Subtract from the previous row R

Question

I have a dataframe like so:

df <- data.frame(start=c(5,4,2),end=c(2,6,3))

start end
    5   2
    4   6
    2   3

And I want the following result:

start end diff
    5   2 
    4   6    1
    2   3   -1

Essentially it is:

end[2] (second row) - start[1] = 6-5=1

and end[3] - start[2] = 3-4 = -1

What is a good way of doing this in R?

Or `library(dplyr);mutate(df, diff= end-lag(start))` – akrun May 26 '15 at 04:11 — akrun, May 26 '15 at 04:11

score 13 · Accepted Answer · edited Mar 04 '17 at 11:16

13

Just a simple vector subtraction should work

df$diff <- c(NA,df[2:nrow(df), 2] - df[1:(nrow(df)-1), 1])

  start end diff
1     5   2   NA
2     4   6    1
3     2   3   -1

edited Mar 04 '17 at 11:16

Bhargav Rao

answered May 26 '15 at 03:00

Missing closing parenthesis at end of df$diff statement. Statement should read: df$diff <- c(NA,df[2:nrow(df), 2] - df[1:(nrow(df)-1), 1]) – makeyourownmaker Mar 03 '17 at 10:27

score 2 · Answer 2 · answered May 26 '15 at 03:24

2

library(data.table)

setDT(df)[,value:=end-shift(start,1,type="lag")]
   start end value
1:     5   2    NA
2:     4   6     1
3:     2   3    -1

answered May 26 '15 at 03:24

user227710

2 Answers2