[R] [External] Identify first row of each ID within a data frame, create a variable first =1 for the first row and first=0 of all other rows

[Richard M. Heiberger]

tmp.ID <- unique(olddata$ID)
Firsts <- match(tmp.ID, olddata$ID)
newdata <- cbind(olddata, First=0)
newdata$First[Firsts] <- 1
newdata

newdata$FirstDay <- 0
for (id in tmp.ID)
  newdata$FirstDay[newdata$ID == id] <- newdata$date[newdata$ID == id][1]
newdata


> On Nov 30, 2024, at 21:27, Sorkin, John <jsorkin using som.umaryland.edu> wrote:
>
> Dear R help folks,
>
> First my apologizes for sending several related questions to the list server. I am trying to learn how to manipulate data in R . . . and am having difficulty getting my program to work. I greatly appreciate the help and support list member give!
>
> I am trying to write a program that will run through a data frame organized by ID and for the first line of each new group of data lines that has the same ID create a new variable first that will be 1 for the first line of the group and 0 for all other lines.
>
> e.g. if my original data is
> olddata
>   ID date
>    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
>
> the new data will be
> newdata
>   ID date  first
>    1     1       1
>    1     1       0
>    1     2       0
>    1     2       0
>    1     3       0
>    1     3       0
>    1     4       0
>    1     4       0
>    1     5       0
>    1     5       0
>    2     5       1
>    2     5       0
>    2     5       0
>    2     6       0
>    2     6       0
>    2     6       0
>    3   10       1
>    3   10       0
>
> When I run the program below, I receive the following error:
> Error in df[, "ID"] : incorrect number of dimensions
>
> My code:
> # Create data.frame
> 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))
> olddata <- data.frame(ID=ID,date=date)
> class(olddata)
> cat("This is the original data frame","\n")
> print(olddata)
>
> # This function is supposed to identify the first row
> # within each level of ID and, for the first row, set
> # the variable first to 1, and for all rows other than
> # the first row set first to 0.
> mydoit <- function(df){
>  value <- ifelse (first(df[,"ID"]),1,0)
>  cat("value=",value,"\n")
>  df[,"first"] <- value
> }
> newdata <- aggregate(olddata,list(olddata[,"ID"]),mydoit)
>
> Thank you,
> 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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