R中的行少了时，用diff（）函数添加新列 [英] Adding new column with diff() function when there is one less row in R

问题描述

如果我有一个像mtcars这样的示例数据框，并且想找到所有行的mtcars $ qsec之间的区别，我可以做diff（mtcars $ qsec）。但是，有没有一种简单的方法可以使diff（mtcars $ qsec）在原始mtcars数据帧中成为新列？我发现这很困难，因为diff（mtcars $ qsec）中的行比其余mtcar少。

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

推荐答案

这里有两种方法。两者都在 diff_qsec 的第一行中放入 NA 并放入 diff（qsec）：

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)))

此外，关于填充的一般问题，请参见：如何从正面填充带有NA的向量？

Also, on the general issue of padding see: How can I pad a vector with NA from the front?

这篇关于R中的行少了时，用diff（）函数添加新列的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！