What is the equivalent of the SumIf function in R

Question

I am new to R and this site, but I searched and didn't find the answer I was looking for.

If I have the following data set "total":

names <- c("a", "b", "c", "d", "a", "b", "c", "d")
x <- cbind(x1 = 3, x2 = c(3:10))
total <- data.frame(names, x)
total

  names x1 x2
1     a  3  3
2     b  3  4
3     c  3  5
4     d  3  6
5     a  3  7
6     b  3  8
7     c  3  9
8     d  3  10

How can I create a new data set that works like the SumIf Excel function with just unique rows?

The answer should be a new data set "summary" that is 4 x 3.

names <- unique(names)
summary <- data.frame(names)
summary$Sumx1 <- ?????
summary$Sumx2 <- ?????
summary

 names Sumx1 Sumx2
1  a     6    10
2  b     6    12
3  c     6    14
4  d     6    16

Answer 1

In base R:

aggregate(. ~ names, data=total, sum)

Answer 2

You can use ddply from the plyr package:

library(plyr)
ddply(total, .(names), summarise, Sumx1 = sum(x1), Sumx2 = sum(x2))

  names Sumx1 Sumx2
1     a     6    10
2     b     6    12
3     c     6    14
4     d     6    16

You can also use data.table:

library(data.table)
DT <- as.data.table(total)
DT[ , lapply(.SD, sum), by = "names"]

   names x1 x2
1:     a  6 10
2:     b  6 12
3:     c  6 14
4:     d  6 16

With the new dplyr package, you can do:

library(dplyr)
total %>%
  group_by(names) %>%
  summarise(Sumx1 = sum(x1), Sumx2 = sum(x2))

  names Sumx1 Sumx2
1     d     6    16
2     c     6    14
3     b     6    12
4     a     6    10