Pascal News

TREEPRINT - A Package to Print Trees
on any Character Printer
Ned Freed
Kevin Carosso
Mathematics Department
Harvey Mudd College
Claremont, Calif. 91711
One of the problems facing a programmer who deals with complex
linked data structures in Pascal is the inability to display such a
structure in a graphical form. Usually it is too much to ask a
system debugging tool to even understand records and pointers, let
alone display a structure using them in the way it would appear in a
good t!xtbook. Likewise very few operating systems have a package
of routines to display structures automatically. Pascal has a
tremendous advantage over many languages in its abl1 i ty to support
definable types and structures. If the environment is incapable of
dealing with these features, they become far less useful.
This lack became apparent to us in the process of writing an
algebraic expression parser which produced internal N-ary trees
There was no way at the time under our operating system debugge;
(VAX/VMS) to get at the data structure we were generating. When the
routines produced an incorrect tree we had no way of finding the
specific error,
Our frustration led to the development of TREEPRINT. Starting
with the algorithm of Jean Vaucher [1], we designed a
general-purpose tool capable of displaying any N-ary tree on any
character output device. The trees are displayed in a pleasant
visual fOrm and in the manner in which they would appear if drawn by
hand. We feel that TREEPRINT is of general use -- hence its
presentation here.
The structure of TREEPRINT is that of an independent collection
of subroutines that any program can call. Unfortunately standard
Pascal does not support this form, while our Pascal environment
does. However, building TREEPRUIT directly into a program should
present no difficulty.
TREEPRINT requires no knowledge of the format of the data
structure it is printing. It has even been used to print a tabular
linked structure within a FORTRAN programl In order to allow this,
two procedures are passed in the call to TREEPRINT. One is used to
·walk" the tree, the other to print identifying labels for a given
node. Other parameters are values such as the size of the nodes,
the width of the page, etc. One of the advantages of this calling
mechanism is that a single version of TREEPRINT can be used to
display wildly different structures, even when they are· within the
same program.
One of the major features of TREEPRINT is its ability to span
pages. A tree that is too wide to fit on one page is printed out in
'stripes. which are taped together edge-to-edge after printing. In
addition trees may optionally be printed either upside-down or
reversed from left-to-right.
The method used by TREEPRINT is detailed in Vaucher's work [1].
In its current implementation additional support for N-ary
structures has been added, as well as full connecting-arc printing
and the reversal features. Basically, TREEPRINT walks the input
tree and constructs an analogous structure of its own which
indicates the positions of every node. The new structure is linked
along the left edge and across the page from left-to-right. Once
this structure is completed, TREEPRINT walks the new structures and
prints it out in order. Once printout is finished, the generated
structure is DISPOSE'd of.
There are only two minor problems in TREEPRINT currently. The
first is that a structure which contains circular loops will hang
the routine. This could be detected in the POSITION phase of
TREEPRINT by checking each new node against all of its ancestors.
However, if used in a non-Pascal application, this might fail due to
problems in comparing pointers. If this check is necessary we
suggest it be implemented in the LOWERNODE procedure passed to
TREEPRINT. This procedure at least understands the type of pointer
it is dealing with.
The second problem is a feature of the POSITION routine which
centers a node above its sons. This tends to make the trees
generated wider than necessary. This is largely a matter of taste
-- some minor changes would remove this.
The listing of TREEPRINT which follows should serve to document
the method of calling the routine. The functions of the
user-supplied procedures are also detailed.
References
[11 Vaucher, Jean, "Pretty-printing of Trees." Software- Practice
~ Experience, Vol. 10, pp. 553-561 (1980).
[2] Myers, Brad, Displaying Data Structures for Interactive
Debugging, Palo Alto: Xerox PARC CSL-80-7 (1980).
{3] Sweet, Richard, Empirical Estimates of Program Entropy,
Appendix B - "Implementation description", Palo Alto: Xerox
PARC CSL-78-3 (1978).
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