如何按组计算计数，然后每组只保留一个

Question

data <- data.frame(val=c(rep(6,10), rep(7, 15), rep(8, 20), rep(9, 25), rep(10, 100), rep(11, 20), rep(12, 15), rep(13, 10)))
data$plus <- data$val + 100

table <- table(data$val)
df1 <- data.frame(val = as.integer(names(table)[1:length(table)]), N = table[1:length(table)])
df2 <- merge(data, df1)
df3 <- do.call(rbind, by(df2, list(df2$val), FUN=function(x) head(x, 1)))

bys val plus: egen max = _N
bys val plus: gen first = _n==1
keep if first==1

有没有一种方法可以简化或使R代码更优雅？

Answer 1

下面是一个使用“data.table”的方法：

library(data.table)
as.data.table(data)[, N := .N, by = val][, .SD[1], by = val]
#    val plus   N
# 1:   6  106  10
# 2:   7  107  15
# 3:   8  108  20
# 4:   9  109  25
# 5:  10  110 100
# 6:  11  111  20
# 7:  12  112  15
# 8:  13  113  10

## Or (@RicardoSaporta)
as.data.table(data)[, list(.N, plus=plus[1]), by = val]

## Or (@DavidArenburg)
unique(as.data.table(data)[, N := .N, by = val], by = "val")

使用“dplyr”，您可以尝试：

library(dplyr)

data %>%
  group_by(val) %>%
  mutate(N = n()) %>%
  slice(1)

在底座R中，我想你可以尝试这样的方法：

do.call(rbind, lapply(split(data, data$val), 
                      function(x) cbind(x, N = nrow(x))[1, ]))