Row index in a dataframe

General Usage
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#1

Hello,

I have a dataframe that i sorted in order to get the maximum (of values in a column).
Now, i want to get the index corresponding to this maximum in the initial dataframe.
I’ve tried a lot of things with rownumber or other but nothing works.
Thanks in advance for your help.

#2

There is no row index in DataFrames.

Here’s something you can do though

julia> df = DataFrame(x = rand(10),y = rand(10))
10×2 DataFrame
 Row │ x         y        
     │ Float64   Float64  
─────┼────────────────────
   1 │ 0.341501  0.914511
   2 │ 0.717087  0.28765
   3 │ 0.403885  0.173076
   4 │ 0.931974  0.269083
   5 │ 0.794214  0.795277
   6 │ 0.930986  0.2953
   7 │ 0.32153   0.315169
   8 │ 0.572526  0.641704
   9 │ 0.752558  0.738108
  10 │ 0.643852  0.992788

julia> rownum = findfirst(==(maximum(df.x)), df.x)
4
#3

Alternatively you can just add a column that will serve as a row index if you prefer.

#4

Just in case, the findmax() and sortperm() functions are quite useful too:

df = DataFrame(x = rand(10), y = rand(10))
findmax(df.x)  # outputs the maximum x-value and its row number
ix = sortperm(df.x, rev=true)
ix[1]          # row number with maximum x value
df.x[ix[1]]    # maximum value in column x
1 Like
#5

Thanks a lot !