254 .What is the difference between top-down and bottom-up, programming? 

Ans: Top-down and Bottom-up Approaches
In Top-down programming approach, we start by identifying the major modules of
the program, decomposing them into their lower modules and iterating until the
desired level of detail is achieved. This is also termed as Step Wise Refinement;
starting from an abstract design, in each step the design is refined to a more concrete
level until we reach a level where no more refinement is needed and the design can
be implemented directly, whereas in Bottom-up programming approach, we identify
modules that are required by programs and decide how to combine these modules to
provide larger ones; to combine those to provide even larger ones, and so on, till we
arrive at one big module which is the whole of the desired program.
In Bottom-up approach, we need to use a lot of intuition to decide exactly what
functionality a module should provide. This is not the case in Top-down approach.