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How to change the first value for each group in data.table object in R?
To change the first value for each group in data.table object, we can use single square brackets for accessing and changing the value to desired value. For example, if we have a data.table object called DT that contains a group column defined by Class and a numerical column defined by Response then the first value of Response for each Class can be set to say 5 by using the command DT[,Response:=c(2,Response[-]),by=Class]
Consider the below data.table object −
Example
library(data.table) Group<-sample(c("A","B","C"),20,replace=TRUE) DT1<-data.table(Group,y) DT1
Output
Group y 1: B 5 2: A 7 3: A 4 4: B 3 5: B 5 6: C 7 7: C 5 8: A 4 9: C 6 10: A 5 11: B 6 12: C 5 13: A 9 14: A 4 15: B 5 16: C 3 17: C 3 18: B 8 19: A 7 20: C 2
Changing first value of y for each Group in DT1 to 2 −
Example
DT1[,y:=c(2,y[-1]),by=Group] DT1
Output
Group y 1: B 2 2: A 2 3: A 4 4: B 3 5: B 5 6: C 2 7: C 5 8: A 4 9: C 6 10: A 5 11: B 6 12: C 5 13: A 9 14: A 4 15: B 5 16: C 3 17: C 3 18: B 8 19: A 7 20: C 2
Example
Class<-sample(c("First","Second","Third","Fourth"),20,replace=TRUE) x<-rpois(20,1) DT2<-data.table(Class,x) DT2
Output
Class x 1: Fourth 1 2: Second 2 3: Second 0 4: Third 0 5: Second 0 6: First 1 7: Third 2 8: First 1 9: Third 0 10: Fourth 3 11: Fourth 0 12: Second 0 13: Third 2 14: Fourth 3 15: Fourth 0 16: Fourth 3 17: Second 0 18: Fourth 1 19: First 0 20: Second 2
Changing first value of x for each Class in DT2 to 3 −
Example
DT2[,x:=c(3,x[-1]),by=Class] DT2
Output
Class x 1: Fourth 3 2: Second 3 3: Second 0 4: Third 3 5: Second 0 6: First 3 7: Third 2 8: First 1 9: Third 0 10: Fourth 3 11: Fourth 0 12: Second 0 13: Third 2 14: Fourth 3 15: Fourth 0 16: Fourth 3 17: Second 0 18: Fourth 1 19: First 0 20: Second 2
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