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

[Tom Woolman]

Check out the dplyr package, specifically the mutate function.

# Create new column based on existing column value 

df <- df %>% mutate(FirstDay = if(ID = 2, 5))

df



Repeat as needed to capture all of the day/firstday combinations you want to account for.

Like everything else in R, there are probably at least a dozen other ways to do this, between base R and all of the library packages available.




On Wednesday, November 27th, 2024 at 11:30 AM, Sorkin, John <jsorkin using som.umaryland.edu> 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. . .
> 
> 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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