R函数返回分组变量的已更改数据

Question

I have a simple example to explain my problem: lets say I use the mtcars dataframe. And I would like to alter the data of mtcars$qsec by using the following function:

TT.test <- function(x)
{
  x<- ifelse(x > 20,x,NA)
  return(x)
}

The catch is I want to alter the mtcars$qsec using this function by grouping on mtcars$gear and mtcars$cyl . So I would like to get a dataframe or matrix that gives me a dataframe back with the altered data. It is fine if the dataframe only contains the three columns considered.

Do you have any suggestions on how to deal with this problem?

Answer 1

If you want to mutate just that column, and group by some others, here is how you can do it with dplyr:

library(dplyr)
mtcars %>% group_by(gear, cyl) %>% mutate(qsec = TT.test(qsec))

Here is the resulting output:

Source: local data frame [32 x 11]
Groups: gear, cyl [8]

     mpg   cyl  disp    hp  drat    wt  qsec    vs    am  gear  carb
   (dbl) (dbl) (dbl) (dbl) (dbl) (dbl) (dbl) (dbl) (dbl) (dbl) (dbl)
1   21.0     6 160.0   110  3.90 2.620    NA     0     1     4     4
2   21.0     6 160.0   110  3.90 2.875    NA     0     1     4     4
3   22.8     4 108.0    93  3.85 2.320    NA     1     1     4     1
4   21.4     6 258.0   110  3.08 3.215    NA     1     0     3     1
5   18.7     8 360.0   175  3.15 3.440    NA     0     0     3     2
6   18.1     6 225.0   105  2.76 3.460 20.22     1     0     3     1
7   14.3     8 360.0   245  3.21 3.570    NA     0     0     3     4
8   24.4     4 146.7    62  3.69 3.190    NA     1     0     4     2
9   22.8     4 140.8    95  3.92 3.150 22.90     1     0     4     2
10  19.2     6 167.6   123  3.92 3.440    NA     1     0     4     4
..   ...   ...   ...   ...   ...   ...   ...   ...   ...   ...   ...

If you want to mutate all columns, you can do something like this:

mtcars %>% group_by(gear, cyl) %>% mutate_each(funs(TT.test))

OR if you want to mutate some select columns (multiple), you can do something like this:

mtcars %>% group_by(gear, cyl) %>% mutate_each(funs(TT.test), c(qsec, hp))