如何根据r中的多个列和行值添加计算行？

Question

If I have a dataframe below and I want to add another row for each group in r:

dataframe before:

Letter   Value
A         1
A         2
A         3
B         1
B         2
B         3

If I just wanted to add another row for each group that averaged those rows, how would I go about that:

dataframe after:

Letter   Value
A         1
A         2
A         3
A         2
B         1
B         2
B         3
B         2

Answer 1

A base R solution could be

do.call(rbind, lapply(split(df, df$Letter),
    function(x) rbind(x, c(unique(x$Letter), mean(x$Value)))))
#    Letter Value
#A.1       A     1
#A.2       A     2
#A.3       A     3
#A.4       A     2
#B.4       B     1
#B.5       B     2
#B.6       B     3
#B.41      B     2

This requires column Letter to be a character vector. If Letter is a factor you can convert from factor to character vector

df$Letter <- as.character(df$Letter)

---

Sample data

df <- read.table(text =
    "Letter   Value
A         1
A         2
A         3
B         1
B         2
B         3", header = T, stringsAsFactors = FALSE)

Answer 2

Calculate the mean of value by letter giving ag , rbind that to the end of DF giving DF2 and sort.

ag <- aggregate(value ~ letter, DF, mean)
DF2 <- rbind(DF, ag)

o <- order(DF2$letter, rownames(DF2))
DF2[o, ]

giving:

  letter value
1      A     1
2      A     2
3      A     3
7      A     2
4      B     1
5      B     2
6      B     3
8      B     2

Note

The input DF in reproducible form is:

DF <- data.frame(letter = rep(c("A", "B"), each = 3), value = 1:3)