Program Design in a Nutshell:


The design recipe for data definitions guides the students to ask the following questions:

       Can you represent the information by a primitive data type?

       Are there several related pieces of information that describe one item? If yes, design a composite data type (struct, class).

       Does the composite data type contain another complex piece of data? Define that data type separately and refer to it. (A Book data item contains an Author data item.)

       Are there several variants of the information that are represented differently, but are related (e.g. a circle, a rectangle, a triangle --- all are shapes)? If yes, design a union type. (In Java, define a common interface.)

       Repeat these steps. This may lead to self-reference, mutual reference, and eventually to a complex collection of classes and interfaces.

       Make examples of data for every data type you design.


The design recipe for functions/methods:

       Write down in English the purpose statement for the function/method, describing what data it will consume, and what values will it produce. Add a contract that specifies the data types for all inputs and the output.

       Make examples of the use of the function/method with the expected outcomes.

       Make an inventory of all data, data parts, and functions/methods available to solve the problem.

       Now design the body of the function/method. If the problem is too complex, use a wish list for tasks to be deferred to helper functions.

       Run tests that evaluate your examples. Add more tests if needed.


If every function produces a new value, the result, then the entire design process is very straightforward:

       Tests are simple, as they only verify that the result matches the expected value.

       Function composition comes naturally --- the result of any function application can be used in further computations.

       The order of computation does not affect the result. (However, a function or a data item must be defined before it can be used.)


To test method that change state (have side effects) you have to do the following:

       Setup: Initialize the data needed to invoke the method (and to verify the results)

       Invoke the method to be tested.

       Test the expected results, and the expected changes (effects).

       Tear-down: Reset the data that has been used to their original values (it the data will be used again in other tests).


The design recipe for abstractions helps us eliminate code repetition and produce a more general solution:

       Mark all places where the similar code segments differ.

       Replace them with parameters and rewrite the solution using them as arguments.

       Rewrite the original solutions to your problems by invoking the generalized solution with the appropriate arguments.

       Make sure that the tests for the original solution still pass.