topic macro do loop and recursive method in SAS Programming

macro do loop and recursive method

pavank — Sat, 24 Jul 2021 06:40:22 GMT

Hi Everyone

Goodmornin

macro function %countdown having a numeric parameter and producing the following output in the log :

Countdown 10...

Countdown 9...

Countdown 8...

Countdown 7...

Countdown 6...

Countdown 5...

Countdown 4...

Countdown 3...

Countdown 2...

Countdown 1...

BOOM !!!

Using a %do loop
Using a recursive method

Re: macro do loop and recursive method

Kurt_Bremser — Sat, 24 Jul 2021 06:46:27 GMT

From the lessons in your course, you should be able to do your homework yourself.

Let us see what you tried.

Re: macro do loop and recursive method

pavank — Sun, 25 Jul 2021 09:46:54 GMT

data count;

var='countdown';
do i=n to 1 by -1;

put vari;
end;
stop;
run;

Could you please give solution

Re: macro do loop and recursive method

Kurt_Bremser — Sun, 25 Jul 2021 10:21:21 GMT

So next, do the same in a macro with a %DO loop and a macro parameter as starting value.

Re: macro do loop and recursive method

pavank — Sun, 25 Jul 2021 10:39:25 GMT

I didn't get any idea how to do that

Re: macro do loop and recursive method

Kurt_Bremser — Sun, 25 Jul 2021 10:50:12 GMT

Were you sleeping through the course lessons?

Converting a data step loop to a macro loop is dead simple (if you paid any attention).

Re: macro do loop and recursive method

pavank — Sun, 25 Jul 2021 11:30:19 GMT

%macro countdown(ds,num);
var='Countdown';
%do i= 10 %to  1 by -1;
output;
end;
stop;
put vari;
run;
%macro;

%countdown(ds,10);
%put(countdown.. &i);

I didn't get output ??

Re: macro do loop and recursive method

pavank — Sun, 25 Jul 2021 13:05:25 GMT

please help for solution

Re: macro do loop and recursive method

Kurt_Bremser — Sun, 25 Jul 2021 13:18:25 GMT

%macro countdown(num);
%do i = &num. %to 1 %by -1;
  %put Countdown &i.;
%end;
%put Boom!;
%mend;

%countdown(10);

Study this closely, so you get a grasp on the very basics of macro programming.

Next, go searching for the concept of recursion in programming, and see how you can apply it to this task.

Re: macro do loop and recursive method

pavank — Sun, 25 Jul 2021 14:02:13 GMT

THANK YOU VERY MUCH

KurtBremser

Re: macro do loop and recursive method

Tom — Sun, 25 Jul 2021 14:32:47 GMT

There is no recursion in that example.

Re: macro do loop and recursive method

Tom — Sun, 25 Jul 2021 14:46:45 GMT

Let's breakdown the assignment into parts:

macro function %countdown

Sounds like they want to to define a SAS macro named COUNTDOWN. Do you know what statement that is?

It is strange that they used the term FUNCTION here. Are they trying to say that the macro only generates part of statement so that it can be used in the middle of a statement or expression, like you would use an actual SAS function call?

having a numeric parameter

SAS parameters are macro variables (or they become macro variables that local to the macro). And macro variables always contain text. I think they mean here that the macro will expect the parameter values to be text strings that SAS code would interpret as numbers. Like 10.

and producing the following output in the log :

Countdown 10...

Countdown 9...

Countdown 8...

Countdown 7...

Countdown 6...

Countdown 5...

Countdown 4...

Countdown 3...

Countdown 2...

Countdown 1...

BOOM !!!

Sounds like they want it to ignore the input parameter and always start the countdown at 10. But let's assume that they really wanted the countdown to start with the value of the input parameter.

You have an answer for a macro that uses a %DO loop.

How do you think you could do it with recursion?
Do you know what recursion is?
Do you know how to program recursion? Hint: make sure that there is a way to prevent the recursion from going on forever.

When done compare to this:

%macro countdown(start);
%if &start > 0 %then %do;
  %put Countdown &start;
  %countdown(%eval(&start-1))
%end;
%else %put BOOM !!!;
%mend countdown;

Re: macro do loop and recursive method

pavank — Sun, 25 Jul 2021 16:03:56 GMT

What is recursive method how to do recursive methof using bu above example

Re: macro do loop and recursive method

Tom — Sun, 25 Jul 2021 16:31:59 GMT

Here is an introduction to the concept of recursion.

https://www.topcoder.com/thrive/articles/An%20Introduction%20to%20Recursion%20Part%20One

The SAS solution for your problems looks very much like the first example.

%macro countdown(start);
%if &start > 0 %then %do;
  %put Countdown &start;
  %countdown(%eval(&start-1))
%end;
%else %put BOOM !!!;
%mend countdown;

Re: macro do loop and recursive method

BrahmanandaRao — Mon, 26 Jul 2021 06:35:13 GMT

Thank you TOM

Re: macro do loop and recursive method

David_Billa — Wed, 28 Jul 2021 15:23:05 GMT

Why by -1 in do loop?

Re: macro do loop and recursive method

PaigeMiller — Wed, 28 Jul 2021 16:14:44 GMT

@David_Billa wrote:
Why by -1 in do loop?

If you want a loop to go from 10 to 9 to 8 to ..., what is the increment that gets you from 10 to 9?

Re: macro do loop and recursive method

Kurt_Bremser — Wed, 28 Jul 2021 16:55:25 GMT

Because it's a countdown.