               Chapter 1: Overview, DISGCL / 3.1
           ----------------------------------------

1.1 Introduction

This manual  describes  the graphics command language DISGCL 
which is an interpreter  based on the graphics software DIS-
LIN. All DISLIN statements  can be written to a  script file
and then be executed with DISGCL,  or can be  entered  in an
interactive mode.

Similar to  programming  languages  such  as Fortran  and C,
high-level  language elements can be used  within DISGCL.
These are variables, operators, expressions,  array operati-
ons, loops, if and switch statements,  user-defined  subrou-
tines and functions, and file I/O routines.

An easy to use interface for data input  is given to include
data into  DISGCL jobs.  The format  of data  files  is very
simple and useful for most DISLIN plotting routines.

Several quickplots are offered by DISGCL which are collecti-
ons of DISLIN statements to display data with one command.

1.2 Syntax of the Command DISGCL

Command:    DISGCL   [filename[.gcl]]   [args]   [options]  

filename    is  the  name  of  a  script file. The extension 
            '.gcl' is optional.

args        are optional arguments that can be passed to GCL 
            scripts.  The arguments are stored in the system
            variables  %ARG1,  %ARG2, ..., %ARGn,  or can be
            requested with the function GETARG (i), 
            1 <= i <= n.  The number of passed arguments  is
            stored in the system variable %NARGS.

options     is an optional  field  of keywords  separated by
            blanks:

-D=device   defines  the  format of the metafile  created by
            DISLIN.  This parameter  will overwrite the key-
            word in the DISLIN routine  METAFL  and can have
            the same values as the parameter in METAFL.

-F=file     defines the file used for data input. This para-
            meter  will overwrite  the file parameter in the
            routine DATFIL.

-I=file     replaces the file parameter of the first INCLUDE
            statement in an DISGCL script file.  This option
            can be used  to initialize  variables  with dif-
            ferent values.

-f          means that the extension '.gcl' is not  added to
            the filename.

-v          prints program version and author.


Notes:

 - If no parameters  are specified,  DISGCL runs in interac-
   tive mode.

 - DISGCL searches  the current  working  directory  for the 
   script file.  If the search fails,  DISGCL  searches  the
   directory defined by the environment variable GCL_PATH.

 - On  UNIX systems,  an DISGCL  script file can be executed
   directly  if the following line is included at the begin-
   ning of the script file:

   #! /path/disgcl -f

   where path is the directory containing the disgcl execut-
   able. 

1.3 Syntax of DISGCL Script Files

DISGCL script files must have the following syntax.

 -  A script file must begin with the string %GCL.

 -  Each line may contain up to 132 characters.

 -  The current statement  can be continued on the next line
    if a masterspace (@) is used at the end of the line.

 -  Lines are allowed to carry trailing comment fields, fol-
    lowing a double slash (//)  or the '#' character.  Empty 
    lines are also be interpreted as comment lines.

 -  Keywords and routine names can be in upper and lowercase
    letters.

 -  String constants  must be  enclosed in a pair  of either
    apostrophes or quotation marks. 

Example:

  %GCL
  SUM = 0
  DO I = 1, 10 
    SUM = SUM + I 
  END DO 
  PRINT SUM 

1.4 Syntax of Statements

The following statements  can be used in DISGCL script files
or can be typed directly at the DISGCL prompt.

 -----------------------------------------------------------
 | %GCL                       | Identifier for GCL script  |
 |                            | files.                     |
 -----------------------------------------------------------
 | //Comment or #Comment      | Comment line and inline    |
 |                            | comments.                  |
 -----------------------------------------------------------
 | routine (parameter list)   | Call of a DISLIN or DISGCL |
 |                            | routine.                   |
 -----------------------------------------------------------
 | CALL routine (list)        | Call of a user-defined     |
 |                            | subroutine.                |
 -----------------------------------------------------------
 | v = function (parameter    | Call of a DISLIN, DISGCL   |
 |                   list)    | or user-defined function.  |
 -----------------------------------------------------------
 | v = expression             | Assigns the value of the   |
 |                            | expression to the variable |
 |                            | v.                         |     
 -----------------------------------------------------------
 | command  [parameter list]  | DISGCL command.            |
 |                            |                            |
 -----------------------------------------------------------
 | vray = { constant list }   | Creates and initializes an |
 |                            | integer or floatingpoint   |
 |                            | array.                     |
 -----------------------------------------------------------
 | IF (expression) statement  | IF statement (conditional  |
 |                            | statement).                |
 -----------------------------------------------------------
 | IF (expression)            | IF construct. Up to 8 IF   |
 |   statements               | constructs can be nested.  |
 | ELSE IF (expression)       | The ELSE IF and the ELSE   |
 |   statements               | parts are optional.        |
 | ELSE                       |                            |
 |   statements               |                            |
 | END IF                     |                            |
 -----------------------------------------------------------
 | DO v = expr1, expr2        | DO loop. Up to 8 loops can |
 |                [,expr3]    | be nested.                 | 
 |   statements               |                            |
 | END DO                     |                            |
 -----------------------------------------------------------
 | WHILE (expr)               | WHILE loop. Up to 8 loops  |
 |   statements               | can be nested.             |
 | END WHILE                  |                            |
 -----------------------------------------------------------
 | SWITCH (iexpr)             | SWITCH statement where     |   
 |   CASE n1:                 | iexpr must be an integer   |
 |     statements             | expression and n1, n2, ... |
 |   CASE n2:                 | integer constants.         |
 |     statements             | Up to 8 SWITCH statements  |
 |   ......                   | can be nested.             |
 |   DEFAULT:                 |                            |
 |     statements             |                            |
 | END SWITCH                 |                            |
 -----------------------------------------------------------
 | label:                     | Label statement.           |
 -----------------------------------------------------------
 | GOTO label                 | GOTO statement.            |
 -----------------------------------------------------------
 | $command                   | Executes a system command. |
 -----------------------------------------------------------
               Figure 1.1: DISGCL Statements


1.5 Data Types

Variables in DISGCL are dynamic. They  don't  have to be de-
clared,  and they can change their types during the lifetime
of a GCL session. The following data types are known by DIS-
GCL:

CHAR       an 8-bit integer in the range -128 to 127.

BYTE       an 8-bit integer in the range 0 to 255.

SHORT      an 16-bit integer in the range -32768 to 32767.

INT        an 32-bit integer in the range -2147483648 to
           2147483647.

FLOAT      an  32-bit  floatingpoint  number  in  the  range
           1.2E-38 to 3.4E+38 and with 7-digit precision.

DOUBLE     an  64-bit  floatingpoint  number  in  the  range
           2.2E-308 to 1.8E+308 and with 15-digit precision.

COMPLEX    a pair  of 32-bit  floatingpoint  numbers  in the
           range 1.2E-38 to 3.4E+38.

STRING     a sequence  of characters.  Strings are stored as
           CHAR arrays terminated with the ASCII value zero.

1.6 Expressions

An expression  is an combination of operands  and operators.
The operands can be constants, variables and functions,  and
may  be scalars  or arrays.  Expressions  can be assigned to 
variables or can be passed as parameters  to subroutines and
functions.

Example:

  a = 60 
  x = exp (sin (a * 3.14159))

1.7 Quickplots

DISGCL  offers several quickplots  which are  collections of
DISLIN routines that can display data with one command.  For 
example, the DISGCL command  PLOT  displays  two-dimensional
curves.

Example:

  x = falloc (100)           
  plot x, sin (x/5)

Note:  All quickplots  have corresponding  widget interfaces
       that can be executed with the command

       disgcl   quickplot 

       where quickplot is the name of a quickplot. The widget
       interfaces for quickplots expect  data in the  form of
       data files described in chapter 8, 'Data Files'. 
