mathematical.coffee - 9 days ago 6

R Question

Suppose I have a data.table:

`x <- data.table(x=runif(3), group=factor(c('a','b','a'), levels=c('a','b','c')))`

I want to know how many rows in

`x`

`group`

`x[, .N, by="group"]`

# group N

# 1: a 2

# 2: b 1

`by="group"`

`group`

Notice how since I don't have any rows of with

`group`

Desired output:

`x[, .N, by="group", ???] # somehow use all levels in `group``

# group N

# 1: a 2

# 2: b 1

# 3: c 0

Answer

If you are willing to run through the factor levels by enumerating them in `i`

(rather than by setting `by="group"`

), this will get you the hoped for results.

```
setkey(x, "group")
x[levels(group), .N, by=.EACHI]
# group N
# 1: a 2
# 2: b 1
# 3: c 0
```