How can I stop the overwriting of lagged columns?

Question

Reference: https://stackoverflow.com/a/28056113/3942806

I am using the code from the link above to create lagged values for two columns.

n<-4
odd2<-setDT(odd)[, paste("OBS_Q", 1:n) := shift(OBS_Q, 1:n)]
odd2<-setDT(odd)[, paste("sac", 1:n) := shift(sac, 1:n)]

This works great! I get 18 columns.

But for convenience, I tried to convert it into a function:

masterlag<-function(df,col,n){
   setDT(df)[, paste(col,sep='_',1:n) := shift(df[[col]], 1:n)]
}

odd3<-masterlag(df=odd,col="OBS_Q",n=4)
odd3<-masterlag(df=odd,col="sac",n=4)

But in this case, the newly created columns of the first one ('OBS_Q') are getting replaced when I used the function the second time ('sac'). So, I am only left with 14 columns instead of 18.

Any pointers as to why?

odd<-structure(list(DATE = 19630101:19630104, PRECIP = c(0, 0, 0,0), 
               OBS_Q = c(1.61, 1.48, 1.4, 1.33), swb = c(1.75, 1.73, 1.7,1.67), 
               gr4j = c(1.9, 1.77, 1.67, 1.58), isba = c(0.83, 0.83,0.83, 0.83), 
               noah = c(1.31, 1.19, 1.24, 1.31), sac = c(1.99,1.8, 1.66, 1.57), 
               swap = c(1.1, 1.05, 1.08, 0.99), vic.mm.day. = c(2.1,1.75, 1.55, 1.43)), 
          .Names = c("DATE", "PRECIP", "OBS_Q", "swb","gr4j", "isba", "noah", "sac", "swap", "vic.mm.day."), 
          class = c("data.table","data.frame"), row.names = c(NA, -4L))

I think you pass the second code on `odd3` ie. `masterlag(df=odd3,col="sac",n=4)[]` — akrun, Jan 21 '15 at 06:13
I guess `setDT` within the function won't change the original `odd` dataset i.e. `masterlag(df=odd,col="OBS_Q",n=4)` If you check `odd`, it will be the same. — akrun, Jan 21 '15 at 06:19
Yes, thanks. It works. I don't need to store it in separate dataframes. — maximusdooku, Jan 21 '15 at 06:33

score 1 · Accepted Answer · answered Jan 21 '15 at 06:42

1

The dataframes are self-updating

odd<-masterlag(df=odd,col="OBS_Q",n=4)
odd<-masterlag(df=odd,col="sac",n=4)

answered Jan 21 '15 at 06:42

maximusdooku

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How can I stop the overwriting of lagged columns?

1 Answers1