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The `ave` function in R is one of those little helper function I feel I should be using more. Investigating its source code showed me another twist about R and the “[” function. But first let’s look at `ave`.

The top of `ave`‘s help page reads:

Group Averages Over Level Combinations of Factors

Subsets of x[] are averaged, where each subset consist of those observations with the same factor levels.

As an example I look at revenue data by product and shop.
```revenue <- c(30,20, 23, 17)
shop <- gl(2,2, labels=c("shop_1", "shop_2"))
```
To answer the question "Which shop sells proportionally more bread?" I need to divide the revenue vector by the sum of revenue per shop, which can be calculated easily by `ave`:
```(shop_revenue <- ave(revenue, shop, FUN=sum))
# [1] 50 50 40 40
(revenue_split_in_shop <- revenue/shop_revenue)
# [1] 0.600 0.400 0.575 0.425 # Shop 1 sells more bread than cake
```
In other words, `ave` has to split the revenue vector by shop and apply the `sum` function to it. Well that's exactly what it does. Here is the source code of `ave`:
```#  Copyright (C) 1995-2012 The R Core Team
ave <- function (x, ..., FUN = mean)
{
if(missing(...))
x[] <- FUN(x)
else {
g <- interaction(...)
split(x,g) <- lapply(split(x, g), FUN)
}
x
}```
However, and this is what intrigued me, if I don't provide a grouping variable (`missing(...)`) it will apply the function `FUN` on `x` itself and write its output to `x[]`. That's actually what the help file to `ave` mentioned in its description. So what does it do? Here is an example again:
```ave(revenue, FUN=sum)
# [1] 90 90 90 90```
I get the sum of revenue repeated as many time as the vector has elements, not just once, as with `sum(revenue)`. The trick is that the output of `FUN(x)` is written into `x[]`, which of course is output of a function call itself "["(x).

I think it is the following sentence in the help file of `"["` (see ?"["), which explains it: Subsetting (except by an empty index) will drop all attributes except names, dim and dimnames.

So there we are. I feel less inclined to use `ave` more, as it is just short for the usual `split, lapply` routine, but I learned something new about the subtleties of R.