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

[David Winsemius]

On 11/27/24 08:30, Sorkin, John wrote:
> 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. . .


My earlier approach incorrectly picked the firs of the ID column rather 
than the first of the `date` column to be repeated withing the indexed 
group so here's the correct code:


> olddata$FirstDay <- unlist( by(olddata, olddata["ID"], FUN= function(x) 
{ rep( x$date[1], times=nrow(x) )}) ) > olddata ID date FirstDay 1 1 1 1 
2 1 1 1 3 1 2 1 4 1 2 1 5 1 3 1 6 1 3 1 7 1 4 1 8 1 4 1 9 1 5 1 10 1 5 1 
11 2 5 5 12 2 5 5 13 2 5 5 14 2 6 5 15 2 6 5 16 2 6 5 17 3 10 10 18 3 10 10

>
> 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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