Notes on reading Pascal source code
-----------------------------------

These are some notes to explain  how Pascal source code is structured,
and to give  some  tips  on  how  to  convert  it  to  other languages
(especially C or BASIC)

A ready-compiled test program should be  available: please run this in
ST low resolution!


Pascal Structure
----------------

Due to the way Pascal is  compiled  the  structure may seem strange to
those used to languages such as BASIC. The basic structure is:

PROGRAM name of program;

TYPE data structure declarations

CONST constant declarations

VAR variable declarations

FUNCTION/PROCEDURE subprogram code

BEGIN
  main program code
END.

This should be fairly familiar to  anyone  with experience of C. Other
notes:

1) All variables and data  structures  must  be declared unlike GFA or
STOS. Then all subroutines are  declared and described, including what
must go in and out from  the  routine (ie. its parameters) and finally
there is the  main  core  of  the  program  itself.  Program cores and
functions are all contained within BEGIN and END commands.

2)  The  number  type  "real"  denotes  any  variable  that  does  not
necessarily hold an integer. "Integer" is  the equivalent of a% in GFA
or a normal variable in STOS.

3) In this example "records" should used for each point on the lattice
- but to aid conversion to BASIC  I have used a 3-dimensional array of
numbers.  It  could  also  be  replaced  by  3  data  arrays  eg.  DIM
xgrad(20,20), ygrad(20,20), val(20,20).

4) Pascal functions may have their  own local variables, and pass back
one value to the main program.  This  is  best converted by means of a
DEF FN statement  (especially  in  GFA  -  STOS  will  probably need a
subroutine)

5) The code is designed for HighSpeed Pascal or Turbo Pascal on the PC
(yuk!)  so  I've  commented  the  lines  that  initialise  Pascal-only
graphics routines. These should be  replaced  by  your own routines in
other languages.

6) The semicolon is used to  separate  statements; just ignore them if
they are missing (this  depends  on  what  Pascal  deems a "statement"
which is different from C or BASIC)

7) Other functions: "trunc" equivalates  to  "int" in BASIC (takes the
integer part) and "mod" finds the modulus.




Steve Tattersall
s.j.tattersall@cms.salford.ac.uk