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

[Sorkin, John]

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;

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