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Do loop understand

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New Contributor
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Do loop understand

Hi Guys ,

I'm new to sas programming and now im trying to understand the sas program wriiten by someone -

can you please give me idea how below statment works ?

%do i=1 %to 60 %by 1;

%do j=&i-1 %to &i-1;

Super User
Posts: 7,868

Re: Do loop understand

Posted in reply to mahinikam

First of all, this is not a do loop, it is a %do loop, a very important distinction.

%do means this is a macro statement, and it is executed by the macro preprocessor, and is used to create dynamic SAS code.

(think of the C preprocessor, if you are familiar with programming and the C language)

%do i=1 %to 60 %by 1;

will create a macro variable i (later to be referenced by &i), assigns it the value 1, execute what is inside the block up to %end;, increase i by 1 and repeat until i exceeds 60

Similarly,

%do j=&i-1 %to &i-1;

will iterate from i-1 to i-1, so it will do only one iteration.

Caution: the macro preprocessor knows only one datatype, namely text. That the calculation &i-1 is possible is a capability of the iterative %do macro statement, usually one would need to use %eval(&i-1) to do calculations with macro variables.

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Trusted Advisor
Posts: 1,586

Re: Do loop understand

Posted in reply to mahinikam

When you see the % as part of the code (like: %do, %to, %macro, %mend, %if etc.)

the code is part of a macro program. A macro program is a kind of program generator.

 

The code you are looking at includes, probably, next lines:

$macro <macro_name> (argument1, argument2,...); /* start definition of a macro program */
     .....
    %do i=1 %to 60 %by 1;
          %do j=&i-1 %to &i-1;
                 ...... more code lines ...
          %end;   /* ending inner loop */
    %end;        /* ending outer loop */
    ..........
%mend <macro_name>; /* ending definition of a macro program */
%macro_name(...arguments...); /* execution of the macro program */

so, the two lines you showd is actaullay producing similar code, again and agian, with slight modification

according to i counting from 1 to 60 (are there 60 segemnet or variables ?), maybe relating to the previous one (as j=&i-1);

 

i and j in this case, in a macro program, are called macro variables. You relate to macro variable by adding prefix & before the name, that is: &i, &j etc. 

Regular Contributor
Posts: 240

Re: Do loop understand

Posted in reply to mahinikam

The second loop

%do j=&i-1 %to &i-1;

seems rather useless as it only performs one iteration.

 

It could be replaced by a simple affectation :

%let j=&i.-1;

 

or

 

%let j=%eval(&i.-1)

 

The difference between both commands is that, with the first, whenever &j. is encountered in

the program, there will only be a subtitution for the current value of i and no evaluation of

the resulting expression, e.g., if &i.=10, &j. will be replaced by 10-1 wich may not be what we want

in certain contexts. Using %eval will ensure that the resulting expression will be evaluated so that

each occurrence of &j will be replaced by the value of &i.-1 (9 in the example).

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