topic Re: advantage of DO UNTIL in SAS Programming

advantage of DO UNTIL

SAS_inquisitive — Thu, 17 Mar 2016 18:40:39 GMT

What advantage can be made of DO UNTIL over DO WHILE in programming because of the fact that DO UNTIL executes at least once?

Re: advantage of DO UNTIL

LinusH — Thu, 17 Mar 2016 19:44:21 GMT

Mainly semantics I guess. And save a line of code or two compared to while, given the situation.
You seen very interested in these matters, any specific problem you wish to solve?
The while and until constructs exists in many programming languages. There must lot of literature to browse that can guide you.

Re: advantage of DO UNTIL

Tom — Thu, 17 Mar 2016 19:52:44 GMT

In general the advantage is not having the repeat the same block of code before the DO loop and inside the DO loop.

So you can convert code like:

< get an item >
do while (not <condition>);
  <do something>
  <get an item>
end;

do until (<condition>);
  <get an item>
  <do something>
end;

Re: advantage of DO UNTIL

ballardw — Thu, 17 Mar 2016 20:04:44 GMT

One generic "difference" is along the lines of "test at the top" and "test at the bottom". "Do While" means the intial value usually is set before the loop and often executes at least once because of the initial value. "Do until" is more often used to check the condition at the end of one execution fo the loop.

Or treat as "do until this condition that is initially true is no longer true" and "Do until this condtion that is initially false is true"

Usually with work one can get the same results with either loop construct.

Be very leery of using Do Until or Do While with non-integer numerics and and Equal comparison especially if calculating the loop control as you might not get "equality" do to a variety of reasons with the sneakiest being precision issues.

Consider this program:

data _null_;
   x=0;
   do until(x=1);
      x=x+0.01;
   end;
run;

Do not run this if you do not know how to interrupt a program.

Re: advantage of DO UNTIL

SAS_inquisitive — Fri, 18 Mar 2016 13:51:36 GMT

@ LinusH. I believe looping is very important part of any programming language. I really want to master on it.

Re: advantage of DO UNTIL

Ron_MacroMaven — Fri, 18 Mar 2016 21:07:29 GMT

getting straight on where the exit test happens is an important aspect of understanding

the difference between while (at top) and until (at bottom).

There are two other important loop constructs:

continue

and

leave

both of which can be implemented in macro loops as well.

Check my paper for details

http://www.sascommunity.org/wiki/Do_which_loop_while_or_until

Ron Fehd loops maven