R: Differences by group and adding

Question

I would like to know how to do this operation simpler.
Imagine I have a data.frame like this one:

set.seed(1)
ID <- rep(1:3,each=4)
XX <- round(run         


        

Answer 1

I think you will need to use lapply() across the relevant columns, as ave() will not take a list in its first argument. Try this:

df[-1] <- lapply(
    df[-1], 
    function(x) ave(x, df$ID, FUN = function(x) c(x[1], diff(x)))
)

which gives the updated df

   ID     XX    ZZ
1   1  0.266 0.266
2   1  0.106 0.744
3   1  0.201 1.719
4   1  0.335 3.632
5   2  0.202 0.202
6   2  0.696 1.796
7   2  0.047 2.835
8   2 -0.284 2.644
9   3  0.629 0.629
10  3 -0.567 0.124
11  3  0.144 0.618
12  3 -0.029 0.708

Data:

df <- structure(list(ID = c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 3L, 3L, 
3L, 3L), XX = c(0.266, 0.372, 0.573, 0.908, 0.202, 0.898, 0.945, 
0.661, 0.629, 0.062, 0.206, 0.177), ZZ = c(0.266, 1.01, 2.729, 
6.361, 0.202, 1.998, 4.833, 7.477, 0.629, 0.753, 1.371, 2.079
)), .Names = c("ID", "XX", "ZZ"), class = "data.frame", row.names = c(NA, 
-12L))