[R] R Processing dataframe by group - equivalent to SAS by group processing with a first. and retain statments

[Jeff Newmiller]

Was wondering when this would be suggested. But the question was about getting the final dataframe...


newdta <- olddta
newdta$FirstDay <- ave(newdata$date, newdata$ID, FUN = \(x) x[1L])

On November 27, 2024 11:13:49 AM PST, Rui Barradas <ruipbarradas using sapo.pt> wrote:
>Às 16:30 de 27/11/2024, Sorkin, John escreveu:
>> I am an old, long time SAS programmer. I need to produce R code that processes a dataframe in a manner that is equivalent to that produced by using a by statement in SAS and an if first.day statement and a retain statement:
>> 
>> I want to take data (olddata) that looks like this
>> ID	Day
>> 1	1
>> 1	1
>> 1	2
>> 1	2
>> 1	3
>> 1	3
>> 1	4
>> 1	4
>> 1	5
>> 1	5
>> 2	5
>> 2	5
>> 2	5
>> 2	6
>> 2	6
>> 2	6
>> 3	10
>> 3	10
>> 
>> and make it look like this:
>> (withing each ID I am copying the first value of Day into a new variable, FirstDay, and propagating the FirstDay value through all rows that have the same ID:
>> 
>> ID	Day	FirstDay
>> 1	1	1
>> 1	1	1
>> 1	2	1
>> 1	2	1
>> 1	3	1
>> 1	3	1
>> 1	4	1
>> 1	4	1
>> 1	5	1
>> 1	5	1
>> 2	5	5
>> 2	5	5
>> 2	5	5
>> 2	6	5
>> 2	6	5
>> 2	6	5
>> 3	10	3
>> 3	10	3
>> 
>> SAS code that can do this is:
>> 
>> proc sort data=olddata;
>>    by ID Day;
>> run;
>> 
>> data newdata;
>>    retain FirstDay;
>>    set olddata;
>>    by ID;
>>    if first.ID then FirstDay=Day;
>> run;
>> 
>> I have NO idea how to do this is R (so I can't post test-code), but below I have R code that creates olddata:
>> 
>> ID <- c(rep(1,10),rep(2,6),rep(3,2))
>> date <- c(rep(1,2),rep(2,2),rep(3,2),rep(4,2),rep(5,2),
>>            rep(5,3),rep(6,3),rep(10,2))
>> date
>> olddata <- data.frame(ID=ID,date=date)
>> olddata
>> 
>> Any suggestions on how to do this would be appreciated. . . I have worked on this for more than 12-hours, despite multiple we searches I have gotten nowhere. . .
>> 
>> Thanks
>> John
>> 
>> 
>> 
>> 
>> John David Sorkin M.D., Ph.D.
>> Professor of Medicine, University of Maryland School of Medicine;
>> Associate Director for Biostatistics and Informatics, Baltimore VA Medical Center Geriatrics Research, Education, and Clinical Center;
>> PI Biostatistics and Informatics Core, University of Maryland School of Medicine Claude D. Pepper Older Americans Independence Center;
>> Senior Statistician University of Maryland Center for Vascular Research;
>> 
>> Division of Gerontology and Paliative Care,
>> 10 North Greene Street
>> GRECC (BT/18/GR)
>> Baltimore, MD 21201-1524
>> Cell phone 443-418-5382
>> 
>> 
>> 
>> ______________________________________________
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>> and provide commented, minimal, self-contained, reproducible code.
>Hello,
>
>Isn't ?ave the simplest way?
>The first one-liner assumes the dates are sorted in ascending order.
>
>
>ave(olddata$date, olddata$ID, FUN = \(x) x[1L])
>#>  [1]  1  1  1  1  1  1  1  1  1  1  5  5  5  5  5  5 10 10
>
>
>If the dates are not sorted,
>
>
>ave(olddata$date, olddata$ID, FUN = \(x) min(x))
>
>
>
>Hope this helps,
>
>Rui Barradas
>
>

-- 
Sent from my phone. Please excuse my brevity.