Question in the DO LOOP regard to the value of X1 in the code

[Jack2012]

%let p0=0.05;
%let p1=0.25;
%let alpha=0.1;
%let beta=0.1; 
%let usern=15;

 ****Stage 1;
  data stage1;
     do n1=5 to &usern-1;/*The totals ACCEPTABLE size is set sas "usern", n1 is the total size to be studied at stage 1*/
     do r1=0;
     term1_p0 = cdf('BINOMIAL', r1, &p0, n1);/*PET: probability of terminate after stage 1 when the drug is ineffective;This is also the Type I ERROR at stage 1*/
     term1_p1 = cdf('BINOMIAL', r1, &p1, n1);/*Probability of early ternation after stage 1 when, in fact, the drug is effective;*/
     if term1_p1=<&beta then output; /*remove solution sets that do not meet the beta requirement; This is the type II ERROR occurred at stage 1*/
     end;
     end;
  run;
  
  *****Stage 2; 
  data stage12; 
  	set stage1;
  	do n2=n1+1 to &usern-1;
  		do r2=r1+1 to n2 while (r1<r2<n2);
  			**term2_p0=0; *****initialize the summation terms for alpha & beta calculations;
  			**term2_p1=0;
  			do x1=r1+1 to min(r2, n1);/*x1: stand for the possible number of responses at stage 2 ONLY*/
  				**dum0=pdf('BINOMIAL', x1, &p0, n1)*cdf('BINOMIAL', r2-x1, &p0, n2-n1);
  				**dum1=pdf('BINOMIAL', x1, &p1, n1)*cdf('BINOMIAL', r2-x1, &p1, n2-n1);
  				**term2_p0= term2_p0+dum0;*recursive formulae used for summation terms;
  				**term2_p1= term2_p1+dum1;
				output;
  			end;
  			output;/*Remove the conditions "1-(term1_p0+term2_p0)=<&alpha and" */
  		end;
  	end;
  run;

Regard to the above code, how could the value of X1 be greater than r2 as shown in below shot?

[Astounding · Accepted solution]

Take a simple case:

do v1 = 1 to 5;
end;
Output;

The final value of v1 is 6, not 5. Each time through the loop, SAS gets to the END statement and adds 1 to v1. So the fifth time through v1 increases from 5 to 6. SAS now notices that the loop is done because v1 exceeds the upper range of the loop.

Similarly, in your loop the loop is over when x1 exceeds min(r2, n1) and the final OUTPUT statement outputs (below the loop) that result.

[Jack2012]

Thank yo so much for your helpl. Got now.