Top Down Design is a problem solving technique where:
Top-down design is a lot like creating a paper outline for a large paper where you start with the main sections, and iteratively refine each part with more and more detail until you are ready to start writing the paper.
When you use top-down design, your resulting program’s structure will match the above idea of an outline: the main function should have calls to a few high-level functions, one for each high-level step; high-level functions have calls to unctions that implement sub-steps of the high-level step; and so on.
When writing a large program, programmers use Iterative Refinement: do some top-down design, write function stubs for this part of code and maybe some implementation, then test. Iteratively, add more functions to accomplish subproblems, and perhaps refine some of the steps using Top-Down design, and test, and so on. The idea is to write some code, test it, then write a little more code, and test it before writing even more. Usually, I write a function, then test it, write another function, test it, … This way if I’m careful about testing, I know that if there is a bug in my program it is with the new code I’ve just added.
We often use prototyping to just put in function stubs so we can test the whole program’s flow of control without having to have a full implementation. For example, here is a stub for a function to compute square root (it doesn’t actually do anything related to the task, but I can call it from other parts of my program to see if the program’s flow matches the design):
def squareRoot(num):
"""
This function computes the square root of a number.
num: the number
returns: the square root of num
"""
print "inside squareRoot"
# TODO: implement this function
return 1 # a bogus return value, but it let's me run the program
# and make calls to this function stub to "see" program flowWe are going to walk through the process of designing a computer game to play Black Jack. Follow along and work with a partner or two using your worksheets.