Assign value to group based on condition in column

Question

I have a data frame that looks like the following:

> df = data.frame(group = c(1,1,1,2,2,2,3,3,3), 
                 date = c(1,2,3,4,5,6,7,8,9),
                 


        

Answer 1

Here is a base R option

 df$newValue = rep(df$date[which(df$value == 4)], table(df$group))

Another alternative using lapply

do.call(rbind, lapply(split(df, df$group), 
  function(x){x$newValue = rep(x$date[which(x$value == 4)], 
                    each = length(x$group)); x}))

#    group date value newValue
#1.1     1    1     3        2
#1.2     1    2     4        2
#1.3     1    3     3        2
#2.4     2    4     4        4
#2.5     2    5     5        4
#2.6     2    6     6        4
#3.7     3    7     6        8
#3.8     3    8     4        8
#3.9     3    9     9        8