Chapter 15. Using PLplot from Tcl

Table of Contents
Motivation for the Tcl Interface to PLplot
Overview of the Tcl Language Binding
The PLplot Tcl Matrix Extension
Contouring and Shading from Tcl
Understanding the Performance Characteristics of Tcl

PLplot has historically had C and Fortran language bindings. PLplot version 5.0 introduces a plethora of new programming options including C++ (described earlier) and several script language bindings. The Tcl interface to PLplot (which the PLplot maintainers regard as the "primary" script language binding) is described in this chapter, with further discussion of Tcl related issues following in additional chapters. But Tcl is certainly not the only script language option. Bindings to Perl, Python, and Scheme (which is actually another compiled language, but still has some of the flavor of a VHLL) are in various stages of completion, and are described in separate chapters. Use the one that suits you best--or try them all!

Motivation for the Tcl Interface to PLplot

The recent emergence of several high quality VHLL script languages such as Tcl, Perl, Python and arguably even some Lisp variants, is having a profound effect upon the art of computer programming. Tasks which have traditionally been handled by C or Fortran, are beginning to be seen in a new light. With relatively fast processors now widely available, many programming jobs are no longer bound by execution time, but by "human time". Rapidity of initial development and continued maintenance, for a surprisingly wide class of applications, is far more important than execution time. Result: in a very short period of time, say from 1993 to 1995, script languages have exploded onto the scene, becoming essential tools for any serious programmer.

Moreover, the entire concept of "speed of execution" needs revising in the face of the gains made in computer hardware in recent years. Saying that script language processing is slower than compiled language processing may be undeniable and simultaneously irrelevant. If the script language processing is fast enough, then it is fast enough. Increasingly, computational researchers are finding that script based tools are indeed fast enough. And if their run time is fast enough, and their development and maintenance time is much much better, then why indeed should they not be used?

Even in a field with several high visibility players, Tcl has distinguished itself as a leading contender. There are many reasons for this, but perhaps the most important, at least as it relates to the PLplot user community, is that Tcl was designed to be extensible and embeddable. The whole purpose of Tcl, as it name (Tool Command Language) indicates, is to be a command language for other tools. In other words, the fact that Tcl is capable of being a standalone shell is interesting, even useful, but nonetheless incidental. The real attraction of Tcl is that it can be the shell language for your code. Tcl can easily be embedded into your code, endowing it immediately with a full featured, consistent and well documented script programming language, providing all the core features you need in a programming language: variables, procedures, control structures, error trapping and recovery, tracing, etc. But that is only the beginning! After that, you can easily extend Tcl by adding commands to the core language, which invoke the capabilities of your tool. It is in this sense that Tcl is a tool command language. It is a command language which you can augment to provide access to the facilities of your tool.

But Tcl is more than just an embeddable, extensible script language for personal use. Tcl is an industry, an internet phenomenon. There are currently at least two high quality books, with more on the way. There is an industry of service providers and educators. Furthermore, literally hundreds of Tcl extensions exist, and are readily available over the net. Perhaps the most notable extension, Tk, provides a fantastic interface to X Windows widget programming, permitting the construction of Motif like user interfaces, with none of the hassles of actually using Motif. Some of these extensions endow Tcl with object oriented facilities philosophically similar to C++ or other object oriented languages. Other extensions provide script level access to system services. Others provide a script interface to sockets, RPC, and other network programming protocols. The list goes on and on. Dive into the Tcl archive, and see what it has for you!

So, the answer to the question "Why do we want a Tcl interface to PLplot?" is very simple. "Because we we are using Tcl anyway, as the command language for our project, and would like to be able to do plotting in the command language just as we do so many other things."

But there is more than just the aesthetics of integration to consider. There are also significant pragmatic considerations. If you generate your PLplot output via function calls from a compiled language, then in order to add new diagnostics to your code, or to refine or embellish existing ones, you have to edit the source, recompile, relink, and rerun the code. If many iterations are required to get the plot right, significant time can be wasted. This can be especially true in the case of C++ code making heavy use of templates, for which many C++ compilers will have program link times measured in minutes rather than seconds, even for trivial program changes.

In contrast, if the diagnostic plot is generated from Tcl, the development cycle looks more like: start the shell (command line or windowing), source a Tcl script, issue the command to generate the plot, notice a bug, edit the Tcl script, resource the script, and regenerate the plot. Notice that compiling, linking, and restarting the program, have all been dropped from the development cycle. The time savings from such a development cycle can be amazing!