Bottom-up design is a programming approach in which a program is written in a series of components or layers rather than all at once. The programmer views each component as a tool to solve the problem at hand. Eventually enough tools are created to solve the problem. The program in its entirety is at the top of the hierarchy. Underneath the program is a layer of less powerful components. These components are themselves made of even lower levels of components, and so on. At the bottom of the hierarchy, primitive operations are found.

For example, the problem to be solved is to create a program that tracks company employees. This program may be made of several smaller programs: insert a new employee, delete an employee, insert an address, change an address, print an employee list, sort employees by department, and so on. Each of these programs...