Adding new column with diff() function when there is one less row in R

Question

If I have a sample data frame like mtcars, and I want to find the difference between mtcars$qsec for all rows, I can do diff(mtcars$qsec). But is there a simple way to make

Answer 1

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?