Editing Bottom-up and top-down design (section)

===Software development===
{{Main|Perl Design Patterns Book}}

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

Top-down approaches emphasize 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. Top-down approaches are implemented by attaching the stubs in place of the module. But these delay 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. But this approach 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. [[Code reuse|Re-usability of code]] is one of the main benefits of a bottom-up approach.<ref name="autogenerated1">{{cite web |url=http://step.psy.cmu.edu/scripts/Attention/Treisman1980.html |title=STEP: Scripts: Attention: Treisman and Gelade 1980 |publisher=Step.psy.cmu.edu |date=March 13, 2003 |access-date=October 21, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20110914130951/http://step.psy.cmu.edu/scripts/Attention/Treisman1980.html |archive-date=September 14, 2011 |df=mdy-all }} [https://facultypsy.hope.edu/psychlabs/exp/visualsearch/readings/TreismanGelade_1980.pdf Article PDF]</ref>{{failed verification|date=November 2024}}

Top-down design was promoted in the 1970s by [[IBM]] researchers [[Harlan Mills]] and [[Niklaus Wirth]]. Mills developed [[structured programming]] concepts for practical use and tested them in a 1969 project to automate the ''[[The New York Times|New York Times]]'' [[The New York Times Archival Library|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. Among other achievements, Niklaus Wirth, the developer of [[Pascal programming language]], wrote the influential paper ''Program Development by Stepwise Refinement''. Since Niklaus Wirth went on to develop languages such as [[Modula]] and [[Oberon (programming language)|Oberon]] (where one could define a module before knowing about the entire program specification), one can infer that top-down programming was not strictly what he promoted. Top-down methods were favored in [[software engineering]] until the late 1980s,<ref name="autogenerated1"/>{{failed verification|date=November 2024}} and [[object-oriented programming]] assisted in demonstrating the idea that both aspects of top-down and bottom-up programming could be used.

Modern [[software design]] approaches usually combine 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 flavor.

<gallery>
Top Down Approach.png|Top-down approach to [[integration testing]]
Bottom Up Approach.png|Bottom-up approach
Sandwich Approach.png|Sandwich approach
Bing Bang Approach.png|Big bang approach
</gallery>