Difference in output for do untill and do looping.

[ankitasharma]

Hi i ran the below codes and they gave me different outputs.i wanted to check how many years it will take for $100 at 3.75% int per annum to double?
For do:
Data x;
Int =0.375;
Total=100;
Do total= 100 to 200;
Year+1;
Total= total +int*total ;
Output; end;
Run; this gave 16 obs
While same query , changing do statement to do until(total ge 200)
The result changed to 19 obs.

Can someone explain what is the logic behind it?
Really appreciate your help.

[FreelanceReinh]

Hi @ankitasharma,

Only the DO-UNTIL approach is correct (assuming that you mean Int=0.0375). With the iterative DO loop (do Total=100 to 200;) Total is incremented by 1 at the end of each iteration (i.e. after the OUTPUT statement has executed), which is not what you want. You can replicate this behavior (just for demonstration) with the DO-UNTIL approach by inserting

total+1;

between the OUTPUT and END statements.

Related quote from the help file "DO Statement, Iterative":

CAUTION:

Avoid changing the index variable within the DO group.

[Tom]

You are going to see a difference starting with the second time through the loop.

When you use DO UNTIL ( total ge 200) the value of total does not change from the end of one iteration of the loop to the start of the second.

But when you use DO TOTAL=100 to 200 (note that the increment amount will be 1 since it is not specified) the value of TOTAL is increased by 1 at the start of each new iteration.

Year	Total In	Total Out		Total In	Total Out
1	100	103.75		100	103.75
2	103.75	107.6406		104.75	108.6781
3	107.6406	111.6771		109.6781	113.7911
4	111.6771	115.865		114.7911	119.0957