Computational Models

• A computational model takes the form of an algorithm, that is, a precise description of the steps that are carried out.
  Two informal examples of algorithms are recipes and instructions for knitting.
• The algorithm takes a set of inputs and eventually turns them into an output.
  In a recipe, the ingredients are the inputs, the finished dish the output.
• In a serial algorithm, only one action can be executed at a time.
  This is true for knitting instructions, for example.
• In a parallel algorithm, multiple actions are executed simultaneously by different parts of the system.
  A recipe may be partly parallel because you can cook something while chopping or stirring something else.
• A computational modeler implements an algorithm in a computer program, which is a set of instructions written in a programming language (or computer language) which particular computers know how to interpret. The actual computer (the hardware) used to run the program (the hardware) should not make any difference (even though this not always true in practice, as we have seen from our experience trying to get StarLogo to work on the Macintosh used for the lecture).
• Most programming languages, such as Basic, C, Pascal, Fortran, Cobol, Logo, Lisp, and Scheme, are designed for implementing serial algorithms.
• Parallel programming languages, for example, StarLogo, are designed for implementing parallel algorithms.
• Since most computers are serial machines which can execute only one instruction at a time, it is necessary to simulate parallelism on them. This can be done by dividing each of the processes which are supposed to happen simultaneously into a number of small steps and interleaving these steps to give the illusion of parallelism. Because we can make the steps as small as we want, we can come as close as we want to simulating parallelism. With a parallel language like StarLogo, the work of implementing the parallel algorithm on the serial machine is done for us by the language. When you write a program in StarLogo, it is as if you are writing it for a parallel computer.