I have a data frame, and I'd like to create a new column that gives the sum of a numeric variable grouped by factors. So something like this:
BEFORE:
data1 <- data.frame(month = c(1, 1, 2, 2, 3, 3),
sex = c("m", "f", "m", "f", "m", "f"),
value = c(10, 20, 30, 40, 50, 60))
AFTER:
data2 <- data.frame(month = c(1, 1, 2, 2, 3, 3),
sex = c("m", "f", "m", "f", "m", "f"),
value = c(10, 20, 30, 40, 50, 60),
sum = c(30, 30, 70, 70, 110, 110))
In Stata you can do this with the egen command quite easily. I've tried the aggregate function, and the ddply function but they create entirely new data frames, and I just want to add a column to the existing one.
You are looking for ave
> data2 <- transform(data1, sum=ave(value, month, FUN=sum))
month sex value sum
1 1 m 10 30
2 1 f 20 30
3 2 m 30 70
4 2 f 40 70
5 3 m 50 110
6 3 f 60 110
data1$sum <- ave(data1$value, data1$month, FUN=sum) is useful if you don't want to use transform
Also data.table is helpful
library(data.table)
DT <- data.table(data1)
DT[, sum:=sum(value), by=month]
UPDATE
We can also use a tidyverse approach which is simple, yet elegant:
> library(tidyverse)
> data1 %>%
group_by(month) %>%
mutate(sum=sum(value))
# A tibble: 6 x 4
# Groups: month [3]
month sex value sum
<dbl> <fct> <dbl> <dbl>
1 1 m 10 30
2 1 f 20 30
3 2 m 30 70
4 2 f 40 70
5 3 m 50 110
6 3 f 60 110