R help - Nov 2024 - Trying to get the prior value of a record from a data.frame . . . data.frame

I need to write code that will give me the previous value of from a data.frame.
I have written the following code using the shift function from data.table . It
does not work. I hope someone can help me correct the code.
###########################
# Try to understand shift #
###########################
if(!require(data.table)) install.packages("data.table")
library(data.table)
# Create data
x <- data.frame(Id=rep(1:10),num=rep(11:20))
cat("This is the input data.frame used in the code
below","\n")
x

for (i in 1:10) {
  cat("x[i,num]",x[i,"num"],"\n")
  # Get previous value of x[i,"num"] 
  zoop<-shift(x[i,"num"], n=1L, type="lag") 
  cat("Previous value of x[,num]=",zoop,"\n")
  }
###############################
# END Try to understand shift #
###############################

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

Hi,

is there a specific reason to use "shift"? I mean, you could easily
achieve what you described by simple indexing:

--- snjp ---

for (i in 1:10) {
         cat("x[i,num]",x[i,"num"],"\n")
         # Get previous value of x[i,"num"]
         zoop<-x[i-1,"num"] # NB! Returns "integer(0)"
for row index 0
         cat("Previous value of x[,num]=",zoop,"\n")
}

--- snip ---

The code above just loops through the *rows* from 1 to 10 and dumps the
"num" value from the previous row. If you want to get the
"num" value
from the row above a specific "Id", then the zoop-line should look
something like this:

zoop<-x[which(x["Id"]==i)-1, "num"]

HTH,

Kimmo


Sorkin, John kirjoitti 29.11.2024 klo 3.25:
> I need to write code that will give me the previous value of from a
data.frame. I have written the following code using the shift function from
data.table . It does not work. I hope someone can help me correct the code.
> ###########################
> # Try to understand shift #
> ###########################
> if(!require(data.table)) install.packages("data.table")
> library(data.table)
> # Create data
> x <- data.frame(Id=rep(1:10),num=rep(11:20))
> cat("This is the input data.frame used in the code
below","\n")
> x
>
> for (i in 1:10) {
>    cat("x[i,num]",x[i,"num"],"\n")
>    # Get previous value of x[i,"num"]
>    zoop<-shift(x[i,"num"], n=1L, type="lag")
>    cat("Previous value of x[,num]=",zoop,"\n")
>    }
> ###############################
> # END Try to understand shift #
> ###############################
>
> 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
>
>
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
https://www.r-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
--
Kimmo Elo
Senior Lecturer | Adjunct professor, Dr.
=======================================================University of Eastern
Finland
Department of Geographical and Historical Studies
P.O. Box 111
FIN-80101 Joensuu
Finland
E-mail: kimmo.elo at uef.fi
ResearchGate: http://www.researchgate.net/profile/Kimmo_Elo
LAWPOL Consortium (PI): https://lawpol.fi/en
========================================================

?s 01:25 de 29/11/2024, Sorkin, John escreveu:
> I need to write code that will give me the previous value of from a
data.frame. I have written the following code using the shift function from
data.table . It does not work. I hope someone can help me correct the code.
> ###########################
> # Try to understand shift #
> ###########################
> if(!require(data.table)) install.packages("data.table")
> library(data.table)
> # Create data
> x <- data.frame(Id=rep(1:10),num=rep(11:20))
> cat("This is the input data.frame used in the code
below","\n")
> x
> 
> for (i in 1:10) {
>    cat("x[i,num]",x[i,"num"],"\n")
>    # Get previous value of x[i,"num"]
>    zoop<-shift(x[i,"num"], n=1L, type="lag")
>    cat("Previous value of x[,num]=",zoop,"\n")
>    }
> ###############################
> # END Try to understand shift #
> ###############################
> 
> 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
> 
> 
> 
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
https://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
Hello,

If you want to shift an entire vector, you don't need the loop, pass the 
vector itself to data.table::shift.



x <- data.frame(Id=rep(1:10),num=rep(11:20))

data.table::shift(x[["num"]], n = 1L)
#>  [1] NA 11 12 13 14 15 16 17 18 19
data.table::shift(x[, "num"], n = 1L)
#>  [1] NA 11 12 13 14 15 16 17 18 19


Hope this helps,

Rui Barradas


-- 
Este e-mail foi analisado pelo software antiv?rus AVG para verificar a presen?a
de v?rus.
www.avg.com

I assume that the responses that John already received to his recent
post met his needs. However, when I read it, I had a slightly
different interpretation. So feel free to ignore the rest of this post
if you like, but here's my interpretation and a simple solution to it.

An example to help explain:

set.seed(453)
df <- data.frame(
      group = sample(letters[1:4],30, rep = TRUE),
      gender = sample(c("M", "F", "NB"), 30, rep =
TRUE),
      value = 1:30
)

df is a data frame with 30 records/rows of 3 columns giving a group
identifier, gender, and value for each record. I kept the values
artificially simple to (I hope) make it easier to understand the
problem and my solution.

The problem: create a new column, prev.value, that gives the value of
the previous record that has the same group and gender as the current
record if any such exist; or NA if no such previous id and gender
combination occur.
I think this is a slightly more complex task than John's original
request, but it is actually straightforward even in base R -- and
undoubtedly also using similar functionality in the Tidyverse or other
packages.

Here is my solution (using the R pipe, "|>", syntax):

df$prev.value <- with(df, {
   f <- paste0(group,gender) ## a simple 'hash' to identify the
combinations
      ## "shift" the values for each group defined by f:
   f |> tapply(value, INDEX = _, FUN = \(x)c(NA, head(x, -1)))  |>
      ## reassemble according to f:
   unsplit(f)
})
df


Cheers,
Bert


On Thu, Nov 28, 2024 at 5:25?PM Sorkin, John <jsorkin at
som.umaryland.edu> wrote:
>
> I need to write code that will give me the previous value of from a
data.frame. I have written the following code using the shift function from
data.table . It does not work. I hope someone can help me correct the code.
> ###########################
> # Try to understand shift #
> ###########################
> if(!require(data.table)) install.packages("data.table")
> library(data.table)
> # Create data
> x <- data.frame(Id=rep(1:10),num=rep(11:20))
> cat("This is the input data.frame used in the code
below","\n")
> x
>
> for (i in 1:10) {
>   cat("x[i,num]",x[i,"num"],"\n")
>   # Get previous value of x[i,"num"]
>   zoop<-shift(x[i,"num"], n=1L, type="lag")
>   cat("Previous value of x[,num]=",zoop,"\n")
>   }
> ###############################
> # END Try to understand shift #
> ###############################
>
> 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
>
>
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
https://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.