Do Index loop: The index variable serves as the start and finish value for the Do Index loop. SAS commands are performed repeatedly until the index variable reaches its ultimate value.

The Do While Loop employs a WHILE condition. This loop executes the block of code while the condition is true and continues to execute it until the condition is false. When the condition becomes false, the loop is ended.

The Do Until loop has an Until condition. This loop executes the block of code while the condition is false and continues to execute it until the condition is true. The loop ends when the condition is met.

If you have to explain using code, let’s suppose we want to compute the SUM and the number of variables.
For the loops, write the code as follows:

Do Index

              DATA ExampleLoop;
                  SUM=0;
              Do VAR = 1 = 10;
              SUM = SUM + VAR;
                   END;
        PROC PRINT DATA = ExampleLoop;
                   Run;

The output would be:

     Obs      SUM         VAR
      1        55          11

Do While

                DATA ExampleLoop;
                    SUM = 0;
                    VAR = 1;
               Do While(VAR<15);
               SUM = SUM + VAR;
                    VAR+1;
                    END;
           PROC PRINT DATA = ExampleLoop;
                     Run;

output:
         Obs      SUM      VAR
          1       105       15

Do Until

               DATA ExampleLoop;
                   SUM = 0;
                   VAR = 1;
               Do Until(VAR>15);
                  SUM=SUM+VAR;
                   VAR+1;
                    END;
                 PROC PRINT;
                     Run;

output:
             Obs             SUM                 VAR
              1              120                 16