Top-down and bottom-up design

''"Top down" redirects here. For the view angle in computer and video games, see top-down perspective. For the computer and video game genre centered in this view angle, see top-down shooter.

Top-down and bottom-up are strategies of information processing, mostly involving software, and by extension other humanistic and scientific systems theory.

In the top-down model an overview of the system is formulated, without going into detail for any part of it. Each part of the system is then refined by designing it in more detail. Each new part may then be refined again, defining it in yet more detail until the entire specification is detailed enough to validate the model. The top-down model is often designed with the assistance of "dark boxes" that make it easier to bring to fulfillment but insufficient and irrelevant in understanding the elementary mechanisms.

By contrast in bottom-up design individual parts of the system are specified in detail. The parts are then linked together to form larger components, which are in turn linked until a complete system is formed. Strategies based on this bottom-up information flow seem potentially necessary and sufficient because they are based on the knowledge of all variables that may affect the elements of the system.

Computer science

Some part of this chapter is from the Perl Design Patterns Book.

In the software development process the top-down and bottom-up approaches play a key-role.

Top-down approaches emphasise planning and a complete understanding of the system. It is inherent that no coding can begin until a sufficient level of detail has been reached in the design of at least some part of the system. This, however, delays testing of the ultimate functional units of a system until significant design is complete. Bottom-up emphasizes coding and early testing, which can begin as soon as the first module has been specified. This approach, however, runs the risk that modules may be coded without having a clear idea of how they link to other parts of the system, and that such linking may not be as easy as first thought. Re-usability of code is one of the main benefits of the bottom-up approach.

Top-down design was promoted in the 1970s by IBM researcher Harlan Mills and Niklaus Wirth. Mills developed structured programming concepts for practical use and tested them in a 1969 project to automate the New York Times morgue index. The engineering and management success of this project led to the spread of the top-down approach through IBM and the rest of the computer industry. Niklaus Wirth, among other achievements the developer of Pascal programming language, wrote the influential paper Program Development by Stepwise Refinement. Top-down methods were favored in software engineering until the rise of object-oriented programming in the late 1980s.

Modern software design approaches usually combine both top-down and bottom-up approaches. Although an understanding of the complete system is usually considered necessary for good design, leading theoretically to a top-down approach, most software projects attempt to make use of existing code to some degree. Pre-existing modules give designs a bottom-up flavour. Some design approaches also use an approach where a partially-functional system is designed and coded to completion, and this system is then expanded to fulfill all the requirements for the project.

Programming

Top-down programming is a programming style, the mainstay of traditional procedural languages, in which design begins by specifying complex pieces and then dividing them into successively smaller pieces. Eventually, the components are specific enough to be coded and the program is written. This is the exact opposite of the bottom-up programming approach which is common in object-oriented languages such as C++ or Java.

The technique for writing a program using top-down methods is to write a main procedure that names all the major functions it will need. Later, the programming team looks at the requirements of each of those functions and the process is repeated. These compartmentalized sub-routines eventually will perform actions so simple they can be easily and concisely coded. When all the various sub-routines have been coded the program is done.

Advantages

Programming team stays focused on the goal
Everyone knows his or her job.
By the time the programming starts there are no questions.
Code is easier to follow, since it is written methodically and with purpose.

Disadvantages

Top-down programming may complicate testing, since nothing executable will even exist until near the end of the project.
Bottom-up programming may allow for unit testing, but until more of the system comes together none of the system can be tested as a whole, often causing complications near the end of the project, "Individually we stand, combined we fall."
All decisions depend on the starting goal of the project, and some decisions cannot be made depending on how specific that description is.

Neuroscience and psychology

This vocabulary is also employed in neuroscience and psychology. The study of visual attention provides an example. If your attention is drawn to a flower in a field, it may be simply that the flower is more visually salient than the surrounding field. The information which caused you to attend to the flower came to you in a bottom-up fashion -- your attention was not contingent upon knowledge of the flower; the outside stimulus was sufficient on its own. Contrast this situation with one in which you are looking for a flower. You have a representation of what you are looking for. When you see the object you are looking for, it is salient. This is an example of the use of top-down information.

External links

Information science | Neuroscience | Software development