series of YYMM

[Ronein]

Hello

User define base month in a structure of YYMM (Macro variable).

Then I want to create automatically 6 macro variables called: Back0,Back1,Back2,Back3,Back4,Back5  which are 0/1/2/3/4/5 months previous months  of base month.

I also want to create automatically 6 macro variables called: month6,month5,month4,month3,month2,month1  which are equal to macro variables Back0,Back1,Back2,Back3,Back4,Back5  (month6 eual to Back0,month5 equal to Back1 and so on).

What is the way to solve the problem with statement k-1?

I know that I can do it in the following way 

%let month6=&back0;
%let month5=&back1;
%let month4=&back2;
%let month3=&back3;
%let month2=&back4;
%let month1=&back5;

But I want to understand what is the problem with k-1 statement and how to solve it?

%let baseMon=1811;
data tbl;
EndMon = intnx('month',input("&baseMon",yymmn4.),0,'b');/*initial start date*/
startMon = intnx('month',input("&baseMon",yymmn4.),-5,'b');/*End date*/
length YYMM $4;
j=0;
k=6;
do until (startMon > EndMon);
  YYMM = put(EndMon,yymmn4.);
  back=cats('back',j);
  month=cats('month',k);
  call symputx(cats('back',j),YYMM);
  call symputx(cats('month',k),YYMM);
  output;
  j+1;
  k-1;/*Here is the problem. When I write k+1 it works but I need k-1 for my purpose*/
  EndMon = intnx('month',EndMon,-1,'b');
end;
keep YYMM back month;
run;
/**Results that I want to get are:**/
%put &back0;/*1811*/
%put &back1;/*1810*/
%put &back2;/*1809*/
%put &back3;/*1808*/
%put &back4;/*1807*/
%put &back5;/*1806*/

%put &month6;/*1811*/
%put &month5;/*1810*/
%put &month4;/*1809*/
%put &month3;/*1808*/
%put &month2;/*1807*/
%put &month1;/*1806*/

[mkeintz · Accepted solution]

---

@Ronein wrote:

Hello

User define base month in a structure of YYMM (Macro variable).

Then I want to create automatically 6 macro variables called: Back0,Back1,Back2,Back3,Back4,Back5  which are 0/1/2/3/4/5 months previous months  of base month.

I also want to create automatically 6 macro variables called: month6,month5,month4,month3,month2,month1  which are equal to macro variables Back0,Back1,Back2,Back3,Back4,Back5  (month6 eual to Back0,month5 equal to Back1 and so on).

What is the way to solve the problem with statement k-1?

I know that I can do it in the following way 

%let month6=&back0;
%let month5=&back1;
%let month4=&back2;
%let month3=&back3;
%let month2=&back4;
%let month1=&back5;

But I want to understand what is the problem with k-1 statement and how to solve it?

  k-1;/*Here is the problem. When I write k+1 it works but I need k-1 for my purpose*/

---

The problem is that the statement

    k+1;

is a summing statement.  Which means that (1) missing k will be treated as a zero, and (2) k will be retained over iterations of the data step.   Since you are doing iterations over a do loop, instead of over multiple data set iterations, you could just as well do k=sum(k,1).

But

   k-1;

is NOT such a statement.  But you could do

k=sum(k,-1);

Or if you like the summing statement format you could do   

   k+(-1);

[Kurt_Bremser]

Too complicated, build the inversion with a simple subtraction:

%let baseMon=201811;
%let number=6;

data _null_;
do i = 0 to &number.;
  em = put(intnx('month',input("&basemon.",yymmn6.),-i,'b'),yymmn6.);
  call symputx(cats('back',i),em);
  call symputx(cats('month',&number. - i),em);
end;
run;