Differentiate Top down and bottom up approach with an example.

Example 1 :These are two design approaches, which can be explained as:
You can imagine a tree like structure , in which when you 
are following top-down appoarch you move from root node to 
leaf node and when you follow bottom-up apprach you follow 
leaf node to root node.

Top-down approach:
In this an overview of the system is first formulated, 
specifying but not detailing any first-level subsystems. 
Each subsystem is then refined in yet greater detail, 
sometimes in many additional subsystem levels, until the 
entire specification is reduced to base elements. 

Bottom-up approach:
In this approach the individual base elements of the system 
are first specified in great detail. These elements are 
then linked together to form larger subsystems, which then 
in turn are linked, sometimes in many levels, until a 
complete top-level system is formed.


Top down design proceeds from the abstract entity to get to
the concrete design. Bottom up design proceeds from the
concrete design to get to the abstract entity.

Top down design is most often used in designing brand new
systems, while bottom up design is sometimes used when one
is reverse engineering a design; i.e. when one is trying to
figure out what somebody else designed in an existing system.

Bottom up design begins the design with the lowest level
modules or subsystems, and progresses upward to the main
program, module, or subsystem. With bottom up design, a
structure chart is necessary to determine the order of
execution, and the development of drivers is necessary to
complete the bottom up approach.

Top down design, on the other hand, begins the design with
the main or top-level module, and progresses downward to the
lowest level modules or subsystems.

Real life sometimes is a combination of top down design and
bottom up design. For instance, data modeling sessions tend
to be iterative, bouncing back and forth between top down
and bottom up modes, as the need arises.

Answered by: siva85     On: 28-Apr-2011

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