根据其他列中的条件添加列

Question

I have this table and I would like to add the colum "New column" based on a condition. The condition is that when in the column 3, we have "Peter", then in the "New col" I want to have the result of the col2. This value would be the same for the same ID. In the "New Col", you can see the desire result.

ID  Col2    Col3    New Col
1   B   Peter   B
1   A   Matt    B
2   B   Peter   B
2   B   Matt    B
2   A   Matt    B
3   C   Peter   C

This is what I have.

for (j in 2:(j-1)){
  if (df$Col3[j] == "Peter"){
    df$Newcol[j] = df$Col2[j]
  } else {
    df$Newcol[j] = df$Newcol[j-1]  
  }
}

But instead of getting the rigth values, I get numbers "6" and "9".

Any suggestions? Thanks a lot

Answer 1

Using data.table

library(data.table)
setDT(data)
data[,new:= if(any(Col3 == "Peter")) Col2[which(Col3 == "Peter")] else NA, by = ID]

#   ID Col2  Col3 new
#1:  1    B Peter   B
#2:  1    A  Matt   B
#3:  2    B Peter   B
#4:  2    B  Matt   B
#5:  2    A  Matt   B
#6:  3    C Peter   C

Using base R lapply

do.call(rbind, 
        lapply(split(data, data$ID), 
        function(x){ if(any(x$Col3 == "Peter")){ 
        x$new = x$Col2[which(x$Col3 == "Peter")]; 
        x}}))

#    ID Col2  Col3 new
#1.1  1    B Peter   B
#1.2  1    A  Matt   B
#2.3  2    B Peter   B
#2.4  2    B  Matt   B
#2.5  2    A  Matt   B
#3    3    C Peter   C

data

data = structure(list(ID = c(1L, 1L, 2L, 2L, 2L, 3L), Col2 = structure(c(2L, 
1L, 2L, 2L, 1L, 3L), .Label = c("A", "B", "C"), class = "factor"), 
    Col3 = structure(c(2L, 1L, 2L, 1L, 1L, 2L), .Label = c("Matt", 
    "Peter"), class = "factor")), .Names = c("ID", "Col2", "Col3"
), class = "data.frame", row.names = c(NA, -6L))