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Posted 05-01-2016 07:54 PM
(1559 views)

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.

7 REPLIES 7

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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.

Learn from the Experts! Check out the huge catalog of free sessions in the Ask the Expert webinar series.

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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

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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.

Learn from the Experts! Check out the huge catalog of free sessions in the Ask the Expert webinar series.

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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;

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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 😞

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Post sample data.

However, I think all you need is a retain.

Add

```
retain y;
***rest of code;
y=y*0.75+x/2;
```

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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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