CS /29/2004 (Recitation Objectives) and Computer Science and Objects and Algorithms

Recitation Objectives ● Occasional projects to support lecture topics, such as studying Google as an introduction to talking about information and the web. ● Improve your programming skills ● Create a good web site (at least 2 linked pages) for yourself ● Learn at least some Java, some html, and some Linux ● An opportunity for you to check in with me about your grade if you wish to do so.

What is Computer Science? ● At the highest level computer science (CS) is the study of computation and information processing, both in hardware and in software. ● Computation can be defined as finding a solution to a problem from given inputs (information received) by means of an algorithm (set of instructions). ● For thousands of years, computing was done with pen and paper, or chalk and slate, or mentally, sometimes with the aid of tables.

Information Processing... ● At its most general, information processing is the changing (processing) of information in any manner that an observer can detect. Computer scientists study how information is used by or changed by computers.

More Computer Science ● In practice, computer science includes a variety of topics relating to computers. These range from the abstract analysis of algorithms, formal grammars, etc. to more concrete subjects like programming languages, software, and computer hardware. ● Computer science is very different from mathematics, programming, software engineering, and computer engineering, although these fields are often confused. ● Computer science has its roots in electrical engineering, linguistics, and mathematics.

Quotations... ● Edgar Dijkstra: “Computer science is no more about computers than astronomy is about telescopes.” ● Richard Feynman: “Computer science is not as old as physics; it lags by a couple of hundred years. However, this does not mean that there is significantly less on the computer scientist's plate than on the physicist's: younger it may be, but it has had a far more intense upbringing!”

What is an Object? ● An object is a unique concrete instance of an abstract data type whose identity is separate from that of other objects, although it can communicate with them using messages.

What is an abstract data type? ● An abstract data type is a conceptual structure that includes both data and the methods to access it. ● Abstract data types are described in terms of the operations they support (their interface) instead of how they are implemented. Users are aware of the interface but not of the implementation. ● When done correctly this means that the implementation can be completely changed without needing to modify any existing code that is using the abstract data type.

Benefits of object-oriented programs... ● When structured correctly, one can build and change chunks of programs independently from each other. ● Another claim for object-oriented program is that this approach models real life problems "naturally" by a sort of simulation.

Objects... ● Object-based systems are typically built using a class-based language, where an instance is "instantiated" as an object of a particular class. Each instance is a variation on a general theme, which is defined by the parent class. For example, my personal plate could be described as an instance of a plate (where plates are a class of dishes).

Objects... ● In small groups, we will assign you a small task. ● Quickly identify the objects involved in the task. ● Pick ONE object and identify the attributes (qualities) of the object and its methods (actions it can do). ● Pick someone who will tell the class the task, the objects, and the attributes and methods of the object you chose.

What is an Algorithm? ● An algorithm is a well-defined set of instructions, finite in number, for accomplishing some task which, given a set of inputs, will result in some recognizable end-state. ● The concept of an algorithm is often illustrated by the example of a recipe, although many algorithms are much more complex; algorithms often have steps that repeat (iterate) or require decisions (such as logic or comparison) until the task is completed.

Formally... ● In formal mathematical terms, an algorithm is considered to be any sequence of operations which can be performed by a Turing-complete system.

Simply... ● Algorithms are the ideas behind the computer programs. ● The algorithm stays the same regardless of the language the program is written in and the location/size/generation/type of the computer on which it is running.

Algorithms... ● Algorithms typically have steps that repeat (iterate) or require decisions (comparison and logic) until the task is completed. ● Functions are often used to implement algorithms.

Different algorithms... ●... may complete the same task with a different set of instructions in more or less time, space, or effort than others. A cooking recipe is an example of an algorithm. Given two different recipes for making potato salad, one may have "peel the potato" before "boil the potato" while the other presents the steps in the reverse order, yet they both call for these steps to be repeated for all potatoes and end when the potato salad is ready to be eaten.

Correctness ● Correctly performing an algorithm will not solve a problem if the algorithm is flawed, or not appropriate to the problem. For example, performing the potato salad algorithm will fail if there are no potatoes present, even if all the motions of preparing the salad are performed as if the potatoes were there.

Some things... ● Computer scientists study include: – Objects, how and how to structure and use them – Algorithms, including correctness, appropriateness, space, time, and difficulty

Your assignment... ● Describe an object: – Pick a SIMPLE task (like making a cup of tea with a teabag). – Describe the task functionally (how would you explain it to a five-year-old). – Now describe it in an object-oriented way: ● Identify the list of objects involved ● For ONE of the objects list its attributes (qualities) and methods (the things it can do)