Overview of the Tcl Language Binding
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## Overview of the Tcl Language Binding

Each of the PLplot calls available to the C or Fortran programmer are also available from Tcl, with the same name and generally the same arguments. Thus for instance, whereas in C you can write:

          plenv( 0., 1., 0., 1., 0, 0 );
          pllab( "(x)", "(y)", "The title of the graph" );

you can now write in Tcl:

          plenv 0 1 0 1 0 0
          pllab "(x)" "(y)" "The title of the graph"

All the normal Tcl rules apply, there is nothing special about the PLplot extension commands. So, you could write the above as:

          set xmin 0; set xmax 1; set ymin 0; set ymax 1
          set just 0; set axis 0
          set xlab (x)
          set ylab (y)
          set title "The title of the graph"
          plenv $xmin $xmax $ymin $ymax $just $axis
          pllab $xlab $ylab $title

for example. Not that there is any reason to be loquacious for its own sake, of course. The point is that you might have things like the plot bounds or axis labels stored in Tcl variables for some other reason (tied to a Tk entry widget maybe, or provided as the result of one of your application specific Tcl extension commands, etc), and just want to use standard Tcl substitution to make the PLplot calls.

Go ahead and try it! Enter `pltcl` to start up the PLplot extended Tcl shell, and type (or paste) in the commands. Or put them in a file and source it. By this point it should be clear how incredibly easy it is to use the PLplot Tcl language binding.

In order to accommodate the ubiquitous requirement for matrix oriented data in scientific applications, and in the PLplot API in particular, PLplot includes a Tcl extension for manipulating matrices in Tcl. This Tcl Matrix Extension provides a straightforward and direct means of representing one and two dimensional matrices in Tcl. The Tcl Matrix Extension is described in detail in the next section, but we mention its existence now just so that we can show how the PLplot Tcl API works. Many of the PLplot Tcl API functions accept Tcl matrices as arguments. For instance, in C you might write:

          float x[100], y[100];

          /* code to initialize x and y */

          plline( 100, x, y );

In Tcl you can write:

          matrix x f 100
          matrix y f 100

          # code to initialize x and y

          plline x y

N.B. Our Tcl binding uses a redacted API which is why the redundant dimension of the x and y arrays must be dropped from the plline call.

Some of the PLplot C function calls use pointer arguments to allow retrieval of PLplot settings. These are implemented in Tcl by changing the value of the variable whose name you provide. For example:

          pltcl> plgxax
          wrong # args: should be "plgxax digmax digits  "
          pltcl> set digmax 0
          0
          pltcl> set digits 0
          0
          pltcl> plgxax digmax digits
          pltcl> puts "digmax=$digmax digits=$digits"
          digmax=4 digits=0

This example shows that each PLplot Tcl command is designed to issue an error if you invoke it incorrectly, which in this case was used to remind us of the correct arguments. We then create two Tcl variables to hold the results. Then we invoke the PLplot `plgxax` function to obtain the label formatting information for the x axis. And finally we print the results.

People familiar with Tcl culture may wonder why the `plg*` series functions don't just pack their results into the standard Tcl result string. The reason is that the user would then have to extract the desired field with either `lindex` or `regexp`, which seems messy. So instead, we designed the PLplot Tcl API to look and feel as much like the C API as could reasonably be managed.

In general then, you can assume that each C function is provided in Tcl with the same name and same arguments (and one or two dimensional arrays in C are replaced by Tcl matrices). There are only a few exceptions to this rule, generally resulting from the complexity of the argument types which are passed to some functions in the C API. Those exceptional functions are described below, all others work in the obvious way (analogous to the examples above).

See the Tcl example programs for extensive demonstrations of the usage of the PLplot Tcl API. To run the Tcl demos:

          % pltcl
          pltcl> source tcldemos.tcl
          pltcl> 1
          pltcl> 2

Alternatively, you can run `plserver` and source `tkdemos.tcl`.

In any event, the Tcl demos provide very good coverage of the Tcl API, and consequently serve as excellent examples of usage. For the most part they draw the same plots as their C counterpart. Moreover, many of them were constructed by literally inserting the C code into the Tcl source file, and performing fairly mechanical transformations on the source. This should provide encouragement to anyone used to using PLplot through one of the compiled interfaces, that they can easily and rapidly become productive with PLplot in Tcl.

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