collapse, by

[gergelyattilakiss]

closed in https://github.com/codedthinking/Kezdi.jl/commit/f82de013c99d3e6a8082338c8cdbdeb2775387d0

[korenmiklos]

Super cool!

julia> df = DataFrame(a = 1:10, g = [:a, :b, :a, :b, :a, :b, :a, :b, :a, :b])
10×2 DataFrame
 Row │ a      g
     │ Int64  Symbol
─────┼───────────────
   1 │     1  a
   2 │     2  b
   3 │     3  a
   4 │     4  b
   5 │     5  a
   6 │     6  b
   7 │     7  a
   8 │     8  b
   9 │     9  a
  10 │    10  b

julia> @collapse df m_a = mean(a), by(g)
2×2 DataFrame
 Row │ g       m_a
     │ Symbol  Float64
─────┼─────────────────
   1 │ a           5.0
   2 │ b           6.0