MethodError: no method matching groupby(::DataFrame, ::Vector{Symbol}, ::Pair{typeof(nrow), Symbol})-CodePudding

I am trying to use the code of this question: DataFrames.jl : count rows by group while defining count column name where I would like to count the number of rows per group. Here is some reproducible code:

using DataFrames

df = DataFrame(group = ["A", "A", "A", "A", "A", "A", "B", "B", "B", "B", "B", "B"],
               subgroup = ["X", "X", "X", "Y", "Y", "Y", "X", "X", "Y", "Y", "Y", "Y"])

12×2 DataFrame
 Row │ group   subgroup 
     │ String  String   
─────┼──────────────────
   1 │ A       X
   2 │ A       X
   3 │ A       X
   4 │ A       Y
   5 │ A       Y
   6 │ A       Y
   7 │ B       X
   8 │ B       X
   9 │ B       Y
  10 │ B       Y
  11 │ B       Y
  12 │ B       Y

When running the following code:

combine(groupby(df, [:group, :subgroup], nrow => :n))

Returns this error:

MethodError: no method matching groupby(::DataFrame, ::Vector{Symbol}, ::Pair{typeof(nrow), Symbol})
Closest candidates are:
  groupby(::AbstractDataFrame, ::Any; sort, skipmissing) at ~/.julia/packages/DataFrames/JZ7x5/src/groupeddataframe/groupeddataframe.jl:211

Stacktrace:
 [1] top-level scope
   @ In[51]:1
 [2] eval
   @ ./boot.jl:368 [inlined]
 [3] include_string(mapexpr::typeof(REPL.softscope), mod::Module, code::String, filename::String)
   @ Base ./loading.jl:1428

I am not sure why this error happens while running the same code. So I was wondering if anyone could explain why this happens and if this is the right way to count the number of rows per group in Julia?

CodePudding user response：

You've just got the brackets wrong:

combine(groupby(df, [:group, :subgroup]), nrow => :n)

Note the first closing parens before nrow - this is the call to groupby, and it is the result of this which you pass as the first argument to combine

CodePudding user response：

Besides the Nil's answer, you can also use the following syntax:

combine(groupby(df, [:group, :subgroup]), :group => (length) => :nrow)
# 4×3 DataFrame
#  Row │ group   subgroup  nrow
#      │ String  String    Int64
# ─────┼─────────────────────────
#    1 │ A       X             3
#    2 │ A       Y             3
#    3 │ B       X             2
#    4 │ B       Y             4

In the above, I'm counting nrows per :group and the subsequent :subgroup. The first row of the result tells me the number of samples (rows) that have the A group label along with the X subgroup label.

*If you think this can't be implied as a different syntax, please tell me; I'll delete this answer.