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

[Rui Barradas]

Às 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
> 
> 
> 
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> 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


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