$LISTVALID
Synopsis
$LISTVALID(exp) $LV(exp)
Parameter
Argument | Description |
---|---|
exp | Any valid expression. |
Description
$LISTVALID determines whether exp is a list, and returns a Boolean value: If exp is a list, $LISTVALID returns 1; if exp is not a list, $LISTVALID returns 0.
A list can be created using $LISTBUILD or $LISTFROMSTRING, or extracted from another list using $LIST. A list containing the empty string ("") as its sole element is a valid list. The empty string ("") by itself is also considered a valid list. (Certain $CHAR non-printing character combinations, such as $CHAR(1), $CHAR(2,1), and $CHAR(3,1,asciicode) can also return a valid empty or one-element list.)
Examples
The following examples all return 1, indicating a valid list:
SET w = $LISTBUILD("Red","Blue","Green") SET x = $LISTBUILD("Red") SET y = $LISTBUILD(365) SET z = $LISTBUILD("") WRITE !,$LISTVALID(w) WRITE !,$LISTVALID(x) WRITE !,$LISTVALID(y) WRITE !,$LISTVALID(z)
The following examples all return 0. Numbers and strings (with the exception of the null string) are not valid lists:
SET x = "Red" SET y = 44 WRITE !,$LISTVALID(x) WRITE !,$LISTVALID(y)
The following examples all return 1. Concatenated, nested, and omitted value lists are all valid lists:
SET w=$LISTBUILD("Apple","Pear") SET x=$LISTBUILD("Walnut","Pecan") SET y=$LISTBUILD("Apple","Pear",$LISTBUILD("Walnut","Pecan")) SET z=$LISTBUILD("Apple","Pear",,"Pecan") WRITE !,$LISTVALID(w_x) ; concatenated WRITE !,$LISTVALID(y) ; nested WRITE !,$LISTVALID(z) ; omitted element
The following examples all return 1. $LISTVALID considers all of the following “empty” lists as valid lists:
WRITE $LISTVALID(""),! WRITE $LISTVALID($LB()),! WRITE $LISTVALID($LB(UndefinedVar)),! WRITE $LISTVALID($LB("")),! WRITE $LISTVALID($LB($CHAR(0))),! WRITE $LISTVALID($LB(,))
See Also
$LIST function
$LISTBUILD function
$LISTDATA function
$LISTFIND function
$LISTFROMSTRING function
$LISTGET function
$LISTLENGTH function
$LISTNEXT function
$LISTSAME function
$LISTTOSTRING function
$LISTUPDATE function