$LISTNEXT (ObjectScript)

Synopsis

$LISTNEXT(list,ptr,value)

Description

$LISTNEXT sequentially returns elements in a list. You initialize ptr to 0 before the first invocation of $LISTNEXT. This causes $LISTNEXT to begin returning elements from the beginning of the list. Each successive invocation of $LISTNEXT advances ptr and returns the next list element value to value. The $LISTNEXT function returns 1, indicating that a list element has been successfully retrieved.

When $LISTNEXT reaches the end of the list, it returns 0, resets ptr to 0, and leaves value unchanged from the previous invocation. Because ptr has been reset to 0, the next invocation of $LISTNEXT would start at the beginning of the list.

You can use $LISTVALID to determine if list is a valid list. An invalid list causes $LISTNEXT to generate a <LIST> error.

When $LISTNEXT encounters an omitted list element (an element with a null value), it returns 1 indicating that a list element has been successfully retrieved, advances ptr to the next element, and resets value to be an undefined variable. This can happen when encountering an omitted list element, such as the second invocation of $LISTNEXT on list=$LB("a",,"b"), or with any of the following valid lists: list=$LB(), list=$LB(UndefinedVar), or list=$LB(,).

$LISTNEXT("",ptr,value) returns 0, and does not advance the pointer or set value. $LISTNEXT($LB(""),ptr,value) returns 1, advances the pointer, and set value to the null string ("").

Example

The following example sequentially returns all the elements in the list. When it encounters an omitted element, the $SELECT returns the default value “omitted”:

   SET list=$LISTBUILD("Red","Blue",,"Green")
   SET ptr=0,count=0
   WHILE $LISTNEXT(list,ptr,value) {
      SET count=count+1
      WRITE !,count,": ",$SELECT($DATA(value):value,1:"omitted")
   }
   WRITE !,"End of list: ",count," elements found"
   QUIT

$LISTNEXT and Performance

An InterSystems IRIS list is the most efficient way to process large numbers of data values. You can use lists to hold large numbers of values for processing, rather than using an array or other data structure. This enables you to avoid the maximum string length.

Using $LISTNEXT to return a large number of elements from a list is substantially more efficient than using $LIST to perform the same operation.

The following example rapidly accesses the elements in mylist:

    SET ptr=0
    WHILE $LISTNEXT(mylist,ptr,value) {
          /* perform some operation on value */
    }

It is substantially faster than the following equivalent example:

    FOR i=1:1:$LISTLENGTH(mylist) {
        SET value=$LIST(mylist,i)
        /* perform some operation on value */
    }

$LISTNEXT and Nested Lists

The following example returns three elements, because $LISTNEXT does not recognize the individual elements in nested lists:

   SET list=$LISTBUILD("Apple","Pear",$LISTBUILD("Walnut","Pecan"))
   SET ptr=0,count=0
   WHILE $LISTNEXT(list,ptr,value) {
      SET count=count+1
      WRITE !,value 
   }
   WRITE !,"End of list: ",count," elements found"
   QUIT

See Also

• $LIST function

• $LISTBUILD function

• $LISTDATA function

• $LISTFIND function

• $LISTFROMSTRING function

• $LISTGET function

• $LISTLENGTH function

• $LISTSAME function

• $LISTTOSTRING function

• $LISTUPDATE function

• $LISTVALID function