Encyclopedia of Computer Science and Technology

simulation 431Further ReadingCharles in Space. Available online. URL: http://www.charlesinspace.com/. Accessed November 14, 2007.Greene, Stephen G. “Entrepreneur Seeks to Promote Excellencethrough Philanthropy.” Chronicle of Philanthropy 16 (February19, 2004): 20.Intentional Software. Available online. URL: http://www.intentsoft.com/. Accessed November 14, 2007.Rosenberg, Scott. “Anything You Can Do, I Can Do Meta: SpaceTourist and Billionaire Programmer Charles SimonyiDesigned Microsoft Office. Now He Wants to ReprogramSoftware.” Technology Review, January 2007. Available online.URL: http://www.technologyreview.com/Infotech/18047/?a=f.Accessed November 14, 2007.SimulaOne of the most interesting applications of computers isthe simulation of systems in which many separate actionsor events are happening simultaneously (see simulation).During the 1950s, Norwegian computer scientist KristenNygaard began to develop a more formal way of describingand designing simulations. A typical simulation consistsof a number of “objects,” such as cars in a traffic flow orcustomers waiting in a bank line. In a bank simulation,for example, the objects (customers) would demand servicefrom particular serving objects (teller windows). Theywould move in a queue and their motion would be capturedat various points of time.Nygaard used his ideas to create symbols and flow diagramsto represent the events going on in a simulation.However, existing computer languages such as Algol 60were designed to carry out procedures sequentially and oneat a time, not simultaneously. This made it difficult to writea program representing a situation in which many cars orcustomers were moving simultaneously.In the early 1960s, Nygaard was joined by Ole-JohanDahl, who had more experience with systems programmingand computer language design. They worked together tocreate a new language that they called Simula, reflectingtheir emphasis on simulation programming. In designingSimula, the authors sought to create a data structure thatwas better suited to simultaneous actions or events. Forexample, in a simulation of automobile traffic, each carwould be an “object” with data such as its location andspeed as well as actions or capabilities such as changingspeed or direction. The data for each object must be maintainedseparately and updated frequently.The Algol 60 language already had a way to define code“blocks” (see procedures and functions) that could containtheir own local data as well as actions to be performed.Further, such blocks could be called repeatedly such thatmany copies could be “open” at the same time. However,these calls were still essentially sequential, not simultaneous.In their new Simula 1 language (introduced in 1965),Dahl and Nygaard created a way to simulate simultaneousprocessing. Even though the computer would (probably)only have a single processor such that only one copy ofa block of code could be executing at a given time, Simulaset up special variables for keeping track of simulatedtime. Control would “jump” from one instance of a block toanother such that all blocks would, for example, have theiractions for the time 20:15 executed, then actions for 20:16would be executed, and so on. A list kept track of processesin time order. Thus, Simula 1 kept all the features of Algolbut made it more suitable for modeling simultaneous events(see multiprocessing).Simula 1 was quite successful as a simulation language,but the authors soon realized that the ability to use separateinvocations of a procedure to create individual “objects”had a more general application to representing data inapplications other than simulations. In creating Simula 67(the version of the language still used today), they thereforeintroduced the formal concept of the class as a specificationthat could be used to create objects of that type. Theyalso introduced the key idea of inheritance (where one classcan be derived from an earlier class), as well as a way that aderived class could redefine a procedure that it had inheritedfrom the original (base) class (see object-orientedprogramming and class).Although Simula 67 would continue to be used primarilyfor simulations rather than as a general-purpose programminglanguage, its object-oriented ideas would proveto be very influential. The designers of Smalltalk and Adawould look to Simula for structural ideas, and the popularC++ language began with an effort to create a “C withclasses” language along the lines of Simula. (See Smalltalk,Ada, and C++.)Further ReadingHolmevik, Jan-Rune. “Compiling Simula: a Study in the History ofComputing and the Construction of the SIMULA ProgrammingLanguages.” STS Report (Trondheim). Available online. URL:http://staff.um.edu.mt/jskl1/simula.html. Accessed August 21,2007.Nygaard, K., and O.-J. Dahl. “The Development of the Simula Languages”in The History of Programming Languages, R. L. Wexelblat,ed. New York: Academic Press, 1981.Pooley, R. Introduction to Programming with Simula. Oxfordshire,U.K.: Alfred Waller, 1987.Sebesta, Robert W. Concepts of Programming Languages. 7th ed.Boston: Addison-Wesley, 2006.Sklenar, J. “Introduction to OOP in Simula.” Available online.URL: http://staff.um.edu.mt/jskl1/talk.html. Accessed August21, 2007.simulationA simulation is a simplified (but adequate) model that representshow a system works. The system can be an existing,real-world one, such as a stock market or a human heart,or a proposed design for a system, such as a new factory oreven a space colony.If a system is simple enough (a cannonball falling froma height, for example), it is possible to use formulas suchas those provided by Newton to get an exact answer. However,many real-world systems involve many discrete entitieswith complex interactions that cannot be captured witha single equation. During the 1940s, scientists encounteredjust this problem in attempting to understand what wouldhappen under various conditions in a nuclear reaction.