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*Conan The Librarian (sorry for the slow response - running on an old VAX)
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Defines a symbolic name for a character string or integer value.
Format
symbol-name =[=] expression
symbol-name[bit-position,size] =[=] replacement-expression
NOTE
Compaq advises against assigning a symbolic name that is
already a DCL command name. Compaq especially discourages
the assignment of symbols such as IF, THEN, ELSE, and GOTO,
which can affect the interpretation of command procedures.
| 1 - Parameters |
symbol-name
Specifies a string of 1 to 255 characters for the symbol name.
The name can contain any alphanumeric characters from the DEC
Multinational character set, the underscore (_), and the dollar
sign ($). However, the name must begin only with an alphabetic
character (uppercase and lowercase characters are equivalent),
an underscore, or a dollar sign. Using one equal sign (=) places
the symbol name in the local symbol table for the current command
level. Using two equal signs (==) places the symbol name in the
global symbol table.
expression
Names the value on the right-hand side of an assignment
statement. This parameter can consist of a character string, an
integer, a symbol name, a lexical function, or a combination of
these entities. The components of the expression are evaluated,
and the result is assigned to the symbol. All literal character
strings must be enclosed in quotation marks (" "). If the
expression contains a symbol, the expression is evaluated using
the symbol's value.
The result of expression evaluation is either a character string
or a signed integer value. If the expression is evaluated as a
string, the symbol is assigned a string value. If the expression
is evaluated as an integer, the symbol is assigned an integer
value. If the integer value exceeds the capacity of the 4-byte
buffer that holds it, no error message is issued.
For a summary of operators used in expressions, details on
how to specify expressions, and details on how expressions are
evaluated, refer to the OpenVMS User's Manual.
DCL uses a buffer that is 1024 bytes long to hold an assignment
statement and to evaluate the expression. The length of the
symbol name, the expression, and the expression's calculations
cannot exceed 1024 bytes.
[bit-position,size]
States that a binary overlay is to be inserted in the current
32-bit value of a symbol name. The current value of the symbol
name is evaluated. Then, the specified number of bits is replaced
by the result of the replacement expression. The bit position is
the location relative to bit 0 at which the overlay is to occur.
If the symbol you are overlaying is an integer, then the bit
position must be less than 32. The sum of the bit position and
the size must be less than or equal to 32.
If the symbol you are overlaying is a string, then the bit
position must be less than 6152. Because each character is
represented using 8 bits, you can begin an overlay at any
character through the 768th character. (The 768th character
starts in bit position 6144.) The sum of the bit position and
the size must be less than or equal to 6152.
The size is the number of bits to be overlaid. If you specify a
size that is greater than 32, DCL reduces the size to 32.
The brackets are required notation; no spaces are allowed between
the symbol name and the left bracket. Specify values for the bit
position and size as integers.
replacement-expression
Specifies the value that is used to overlay the symbol you are
modifying. Specify the replacement expression as an integer.
If the symbol you are modifying is an integer, the replacement
expression defines a bit pattern that is overlaid on the value
assigned to the symbol. If the symbol you are modifying is
a character string, the result of the replacement expression
defines a bit pattern that is overlaid on the specified bits
of the character string. If the symbol you are modifying is
undefined, the result of the replacement expression is overlaid
on a null string.
| 2 - Examples |
1.$ LIST == "DIRECTORY"
The assignment statement in this example assigns the user-
defined synonym LIST as a global symbol definition for the DCL
command DIRECTORY.
2.$ COUNT = 0
$ LOOP:
$ COUNT = COUNT + 1
$ IF P'COUNT' .EQS. "" THEN EXIT
$ APPEND/NEW &P'COUNT' SAVE.ALL
$ DELETE &P'COUNT';*
$ IF COUNT .LT. 8 THEN GOTO LOOP
$ EXIT
This command procedure, COPYDEL.COM, appends files (specified
as parameters) to a file called SAVE.ALL. After a file has been
appended, the command procedure deletes the file. Up to eight
file names can be passed to the command procedure. The file
names are assigned to the symbols P1, P2, and so on.
The command procedure uses a counter to refer to parameters
that are passed to it. Each time through the loop, the
procedure uses an IF command to check whether the value of
the current parameter is a null string. When the IF command is
scanned, the current value of the symbol COUNT is concatenated
with the letter P. The first time through the loop, the IF
command tests P1; the second time through the loop it tests
P2, and so on. After the expression P`COUNT' is evaluated, the
substitution of the file names that correspond to P1, P2, and
so on is automatic within the context of the IF command.
The APPEND and DELETE commands do not perform any substitution
automatically, because they expect and require file
specifications as input parameters. The ampersand (&) precedes
the P`COUNT' expression for these commands to force the
appropriate symbol substitution. When these commands are
initially scanned each time through the loop, COUNT is
substituted with its current value. Then, when the commands
execute, the ampersand causes another substitution: the first
file specification is substituted for P1, the second file
specification is substituted for P2, and so on.
To invoke this procedure, use the following command:
$ @COPYDEL ALAMO.TXT BEST.DOC
The files ALAMO.TXT and BEST.DOC are each appended to the file
SAVE.ALL and are then deleted.
3.$ A = 25
$ CODE = 4 + F$INTEGER("6") - A
$ SHOW SYMBOL CODE
CODE = -15 HEX = FFFFFFF1 Octal = 1777761
This example contains two assignment statements. The first
assignment statement assigns the value 25 to the symbol A. The
second assignment statement evaluates an expression containing
an integer (4), a lexical function (F$INTEGER("6")), and the
symbol A. The result of the expression, -15, is assigned to the
symbol CODE.
4.$ FILENAME = "JOBSEARCH" - "JOB"
$ FILETYPE = ".OBJ"
$ FILESPEC = FILENAME + FILETYPE
$ TYPE 'FILESPEC'
The first command in this example assigns the symbol FILENAME
the value "SEARCH". Notice that the string "SEARCH" is the
result of the string reduction operation performed by the
expression. The second command assigns the symbol FILETYPE
the character string ".OBJ".
The symbols FILENAME and FILETYPE are then added together in an
expression assigned to the symbol FILESPEC. Because the values
of the symbols FILENAME and FILETYPE are concatenated, the
resultant value assigned to FILESPEC is the character string
"SEARCH.OBJ". The symbol FILESPEC is then used as a parameter
for the TYPE command. The single quotation marks (` ') request
the command interpreter to replace the symbol FILESPEC with its
value SEARCH.OBJ. Thus, the TYPE command types the file named
SEARCH.OBJ.
5.$ BELL[0,32] = %X07
$ SHOW SYMBOL BELL
BELL = ""
In this example, the symbol BELL is created with an arithmetic
overlay assignment statement. Because the symbol BELL is
previously undefined, the hexadecimal value 7 is inserted
over a null character string and is interpreted as the ASCII
code for the bell character on a terminal. When you issue the
command SHOW SYMBOL BELL, the terminal beeps.
If the symbol BELL had been previously defined with an integer
value, the result of displaying BELL would have been to show
its new integer value.
6.$ $=34
%DCL-W-NOCOMD, no command on line - reenter with alphabetic first
character
$ $$=34
$ SHOW SYMBOL $$
%DCL-W-UNDSYM, undefined symbol - check validity and spelling
$ SHOW SYMBOL $
$ = 34 Hex = 00000022 Octal = 00000000042
If you begin a symbol name with the dollar sign ($), use two
dollar signs ($$) because DCL discards the first instance of
the dollar sign.
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