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

[Ebert,Timothy Aaron]

Here is another version using for loops.

newdata3 <- olddata |>
  dplyr::arrange(ID, date)
newdata3$firstday <- NA
for (i in 1:nrow(newdata3)) {
  if (i == 1) {
    dayz <- newdata3$date[i]
  } else {
    if (newdata3$ID[i] != newdata3$ID[i - 1]) {
      dayz <- newdata3$date[i]
    }
  }
  newdata3$firstday[i] <- dayz
}

newdata3

-----Original Message-----
From: R-help <r-help-bounces using r-project.org> On Behalf Of Tom Woolman
Sent: Wednesday, November 27, 2024 12:05 PM
To: Sorkin, John <jsorkin using som.umaryland.edu>
Cc: r-help using r-project.org (r-help using r-project.org) <r-help using r-project.org>
Subject: Re: [R] R Processing dataframe by group - equivalent to SAS by group processing with a first. and retain statments

[External Email]

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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