Design Issues

Although C lacks the most powerful features of C++ as far as concepts and data organization is concerned, Stroustrup says: ``Remember that much programming can be simply and clearly done using only primitive, data structures, plain functions, and a few library classes.'' This means that one can write many useful and well-written programs in C. Here are a few pointers that will assist you in structuring your C program:

• Identify groups of variables having common function (e.g. basis set, etc.) and organize them into structures. Use several levels of hierarchy if necessary (e.g. a basis set is a collection of basis functions each of which may be described by a structure). This is called ``hierarchical ordering''.
• Think as generally as possible. What you may not need today will be asked for tomorrow. Design data structures that are flexible and modular, i.e. one can be easily modified without affecting the others (e.g. you do not want the structure describing basis sets to know anything about the type of basis functions it contains so that plane waves can be used as easily as Gaussians).
• Write ``constructors'' for the structures, i.e. functions which will initialize data in the structures (e.g. read basis set information). Make as many ``constructors'' as necessary (e.g. basis set info can be read from the checkpoint file or from pbasis.dat). If it is difficult or impossible to write a ``constructor'' for some data structure is a sign that your data hierachy is poorly designed and there are mutual dependencies. Spend more time designing the system. If it doesn't help, then use source code comments heavily to describe the relationships not reflected in the code itself.
• Use global variables sparringly. Placing a variable into global scope leaves it unprotected against ``unauthorized'' use or modification (we are not talking about security here; it is a good idea to protect data from the programmer, because if you do not want some data A to be modified by function B, do not make A available to B) and may also have impact on program's performance. Sometimes it is a good idea to use global data to reduce the cost of passing that data to a function. However, the same effect may be achieved by organizing that data into a local structure and passing the structure instead.
• Learn how to use static variables local to a source file, it is a very powerful tool to protect data in a C program.
• Organize the source code such as to emphasize further the structure of the program (see section 5.3).

More material on data organization may be found in the Stroustrup's book.