In this panel, we will explore the use of auto-graded programming assignments to support timely and effective feedback to learners via small, scaffolded programming tasks. Program comprehension involves processing at different levels [5,10]. As students proceed through a program, associative processes take place and are described as information as the current statements activate information from the previous statements and from memory of prior knowledge. The most frequently inferred relations by the students are those that provide a coherent understanding of the state changes and outputs of a program [3] as well as the purpose of a piece of code [9]. The resulting, interconnected representation of the program goes beyond the syntax of tokens and statements. The outcome of successful comprehension is a representation that captures the meaning of each statement as students infer the operations of a statement, in terms of the underlying data and control flow, given its function in the context of solving a problem [4]. Although models of program comprehension generally agree regarding the processes by which a student arrives at a mental representation of a program, it is less clear how student-initiated processes play a role in program comprehension, and how they combine with such passive processes to result in comprehension.

No abstract available.

Transient spam-bots are hijacked computers that are connected to the Internet for short periods of time, during which they send large amounts of spam. These spam-bots have become a principle source of spam; against which, static countermeasures such as DNS Black Lists are largely ineffective, and content-based filters provide temporary relief without ongoing tuning and upgrading---a never-ending cat-and-mouse game.

A snapshot is an important object in distributed computing whose implementation in asynchronous systems has been studied extensively. It consists of a collection of m >1 components, each storing a value, and supports two atomic operations: an UPDATE of a specified component's value and a SCAN of all components to determine their values at some point in time.In this paper, we investigate implementations of a multiwriter snapshot object in a synchronous shared memory model. In this setting, we show that a snapshot object can be efficiently implemented and prove a tight tradeoff between the complexity of the SCAN and the UPDATE operations. First, we describe a wait-free implementation that performs UPDATE in O(1) time and SCAN in O(m) time, using only slightly more than twice the amount of space needed to simply store the m values. We also describe a variant that performs UPDATE in O(1) time and SCAN in O(n) time.Second, we describe a wait-free implementation that performs UPDATE in O(log m) time and SCAN in O(1) time, and a variant that performs UPDATE in O(log n) time and SCAN in O(1) time.Third, we show how to combine these implementations to realize two implementations that perform UPDATE in Θ(log(m/c)) time and SCAN in Θ(c) time, for 1≤c≤m, or perform UPDATE in Θ(log(n/c)) time and SCAN in Θ(c) time, for 1≤c≤n. This implies that Time[UPDATE] ∈O(log(minm,n/Time[SCAN])). We also prove that Time[UPDATE] ∈ Ω(log(minm,n/Time[SCAN]) ), which matches our upper bound.

Recent work has demonstrated a peer-to-peer storage system that locates data objects using O(logN) messages by placing objects on nodes according to pseudo-randomly chosen IDs. While elegant, this approach constrains system functionality and flexibility: files are immutable, directories and symbolic names are not supported, data location is fixed, and access locality is not exploited. This paper presents Mammoth, a peer-to-peer hierarchical file system that, unlike alternative approaches, supports a traditional file-system API, allows files and directories to be stored on any node, and adapts storage location to exploit locality, balance load, and ensure availability. Our approach handles all coordination at the granularity of files instead of nodes. In effect, the nodes that store a particular file act as its server independently of other nodes in the system. The resulting system is highly available and robust to failure. Our experiments with our prototype have yielded good results, but an important question remains: how the system will perform on a massive scale. We discuss the key issues, some of which we have addressed and others that remain open.

The 2-way quantum finite automaton introduced by Kondacs and Watrous [Proceedings of the 38th Annual Symposium on Foundations of Computer Science, 1997, IEEE Computer Society, pp. 66--75] can accept nonregular languages with bounded error in polynomial time. If we restrict the head of the automaton to moving classically and to moving only in one direction, the acceptance power of this 1-way quantum finite automaton is reduced to a proper subset of the regular languages. In this paper we study two different models of 1-way quantum finite automata. The first model, termed measure-once quantum finite automata, was introduced by Moore and Crutchfield [Theoret. Comput. Sci., 237 (2000), pp. 275--306], and the second model, termed measure-many quantum finite automata, was introduced by Kondacs and Watrous [Proceedings of the38th Annual Symposium on Foundations of Computer Science, 1997, IEEE Computer Society, pp. 66--75]. We characterize the measure-once model when it is restricted to accepting with bounded error and show that, without that restriction, it can solve the word problem over the free group. We also show that it can be simulated by a probabilistic finite automaton and describe an algorithm that determines if two measure-once automata are equivalent. We prove several closure properties of the classes of languages accepted by measure-many automata, including inverse homomorphisms, and provide a new necessary condition for a language to be accepted by the measure-many model with bounded error. Finally, we show that piecewise testable sets can be accepted with bounded error by a measure-many quantum finite automaton, introducing new construction techniques for quantum automata in the process.