[R] Trying to get the prior value of a record from a data.frame . . . data.frame

[Kimmo Elo]

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
>
>
>
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--
Kimmo Elo
Senior Lecturer | Adjunct professor, Dr.
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University of Eastern Finland
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E-mail: kimmo.elo using uef.fi
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