the %WHILE and %UNTIL specifications cannot be added to the increments

[Sajid01]

Hello
I was trying to learn about Macros in SAS. While reading the paper "Five Ways to Create Macro Variables: A Short Introduction to the Macro Language by Arthur L. Carpenter,  I got stuck at the statement in the section on Iterative % DO loops page 4.

the %WHILE and %UNTIL specifications cannot be added to the increments

The author is an expert and there must be great significance in the above statement. I am unable to comprehend it.
Can anybody please help me understand this?
It is my understanding that %Do %WHILE and %DO %Until constructs can be used in macros.
Thanks in anticipation.

[Patrick · Accepted solution]

The SAS datastep allows for constructs as below.

data _null_;
  do i=1 to 99 until(var>99);
    var=i*i;
  end;
  put i= var=;
run;

The same is not possible with SAS macro language. Below code will return a syntax error.

%macro demo1();
  %local var;
  %do i=1 %to 99 %until(&var>99);
    %let var= %eval(&i*&i);
  %end;
  %put &=i &=var;
%mend;
%demo1();

To express such logic on SAS macro level you need to implement differently - for example as below.

%macro demo2();
  %local i;
  %do %until(&var>99);
    %let i=%eval(&i+1);
    %let var= %eval(&i*&i);
  %end;
  %put &=i &=var;
%mend;
%demo2();

[mkeintz]

In normal SAS data step code, you could do something like this, which simultaneously uses both a do index range and an until condition as limits on the number of do loop iterations:

data _null_;
  do i=1 to 90 until (rannum>0.9);
    rannum=ranuni(4150913);
    put i= rannum=;
  end;
run;

The loop will stop once i is incremented beyond 90, or rannum exceeds 0.9, whichever happens first.

But if you write the intuitively analogous macro code, you get the error messages in the log below:

667  %macro t;
668    %do i=1 %to 90 %until (&rannum > 0.9);
669      %let rannum=%sysfunc(ranuni(4150913));
670    %end;
671  %mend t;
672  %t ;
ERROR: Improper use of macro reserved word until.
WARNING: Apparent symbolic reference RANNUM not resolved.
ERROR: A character operand was found in the %EVAL function or %IF condition where a numeric
       operand is required. The condition was: 90 %until (&rannum > 0.9)
ERROR: The %TO value of the %DO I loop is invalid.
ERROR: The macro T will stop executing.

Instead, you have to include both the index limit and the until condition into a compound until condition:

%macro t;
  %let i=0;
  %do %until (&rannum > 0.9 OR &i=90);
    %let i=%eval(&i+1);
    %let rannum=%sysfunc(ranuni(4150913));
    %put &=i &=rannum;
  %end;
%mend t;
%t ;

[Sajid01]

Thanks @Patrick  and @mkeintz for helping me understand it.