The ZWRITE command displays the current value of one or more local , alias variables, ISVs, or global variables. ZWRITE formats its output so that each item in the display forms a valid argument to a SET @ command. This means ZWRITE encloses string values in quotes and represents non-graphic (control) characters in $CHAR() syntax.
The format of the ZWRITE command is:
ZWR[ITE][:tvexpr] [zwrglvn[,...]]
The optional truth-valued expression immediately following the command is a command postconditional that controls whether or not GT.M executes the command.
The optional global or local variable name specifies the variable for ZWRITE to display.
ZWRITE accepts several alternative syntaxes in place of subscripts; ZWRITE also accepts arguments specifying naked references to globals. Because ZWRITE is primarily a debugging tool, ZWRITE does not affect the naked indicator.
ZWRITE accepts null subscripts in its arguments, when these are allowed, and reports array nodes that have null subscripts.
A ZWRITE with no arguments displays all the currently available local variables; in this case, at least two (2) spaces must follow the command to separate it from the next command on the line.
If the specified global or local variable name is unsubscripted, ZWRITE displays the unsubscripted variable and all subscripted descendants.
If an asterisk (*) appears in the space normally occupied by the last subscript in a subscripted variable name, ZWRITE displays all variable nodes descended from the previously specified subscripts.
ZWRITE accepts GT.M pattern-match syntax in place of both variable names and subscripts.
ZWRITE <name>(), where <name> is a local or a global is treated as a synonym for ZWRITE <name>.
A colon acts as a range operator for subscript values; ZWRITE displays all subscripts of the variable starting with the value on the left side of the colon and ending with the value on the right side of the colon; if the range delimiter has no left-hand value, or has the empty string as the left-hand value, the display begins at the first subscript; if the range delimiter has no right-hand value or has the empty string as the right-hand value, the display ends at the last subscript at that level; if the range delimiter has no values or empty strings on either side, ZWRITE displays all subscripts at that level; an empty subscript level also displays all subscripts at that level.
An indirection operator and an expression atom evaluating to a list of one or more ZWRITE arguments form a legal argument for a ZWRITE.
Long ZWRITE format records can be loaded.
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ZWRITE as applied to local variables and ZSHOW "V" are conceptually similar, with two differences:
ZWRITE allows the use of patterns to specify variables and subscripts to display whereas ZSHOW "V" applies to all local variables.
ZSHOW "V" optionally allows the output to be directed to a global or local variable, whereas ZWRITE always directs its output to the current output device.
ZWRITE and ZSHOW "V" dump the values of alias variables, alias container variables, and the associated data as described below, in ZWRITE format. In the ZWRITE format, the contents of an array are displayed with the name associated with that array that appears first in the lexical ordering of names. GT.M displays both the unsubscripted and subscripted nodes and values, appending a notational space-semicolon-asterisk (";*") sequence to the unsubscripted value, if any. The ZWRITE format output can be read into a GT.M process with the commands Read x and Set @x (where x is any name) executed in a loop. ";*" acts as a comment ignored by the SET command. In the following example, since A and C are aliases associated with the same array, the nodes of that array are output with A, which occurs lexically before C, even though the values were assigned to C:
GTM>Set C=1,C("Malvern")="Wales",*A=C,*B(-3.14)=C
GTM>ZSHow "V" ; ZWRite would produce the same output
A=1 ;*
A("Malvern")="Wales"
*B(-3.14)=A
*C=A
GTM>ZWRite C ; Only one is name associated with the array on this ZWRite command
C=1 ;*
C("Malvern")="Wales"
GTM>
Continuing the example, if the variables selected for the ZWRITE command do not include any of the the associated alias variables, the output shows only the reference, not the data:
GTM>ZWRITE B ; B only has a container *B(-3.14)=A GTM>
When ZWRITE / ZSHOW "V" encounters an alias container for an array with no current alias variable, it uses a name $ZWRTACn as the made-up name of an alias variable for that array, where n is an arbitrary but unique integer. The SET command recognizes this special name, thus enabling the output of a ZWRITE / ZSHOW "V" to be used to recreate alias containers without associated alias variables. Continuing the above example:
GTM>Kill *A,*C ; Delete alias variables and associations, leaving only the container
GTM>ZWRite
$ZWRTAC=""
*B(-3.14)=$ZWRTAC1
$ZWRTAC1=3 ;*
$ZWRTAC1("Malvern")="Wales"
$ZWRTAC=""
GTM>
ZWRITE produces $ZWRTACn names to serve as data cell anchors which SET @ accepts as valid set left targets. $ZWRTACn names are created and destroyed when using ZWRITE output to drive restoration of a previously captured variable state. Except for their appearance in ZWRITE output and as left-hand side SET @ targets, they have no function. Other than SET, no other commands can use $ZWRTAC* in their syntax. Although $ZWRTACn superficially looks like an intrinsic special variable (ISV), they are not ISVs. $ZWRTACn with no subscripts can serve as the target (left side of the equals-sign) of a SET * command. SET $ZWRTAC (no trailing integer) deletes all data cell associations with the $ZWRTAC prefixed aliases. GT.M only recognizes the upper-case unabbreviated name and prefix $ZWRTAC.
When ZWRITE displays values for an alias variable, it appends a " ;*" to the name which visually tags the alias without interfering with use of ZWRITE output as arguments to a SET @. ZWRITE can only identify alias variables when at least two aliases for the same data match its argument. When ZWRITE encounters an alias container variable with no current associated alias, it uses the ZWRTAC mechanism to expose the data; SET @ can restore data exposed with the ZWRTAC mechanism.
![[Caution]](images/caution.jpg) |
Caution |
|---|---|
FIS strongly recommends that you should not create or manipulate your own $ZWRTACn "variables". They are not part of the supported functionality for implementing alias variables and containers, but are rather a part of the underlying implementation that is visible to you, the GT.M user. FIS can arbitrarily, for its own convenience change the use of $ZWRTAC in GT.M at any time. They are only documented here since you may see them in the output of ZWRITE and ZSHOW "V". |
Example:
GTM>ZWRITE ^?1"%"2U(0:":",)
This command displays the descendants of all subscripts between 0 and ":" of all global names starting with a "%" and having two upper case letters -- for example, "%AB".
Example:
GTM>ZWRITE A(,:,3)
This command displays all of the third level nodes with a subscript of 3 for local variable A.
Example:
ZWRITE ?1"A".E(?1"X"3N)
This displays data for any local variables starting with "A", optionally followed by any characters, and having any subscripts starting with "X" followed by three numerics.
Example:
GTM>Set A=1,*B=A ; Create an array and an alias association
GTM>ZWRite ; Show that the array and association exist
A=1 ;*
*B=A