Suppose I have the following dataframe
using DataFrames
df = DataFrame(A = 1:10, B = ["a","a","b","b","b","c","c","c","c","d"])
grouped_df = groupby(df, "B")
I would have four groups. How can I drop the groups that have fewer than, say, 2 rows? For example, how can I keep only groups a,b, and c? I can easily do it with a for loop, but I don't think the optimal way.
If you want the result to be still grouped then filter is simplest:
julia> filter(x -> nrow(x) > 1, grouped_df)
GroupedDataFrame with 3 groups based on key: B
First Group (2 rows): B = "a"
Row │ A B
│ Int64 String
─────┼───────────────
1 │ 1 a
2 │ 2 a
⋮
Last Group (4 rows): B = "c"
Row │ A B
│ Int64 String
─────┼───────────────
1 │ 6 c
2 │ 7 c
3 │ 8 c
4 │ 9 c
If you want to get a data frame as a result of one operation then do e.g.:
julia> combine(grouped_df, x -> nrow(x) < 2 ? DataFrame() : x)
9×2 DataFrame
Row │ B A
│ String Int64
─────┼───────────────
1 │ a 1
2 │ a 2
3 │ b 3
4 │ b 4
5 │ b 5
6 │ c 6
7 │ c 7
8 │ c 8
9 │ c 9