lg929 lg929 - 2 months ago 18
R Question

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

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 diff(mtcars$qsec) a new column in the original mtcars data frame? I'm finding it difficult because there's one less row in diff(mtcars$qsec) than the rest of mtcars.

> head(mtcars,3)

mpg cyl disp hp drat wt qsec vs am gear carb
Mazda RX4 21.0 6 160.0 110 3.90 2.620 16.46 0 1 4 4
Mazda RX4 Wag 21.0 6 160.0 110 3.90 2.875 17.02 0 1 4 4
Datsun 710 22.8 4 108.0 93 3.85 2.320 18.61 1 1 4 1

Answer

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?

Comments