The PLplot Tcl Matrix Extension

Tcl does many things well, but handling collections of numbers is not one of them. You could make lists, but for data sets of sizes relevant to scientific graphics which is the primary domain of applicability for PLplot, the extraction time is excessive and burdensome. You could use Tcl arrays, but the storage overhead is astronomical and the lookup time, while better than list manipulation, is still prohibitive.

To cope with this, a Tcl Matrix extension was created for the purpose of making it feasible to work with large collections of numbers in Tcl, in a way which is storage efficient, reasonably efficient for accesses from Tcl, and reasonably compatible with practices used in compiled code.

Using Tcl Matrices from Tcl

Much like the Tk widget creation commands, the Tcl matrix command considers its first argument to be the name of a new command to be created, and the rest of the arguments to be modifiers. After the name, the next argument can be float or int or contractions thereof. Next follow a variable number of size arguments which determine the size of the matrix in each of its dimensions. For example:

	matrix x f 100
	matrix y i 64 64

constructs two matrices. x is a float matrix, with one dimension and 100 elements. y is an integer matrix, and has 2 dimensions each of size 64.

Additionally, an initializer may be specified, with a syntax familiar from C. For example:

	matrix x f 4 = { 1.5, 2.5, 3.5, 4.5 }

A Tcl matrix is a command, and as longtime Tcl users know, Tcl commands are globally accessible. The PLplot Tcl Matrix extension attempts to lessen the impact of this by registering a variable in the local scope, and tracing it for insets, and deleting the actual matrix command when the variable goes out of scope. In this way, a Tcl matrix appears to work sort of like a variable. It is, however, just an illusion, so you have to keep this in mind. In particular, you may want the matrix to outlive the scope in which it was created. For example, you may want to create a matrix, load it with data, and then pass it off to a Tk megawidget for display in a spreadsheet like form. The proc which launches the Tk megawidget will complete, but the megawidget, and the associated Tcl matrix are supposed to hang around until they are explicitly destroyed. To achieve this effect, create the Tcl matrix with the -persist flag. If present (can be anywhere on the line), the matrix is not automatically deleted when the scope of the current proc (method) ends. Instead, you must explicitly clean up by using either the 'delete' matrix command or renaming the matrix command name to {}. Now works correctly from within [incr Tcl].

As mentioned above, the result of creating a matrix is that a new command of the given name is added to the interpreter. You can then evaluate the command, providing indices as arguments, to extract the data. For example:

	pltcl> matrix x f = {1.5, 2.5, 3.5, 4.5}
	insufficient dimensions given for Matrix operator "x"
	pltcl> matrix x f 4 = {1.5, 2.5, 3.5, 4.5}
	pltcl> x 0
	pltcl> x 1
	pltcl> x 3
	pltcl> x *
	1.500000 2.500000 3.500000 4.500000
	pltcl> puts "x\[1\]=[x 1]"
	pltcl> puts "x\[*\] = :[x *]:"
	x[*] = :1.500000 2.500000 3.500000 4.500000:
	pltcl> foreach v [x *] { puts $v }
	pltcl> for {set i 0} {$i < 4} {incr i} {
	if {[x $i] < 3} {puts [x $i]} }

Note from the above that the output of evaluating a matrix indexing operation is suitable for use in condition processing, list processing, etc.

You can assign to matrix locations in a similar way:

	pltcl> x 2 = 7
	pltcl> puts ":[x *]:"
	:1.500000 2.500000 7.000000 4.500000:
	pltcl> x * = 3
	pltcl> puts ":[x *]:"

Note that the * provides a means of obtaining an index range, and that it must be separated from the = by a space. Future versions of the Tcl Matrix extension may allow alternative ways of specifying index ranges and may assign the obvious meaning to an expression of the form:

	x *= 3

However this has not been implemented yet...

In any event, the matrix command also supports an info subcommand which reports the number of elements in each dimension:

	pltcl> x info
	pltcl> matrix y i 8 10
	pltcl> y info
	8 10 

Using Tcl Matrices from C

Normally you will create a matrix in Tcl, and then want to pass it to C in order to have the data filled in, or existing data to be used in a computation, etc. To do this, pass the name of the matrix command as an argument to your C Tcl command procedure. The C code should include tclMatrix.h, which has a definition for the tclMatrix structure. You fetch a pointer to the tclMatrix structure using the Tcl_GetMatrixPtr function.

For example, in Tcl:

	matrix x f 100
	wacky x

and in C:

	int wackyCmd( ClientData clientData, Tcl_Interp *interp,
	int argc, char *argv[] )
	tclMatrix *w;

	w = Tcl_GetMatrixPtr( interp, argv[1] );

To learn about what else you can do with the matrix once inside compiled code, read tclMatrix.h to learn the definition of the tclMatrix structure, and see the examples in files like tclAPI.c which show many various uses of the Tcl matrix.

Using Tcl Matrices from C++

Using a Tcl matrix from C++ is very much like using it from C, except that tclMatrix.h contains some C++ wrapper classes which are somewhat more convenient than using the indexing macros which one has to use in C. For example, here is a tiny snippet from one of the authors codes in which Tcl matrices are passed in from Tcl to a C++ routine which is supposed to fill them in with values from some matrices used in the compiled side of the code:

	if (item == "vertex_coords") {
	tclMatrix *matxg = Tcl_GetMatrixPtr( interp, argv[1] );
	tclMatrix *matyg = Tcl_GetMatrixPtr( interp, argv[2] );

	Mat2<float> xg(ncu, ncv), yg(ncu, ncv);
	cg->Get_Vertex_Coords( xg, yg );

	TclMatFloat txg( matxg ), tyg( matyg );

	for( i=0; i < ncu; i++ )
	for( j=0; j < ncv; j++ ) {
	txg(i,j) = xg(i,j);
	tyg(i,j) = yg(i,j);

There are other things you can do too, see the definitions of the TclMatFloat and TclMatInt classes in tclMatrix.h.

Extending the Tcl Matrix facility

The Tcl matrix facility provides creation, indexing, and information gathering facilities. However, considering the scientifically inclined PLplot user base, it is clear that some users will demand more. Consequently there is a mechanism for augmenting the Tcl matrix facility with your own, user defined, extension subcommands. Consider xtk04.c. In this extended wish, we want to be able to determine the minimum and maximum values stored in a matrix. Doing this in Tcl would involve nested loops, which in Tcl would be prohibitively slow. We could register a Tcl extension command to do it, but since the only sensible data for such a command would be a Tcl matrix, it seems nice to provide this facility as an actual subcommand of the matrix. However, the PLplot maintainers cannot foresee every need, so a mechanism is provided to register subcommands for use with matrix objects.

The way to register matrix extension subcommands is to call Tcl_MatrixInstallXtnsn:

	typedef int (*tclMatrixXtnsnProc) ( tclMatrix *pm, Tcl_Interp *interp,
	int argc, char *argv[] );

	int Tcl_MatrixInstallXtnsn( char *cmd, tclMatrixXtnsnProc proc );

In other words, make a function for handling the matrix extension subcommand, with the same function signature (prototype) as tclMatrixXtnsnProc, and register the subcommand name along with the function pointer. For example, xtk04.c has:

	int mat_max( tclMatrix *pm, Tcl_Interp *interp,
	int argc, char *argv[] )
	float max = pm->fdata[0];
	int i;
	for( i=1; i < pm->len; i++ )
	if (pm->fdata[i] > max)
	max = pm->fdata[i];

	sprintf( interp->result, "%f", max );
	return TCL_OK;

	int mat_min( tclMatrix *pm, Tcl_Interp *interp,
	int argc, char *argv[] )
	float min = pm->fdata[0];
	int i;
	for( i=1; i < pm->len; i++ )
	if (pm->fdata[i] < min)
	min = pm->fdata[i];

	sprintf( interp->result, "%f", min );
	return TCL_OK;

Then, inside the application initialization function (Tcl_AppInit() to long time Tcl users):

	Tcl_MatrixInstallXtnsn( "max", mat_max );
	Tcl_MatrixInstallXtnsn( "min", mat_min );

Then we can do things like:

	dino 65: xtk04
	% matrix x f 4 = {1, 2, 3, 1.5}
	% x min
	% x max

Your imagination is your only limit for what you can do with this. You could add an FFT subcommand, matrix math, BLAS, whatever.