Fill data.table columns based on values of a column of a different data.table

Asked Feb 28 '17 at 21:41

Active Feb 28 '17 at 22:10

Viewed 222 times

I have two data tables

library(data.table)
dt1 = data.table(ID=c(1,2,3),var=c("var1","var2", "var3"),N=c(12,8,19))
    ID  var  N
1:  1 var1 12
2:  2 var2  8
3:  3 var3 19

dt2 = data.table(ID=c(1,2,3),var1=NA,var2=NA,var3=NA)
   ID var1 var2 var3
1:  1   NA   NA   NA
2:  2   NA   NA   NA
3:  3   NA   NA   NA

I would like to fill the var1, var2, var3 columns of dt2 will the values of "N" of dt1, for the corresponding IDs. So, ideally I would like to have

   ID var1 var2 var3
1:  1   12   NA   NA
2:  2   NA    8   NA
3:  3   NA   NA   19

I wasn't able to think of something, except looping row by row... but I hope that there is a solution with data.table.

edited Feb 28 '17 at 22:10

Frank

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asked Feb 28 '17 at 21:41

User800701

1

That's just `dt2 <- dcast(dt1, ID ~ var)` I'll see if I can find a dupe... maybe http://stackoverflow.com/q/6902087/ – Frank Feb 28 '17 at 22:11
1

@Frank is right. Unless your example is over-simplified? – SymbolixAU Feb 28 '17 at 22:12
Thanx @Frank! It seems you don't need data.table at all... – User800701 Feb 28 '17 at 22:20
2

Actually, `dcast` is a data.table function (modified for data.tables from the original version in the reshape2 library). – Frank Feb 28 '17 at 22:22

Fill data.table columns based on values of a column of a different data.table

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