do loop, retain and lag?

[sophia_SAS]

Dear SAS community, 

I have a very simple calculation problem that I can't quite seem to solve using a combination of do loops, lag functions and a retain statement.  In general, I would like to calculate a new denominator based on values from a previous numerator observation.    For example on Day 2 - the denom = 499 because on Day 1 denom of 500 - numer of 1 = 499.  For day 3, the denom = 497 because the day 2 denom of 499 - day 2 numer of 2 = 497. Below is what I would like my final dataset to look like. Thoughts?

Thank you!

data want;

input day numer denom;

datalines;;

1 1 500

2 2 499

3 1 497

4 1 496

5 2 495

;;

[Reeza · Accepted solution]

It's not clear what you start with. 

Perhaps this will help. 

Retain denom2;

Y = lag(numer);

if _n_ ne 1 then denom2= denom2 - y;

Else denom2=denom;

[sophia_SAS]

This is great! Thank you so much!

[Astounding]

I would look at it in this way:

data want;

set have;

new_denom = 500 - total_numer;

total_numer + numer;

run;

Adjustments could be made if you don't want to start at 500 each time.  But the subtraction has to take place first, before recalculating the TOTAL_NUMER.

[sophia_SAS]

This also works great! Thank you both for responding so quickly.