根据列 x 的值对数据框进行子集化。 只想要R中的前两位

Question

here is the data frame

p <- c(1, 3, 45, 1, 1, 54, 6, 6, 2)
x <- c("a", "b", "a", "a", "b", "c", "a", "b", "b")
df <- data.frame(p, x)

I want to subset the data frame such that I get a new data frame with only the top two"x" based on the count of "x".

Answer 1

One of the simplest ways to achieve what you want to do is with the package data.table . You can read more about it here . Basically, it allows for fast and easy aggregation of your data.

Please note that I modified your initial data by appending the elements 10 and c to p and x , respectively. This way, you won't see a NA when filtering the top two observations.

The idea is to sort your dataset and then operate the function .SD which is a convenient way for subsetting/filtering/extracting observations.

Please, see the code below.

library(data.table)

p <- c(1, 3, 45, 1, 1, 54, 6, 6, 2, 10)  
x <- c("a", "b", "a", "a", "b", "c", "a", "b", "b", "c") 
df <- data.table(p, x)

# Sort by the group x and then by p in descending order
setorder( df, x, -p )

# Extract the first two rows by group "x"
top_two <- df[ , .SD[ 1:2 ], by = x ]
top_two
#>    x  p
#> 1: a 45
#> 2: a  6
#> 3: b  6
#> 4: b  3
#> 5: c 54
#> 6: c 10

^{Created on 2021-02-16 by the reprex package (v1.0.0)}

Does this work for you?

Answer 2

Using dplyr:

library(dplyr)
df %>% 
  add_count(x) %>% 
  slice_max(n, n = 2)

   p x n
1  1 a 4
2  3 b 4
3 45 a 4
4  1 a 4
5  1 b 4
6  6 a 4
7  6 b 4
8  2 b 4