R ddp从下一行求和

Question

I want to sum the column value from a row with the next one.

> df

+----+------+--------+------+
| id |  Val | Factor | Col  |
+----+------+--------+------+
|  1 |   15 |      1 |    7 |
|  3 |   20 |      1 |    4 |
|  2 |   35 |      2 |    8 | 
|  7 |   35 |      1 |   12 |
|  5 |   40 |      1 |   11 |
|  6 |   45 |      2 |   13 |
|  4 |   55 |      1 |    4 |
|  8 |   60 |      1 |    7 |
|  9 |   15 |      2 |   12 |
..........

I would like to have the mean of sum of the Row$Val + nextRow$Val based on their id and Col . I can't assume that the id or Col are consecutive.

I am using ddply to summarize my df. I have tried

> ddply(df, .(Factor), summarize, 
       max(Val), 
       sum(Val), 
       mean(Val + df[df$id == id+1 & df$Col = Col]$Val)
       )

> "longer object length is not a multiple of shorter object length"

Answer 1

You can use rollapply from the zoo package. Since you want mean of only two consecutive rows , you can try

library(zoo)
rollapply(df[order(df$id), 2], 2, function(x) sum(x)/2)

#[1] 17.5 27.5 35.0 37.5 42.5 50.0 57.5 37.5

Answer 2

You can build a vector of values with

sapply(df$id, function(x){mean(c(
    subset(df, id == x, select = Val, drop = TRUE), 
    subset(df, id == x+1, select = Val, drop = TRUE)
    ))})

You could simplify, but I tried to make it as readable as possible.

Answer 3

You can do something like this with dplyr package:

library(dplyr)
df <- arrange(df, id)
mean(df$Val + lead(df$Val), na.rm = TRUE)
[1] 76.25