Here are two approaches. Both put an NA
in the first row of diff_qsec
and put diff(qsec)
in the remaining rows:
library(dplyr)
mtcars %>% mutate(diff_qsec = qsec - lag(qsec)) # dplyr has its own version of lag
transform(mtcars, diff_qsec = c(NA, diff(qsec)))
Also, on the general issue of padding see: How can I pad a vector with NA from the front?