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Efficient way to apply a grouping vector in R

Suppose I have some grouping information in the form of a grouping vector:

group = c(1,1,2,2,3,3,3)

So this is saying I have three groups: two groups of size 2 and one group of size 3. Now suppose I have a vector (I've added random numbers)

x = c(1.5, 3.1, 5.4, -4.5, 2.2, 4.4, 1.1)

Is there an efficient way in R to loop over this vector and applying certain functions within the group?

For example, summation within each group, using a for loop would be:

sums = rep(0,3)
for (i in 1:3){
grp_ids = which(group == i)
sums[i] = sum(x[grp_ids])
}

Is there an easier way to do this?

Solution

  • You can use group_by from {dplyr}:

    library(dplyr)

group = c(1, 1, 2, 2, 3, 3, 3)
x = c(1.5, 3.1, 5.4, -4.5, 2.2, 4.4, 1.1)

df <- data.frame(group, x)

result <- df %>%
  group_by(group) %>%
  summarize(sums = sum(x))

> print(result)
# A tibble: 3 × 2
  group  sums
  <dbl> <dbl>
1     1   4.6
2     2   0.9
3     3   7.7