are strictly weaker than the non-erasing random context grammars

Generating pictures using random forbidding context

We use random context picture grammars to generate pictures through successive refinement. At any stage a picture consists of a shape divided into smaller shapes, each containing a variable or terminal. A variable may be rewritten according to a production of the underlying grammar, which entails either dividing the shape containing it into smaller shapes, or substituting a variable or terminal for it. A production may depend on context randomly distributed in the intermediate picture. Context is classified as either permitting or forbidding, the former enabling the application of a production, the latter inhibiting it. For visualization purposes every terminal is associated with a color, and its shape filled with that color. We show examples of pictures generated with random context picture grammars. Then we concentrate on grammars which use permitting context only and present a pumping lemma for the corresponding picture sets.

Abstracting and Narrating Novice Programs Using Regular Expressions

The Syntax-Free Approach (SFA) to teaching programming was proposed in response to the perceived challenges often faced by novice programmers while taking their first course in programming. The idea of the SFA is to provide a level of abstraction over the language syntax and teach programming to novices as algorithms instead of lines of code. In this paper, we report the development of a tool that translates novice programs into detailed textual algorithms using regular expressions. We refer to these algorithms as narrations. These narrations are syntax-free, can improve readability and aid the comprehension of programs. The technique described can also be employed for automatic generation of hints or tips for novice programmers during classroom or laboratory sessions.

Requirements for secure graphical password schemes

Visual or graphical passwords have been proposed as possible alternatives to alphanumeric passwords. People can remember these better and for longer periods than they can alphanumeric passwords. Visual passwords can, however, not yet be readily adopted for use due to the disadvantages associated with them. There has therefore been a move in literature to discussions on efficient implementation of visual password schemes. This paper analyses the current challenges in the design of visual password schemes as well as the factors affecting security and usability of such schemes. The paper further presents requirements of a visual password scheme providing ease of use, a large enough password space and secure password images.

A property of random context picture grammars

We use random context picture grammers to generate pictures through successive refinement. The productions of such a grammar are context free, but their application is regulated by context randomly distributed in the developing picture. Grammars using this relatively weak context often succeed where context-free grammars fail, e.g., in generating the typical iteration sequence of the Sierpinski carpet. On the other hand, it proved possible to develop iteration theorems for three subclasses of these grammars; finding necessary conditions is problematic in the case of most models of context-free picture grammars with context-sensing ability, since they consider a variable and its context as a connected unit.We present a property of all picture sets generated with random context picture grammers, and then construct a picture set that does not belong to this class.

Introducing Code Adviser: A DFA-driven Electronic Programming Tutor

In this paper, we demonstrate a software system called Code Adviser that attempts to understand and find semantic bugs in student programs written in C++ programming language. To do this, Code Adviser has to take a model solution from a lecturer (or expert), generate many variations of the model solution, and compare student programs to the most similar model solution. The student’s program to be checked for correctness is normalized, granulated and abstracted to a string of semantic tokens — we call this the abstraction stage. Similarly, the model solutions are taken through the abstraction stage and the program strings representing all model solutions are abstracted to deterministic finite automaton (DFA) for the programming problem. Code Adviser then uses some algorithms to make inference on student’s program correctness. If the student’s program string is accepted by the problem’s DFA, it is reported as correct. Else, we make inferences on what the bug could be. Code Adviser is a promising proof of concept, and more work is currently being done to improve its inference and make it available to student programmers.

Bag Context Tree Grammars

We introduce bag context, a device for regulated rewriting in tree grammars. Rather than being part of the developing tree, bag context (bc) evolves on its own during a derivation. We show that the class of bc tree languages is the closure of the class of random context tree languages under linear top-down tree transductions. Further, an interchange theorem for subtrees of dense trees in bc tree languages is established. This result implies that the class of bc tree languages is incomparable with the class of branching synchronization tree languages.

Necessary conditions for subclasses of random context languages

Random context grammars belong to the class of context-free grammars with regulated rewriting. Their productions depend on context that may be randomly distributed in a sentential form. Context is classified as either permitting or forbidding, where permitting context enables the application of a production and forbidding context inhibits it. We have proven a pumping lemma for random permitting context languages and a shrinking lemma for random forbidding context languages. We now present new necessary conditions for both these classes of languages and illustrate them with examples. We also present and illustrate a new necessary condition for context-free languages.

Code Pathfinder: A Stepwise Programming E-Tutor Using Plan Mirroring

A significant problem in Computer Science Education is introducing students to programming. Many novice programmers show difficulties in mastering the basics of writing programs. Many students may abandon their study of Computer Science due to these problems. Intelligent Tutoring Systems have been developed to provide guidance and feedback to students. Previous systems require the instructor to prepare extra documentation for the software to function. This creates more work for lecturers who wish to implement such a system. We have developed an Intelligent Tutoring System that will guide a student step-by-step through the writing of simple programs in the language of C++. It will also provide feedback on any mistakes they make. This system will require only a correct version of the code for it to develop its feedback scheme.