创建一个新的 Dataframe ，然后将其堆叠在原始 Dataframe 下。如果要手工编码（因为似乎没有可编程模式），我喜欢tribble()函数用于按行创建表：

library(tibble)
# ID=B2, number= B1's number
# ID=C7, number= C4's number
# ID=A3, number= (A1's number+A2's number)/2
# ID=C9, number= A2's number
rbind(df, 
  tribble(
    ~ID, ~number,
    "B2", df[df$ID == "B1", "number"],
    "C7", df[df$ID == "C4", "number"],
    "A3", (df[df$ID == "A1", "number"] + df[df$ID == "A2", "number"]) / 2,
    "C9", df[df$ID == "A2", "number"],
  )
)
#   ID number
# 1 A1    5.0
# 2 A2   10.0
# 3 B1    7.0
# 4 C4    3.0
# 5 D8    2.0
# 6 B2    7.0
# 7 C7    3.0
# 8 A3    7.5
# 9 C9   10.0

您可以使用with()和上面的方法来保存大量的输入：

with(df, rbind(df, 
  tribble(
    ~ID, ~number,
    "B2", number[ID == "B1"],
    "C7", number[ID == "C4"],
    "A3", (number[ID == "A1"] + number[ID == "A2"]) / 2,
    "C9", number[ID == "A2"],
  )
))

我们可以使用bind_rows并在创建这些行的 Dataframe 后绑定df和所需的行：

library(dplyr)

bind_rows(df, data.frame(
  ID = c("B2", "C7", "A3", "C9"),
  number = c(df$number[3], df$number[4], (df$number[1] + df$number[2])/2, df$number[2])
))

ID number
1 A1    5.0
2 A2   10.0
3 B1    7.0
4 C4    3.0
5 D8    2.0
6 B2    7.0
7 C7    3.0
8 A3    7.5
9 C9   10.0

另一种选择是使用add_row()：

df %>%
  add_row(ID = "B2", number = df$number[3]) %>%
  add_row(ID = "C7", number = df$number[4]) %>%
  add_row(ID = "A3", number = (df$number[1] + df$number[2])/2) %>%
  add_row(ID = "C9", number = df$number[2])

 ID number
1 A1    5.0
2 A2   10.0
3 B1    7.0
4 C4    3.0
5 D8    2.0
6 B2    7.0
7 C7    3.0
8 A3    7.5
9 C9   10.0