使用频率在R中提取变量

Question

Suppose I have a dataframe:

 x  y
 a  1
 b  2
 a  3
 a  4
 b  5
 c  6
 a  7
 d  8
 a  9
 b 10
 e 12
 b 13
 c 15

I want to create another dataframe that includes only the x values that occur at least 3 times ( a and b , in this case), and their highest corresponding y values.

So I want the output as:

x   y
a   9
b   13

Here 9 and 13 are the highest values of a and b respectively

I tried using:

sort-(table(x,y)) 

but it did not work.

Answer 1

The data.table package is great for this. If df is the original data, you can do

library(data.table)
setDT(df)[, .(y = max(y)[.N >= 3]), by=x]
#    x  y
# 1: a  9
# 2: b 13

.N is an integer that tells us how many rows are in each group (which we've set to x here). So we just subset max(y) such that .N is at least three.

Answer 2

Here's one way, using subset to omit any x that occur less than 3 times, and then aggregate to find the maximum value by group:

d <- read.table(text='x y
a 1
b 2
a 3
a 4
b 5
c 6
a 7
d 8
a 9
b 10
e 12
b 13
c 15', header=TRUE)


with(subset(d, x %in% names(which(table(d$x) >= 3))),
     aggregate(list(y=y), list(x=x), max))

#   x  y
# 1 a  9
# 2 b 13

And for good measure, a dplyr approach:

library(dplyr)
d %>% 
  group_by(x) %>% 
  filter(n() >= 3) %>% 
  summarise(max(y))


# Source: local data frame [2 x 2]
# 
#    x max(y)
# 1 a      9
# 2 b     13