ISNULL Function
Determines whether a pointer element of a structure
is null.
Syntax
numeric-variable = ISNULL(pointer-element);
Required Arguments
- numeric-variable
-
specifies a numeric
value.
- pointer-element
-
specifies a variable
that contains the address of another variable.
Examples
Example 1: Generating a Linked List
In the following example,
the LINKLIST structure and GET_LIST function are defined by using
PROC PROTO. The GET_LIST function is an external C routine that generates
a linked list with as many elements as requested.
struct linklist{
double value;
struct linklist * next;
};
struct linklist * get_list(int);Example 2: Using the ISNULL C Helper Function in a Loop
The following code segment
shows that the ISNULL C helper function loops over the linked list
that is created by GET_LIST and writes out the elements.
proc proto package=sasuser.mylib.str2;
struct linklist{
double value;
struct linklist * next;
};
struct linklist * get_list(int);
externc get_list;
struct linklist * get_list(int len){
int i;
struct linklist * list=0;
list=(struct linklist*)
malloc(len*sizeof(struct linklist));
for (i=0;i<len-1;i++){
list[i].value=i;
list[i].next=&list[i+1];
}
list[i].value=i;
list[i].next=0;
return list;
}
externcend;
run;
options pageno=1 nodate ls=80 ps=64;
proc fcmp libname=sasuser.mylib;
struct linklist list;
list=get_list(3);
put list.value=;
do while (^isnull(list.next));
list = list.next;
put list.value=;
end;
run;Results from Using the ISNULL C Helper Function
The SAS System 1
The FCMP Procedure
list.value=0
list.value=1
list.value=2