Change variable value-- repeated IDs

Question

I've this data set

id <- c(0,0,1,1,2,2,3,3,4,4)
gender <- c("m","m","f","f","f","f","m","m","m","m")
x1 <-c(1,1,1,1,2,2,3,3,10,10)
x2 <- c(3,7,5,6,9,15,10,15,12,20)

alldata <- data.frame(id,gender,x1,x2)

which looks like:

id  gender  x1  x2

0   m   1   3
0   m   1   7
1   f   1   5
1   f   1   6
2   f   2   9
2   f   2   15
3   m   3   10
3   m   3   15
4   m   10  12
4   m   10  20

Notice that for each unique id x1 are similar, but x2 are different. I need to sort data by id and x2 (from smallest to largest) and then for each unique id I need to set x1(for the second record) = x2 (for the first record).

The data would look like:

id  gender  x1  x2

0   m   1   3
0   m   3   7
1   f   1   5
1   f   5   6
2   f   2   9
2   f   9   15
3   m   3   10
3   m   10  15
4   m   10  12
4   m   12  20

Answer 1

I found this easier using data.table

> library(data.table)
> dt = data.table(alldata)
> setkey(dt, id, x2)    #sort the data

This next line says: within each ID for x1, take the first value of x1, then every remaining value take from x2 as needed.

> dt[,x1 := c(x1[1], x2)[1:.N],keyby=id] 
> dt
    id gender x1 x2
 1:  0      m  1  3
 2:  0      m  3  7
 3:  1      f  1  5
 4:  1      f  5  6
 5:  2      f  2  9
 6:  2      f  9 15
 7:  3      m  3 10
 8:  3      m 10 15
 9:  4      m 10 12
10:  4      m 12 20