Test if a macro variable has missing value

[UdayGuntupalli]

All, 
 I have a simple question. How can one perform a logical test to validate if a macro variable has missing value ? 

For e.g.: 

%Let a = .; 
%Put &a.; 
%If &a = %str() %Then 
%Do;
%Put "1";
%End;

 I thought something like this, but doesn't get me too far. 

[Astounding]

There are several ways, and an entire paper has been published on the subject. My favorite way:

%if %length(&a) %then %do;

[Astounding · Accepted solution]

That's for a nonnull value. For a null value:

%if %length(&a)=0 %then %do;

[UdayGuntupalli]

@Astounding ,
   Thank you for your response. May I ask you to offer a little more clarity , 

%Let a = . ;
%if %length(&a) %then 
	%do;
		%put 1; 
	%end;
	%else;
		%put 0; 
	%end; 

  This returns an error. Can you point as to what is wrong ? Additionally, can you point me to the paper you are referring to ? 

[FreelanceReinh]

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@UdayGuntupalli wrote:
(...) Additionally, can you point me to the paper you are referring to ? 

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Just for completeness, this is (most likely) the paper Astounding referred to: https://support.sas.com/resources/papers/proceedings09/022-2009.pdf

[Astounding]

In macro language . is a character and not a missing value.

This statement is incorrect:

%else;

It should be:

%else %do;

Authors of the paper are Chung and King. For me, it's too difficult to find and post links using a mobile phone.

[Ksharp]

%If &a =  %Then %Do;%Put "1";%End;