Contributor
Posts: 21

# How to calculate new variable n using the new variable n-1??

[ Edited ]

Hi guys,

I'm trying to calculate a new variable WANT(n), based on the previous observation's variable WANT(n-1).

For example, if n=1, WANT=HAVE/2;

else if n>1 then WANT(n)=WANT(n-1)*0.75+HAVE/2;

I have no idea which code to start. Could anyone give me some idea? Thanks in advance.

Community Manager
Posts: 3,424

## Re: How to calculate geodesic distance in SAS

I'm not sure if your subject (geodesic distance) is what you meant in your question.  If so:

Check out this communities article: How to calculate geodesic distance in SAS by @MikeZdeb!

But it sounds like you need an approach that uses the LAG function (to store the value from a current obs to calculate a new value in the next record).  There are lots of papers and articles about this -- here is one.

Contributor
Posts: 21

## Re: How to calculate geodesic distance in SAS

Chris,

Thank you for your reply. I changed my title. It was automatically filled in by sas when I typed my title.

My question is not only a LAG function question. I think it's more of a LOOP function question, but I don't know how to use it, since the lagged variable I need to use to construct my new variable also need to be constructed the same way, because it's the same new variable, if this makes sense.

Regards,

Haimeng

Community Manager
Posts: 3,424

## Re: How to calculate new variable n using the new variable n-1??

[ Edited ]

I think that the implicit loop in the DATA step takes care of this for you.  Here's a simple example.

``````data test;
set sashelp.class(rename=(weight=have) keep=weight);
prev_have=lag(have);
if _n_ = 1 then want=have/2;
else want=prev_have*0.75 + have/2;
run;``````

Run that and check the math to see if it's what you want.  Note that you must always load up prev_have using the lag function, unconditionally .  That makes the prev obs value available for calculation in the next iteration.

Posts: 1,270

## Re: How to calculate geodesic distance in SAS

Please provide a sample data set. Based on your requirement this may be you are looking for

data have;
input x;
datalines;
2
4
6
8
;

data want;
set have;
y=lag(x)*0.75+x/2;
if _n_=1 then y=x/2;
run;

Contributor
Posts: 21

## Re: How to calculate geodesic distance in SAS

stat_sas, what I need is Y=lag(Y)*0.75+x/2, not y=lag(x)*0.75+x/2. That's my problem

Super User
Posts: 23,681

## Re: How to calculate geodesic distance in SAS

Post sample data.

However, I think all you need is a retain.

``````retain y;

***rest of code;

y=y*0.75+x/2;``````

Posts: 1,270

## Re: How to calculate geodesic distance in SAS

As mentioned earlier by Reeza, retain statement will be required to get previous value of y for calcuation. Please try the following code:

data want;
set have;
retain y;
if _n_=1 then y=x/2;
else y=y*0.75+x/2;
run;

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