Inconsistent results when using split-apply-combine for DataFrames

New to Julia
#1

I’m looking to add a column to my DataFrame combo2 that represents the ranking of DK_points by each Pos (position) group with the following code.

sort!(combo2, [:Pos, :DK_points], rev=[true, true])
dk_rank = []

for pos in groupby(combo2, :Pos)
    append!(dk_rank, 1:size(pos, 1))
end
insertcols!(combo2, 6, DK_rank=dk_rank)

However, one particular value for Pos, “QB”, gives unusual results with the ranking starting at 14 rather than the desired 1.

by(combo2, :Pos, y->DataFrame(minRank = minimum(y[:,:DK_rank])))

	Pos	minRank
String	Int64
1	QB	14
2	RB	1
3	WR	1
4	TE	1
5	DST	1

Very confused why this would happen.

Also, wondering if there are better ways to accomplish adding a ranking for multiple columns in a data set by each group of another particular column.

#2

First, how you can do it more easily is:

by(combo2, :Pos, y -> (DK_rank=axes(y, 1)))

if your data frame is sorted
or

by(combo2, :Pos, y -> (DK_rank=sortperm(y, :DK_points, rev=true)))

if it is not sorted yet.

Now the problem you encounter is most likely related to sorting (but I am not sure). Can you please give the result of the following operation (or share your source data frame):

by(combo2, :Pos, y->DataFrame(len=nrow(y), rankrange = extrema(y[:,:DK_rank], isok=issorted(y[:,:DK_rank]))))
#3

Additionally - I have just checked your code on some random data and I could not reproduce the problem.

#4 
	Pos	len	rankrange	isok
String	Int64	Tuple…	Bool
1	QB	39	(14, 52)	true
2	RB	53	(1, 56)	false
3	WR	56	(1, 39)	false
4	TE	52	(1, 53)	false
5	DST	33	(1, 33)	true

Appears that the append order is out of whack. But not really sure why.

#5

can you send me the data privately if you cannot share them openly? (as I have said - I have checked your codes on random data and they are OK). Also please confirm if the two methods I suggested work correctly on your data.

#6

If I check for “sortedness” just on DK_points, seems sorting indeed didn’t work correctly.

sort!(combo2, [:Pos, :DK_points], rev=[true, true])
by(combo2, :Pos, y->DataFrame(len=nrow(y), isok=issorted(y[:,:DK_points])))

Pos	len	isok
String	Int64	Bool
1	QB	39	false
2	RB	53	false
3	WR	56	false
4	TE	52	false
5	DST	33	false
#7

With both of the methods you provided, if I insert the rank column back to the dataframe using

m = by(combo2, :Pos, y -> (DK_rank=sortperm(y, :DK_points, rev=true)))[:, :x1]
insertcols!(combo2, 6, DK_rank=m)

I get the same result as before

#8

Please make sure that your combo2 data frame is sorted. In your post :Pos column does not seem to be sorted in reverse order. The order should be like:

5×2 DataFrame
│ Row │ Pos    │ minRank │
│     │ String │ Int64   │
├─────┼────────┼─────────┤
│ 1   │ WR     │ 1       │
│ 2   │ TE     │ 1       │
│ 3   │ RB     │ 1       │
│ 4   │ QB     │ 1       │
│ 5   │ DST    │ 1       │