data.table: group-by, sum, name new column, slice columns 一步到位

Question

这看起来应该很容易，但我一直无法弄清楚如何去做。使用 data.table 我想将一列 C 与另一列 A 相加，然后只保留这两列。同时，我希望能够为新列命名。我的尝试和期望的输出：

library(data.table)
dt <- data.table(A= c('a', 'b', 'b', 'c', 'c'), B=c('19', '20', '21', '22', '23'),
C=c(150,250,20,220,130))

# Desired Output - is there a way to do this in one step using data.table? #
new.data <- dt[, sum(C), by=A]
setnames(new.data,'V1', 'C.total')
new.data
   A C.total
1: a     150
2: b     270
3: c     350

# Attempt 1: Problem is that columns B and C kept, extra rows kept #
new.data <- dt[, 'C.total' := sum(C), by=A]
new.data
   A  B   C C.total
1: a 19 150     150
2: b 20 250     270
3: b 21  20     270
4: c 22 220     350
5: c 23 130     350

# Attempt 2: Problem is that new column not named #
new.data <- dt[, sum(C), by=A]
new.data
   A  V1
1: a 150
2: b 270
3: c 350

Answer 1

使用list（或.）：

> dt[, list(C.total = sum(C)), by=A]
   A C.total
1: a     150
2: b     270
3: c     350

data.table: group-by, sum, name new column, slice columns 一步到位

data.table: group-by, sum, name new column, and slice columns in one step

group-by

r

rename

data.table