Adding a numerical index to a categorical index in Pandas

Question

Say for example that we have the data set that looks like

client	client case	client payment
foo	1	200
foo	1	250
foo	1	225
foo	2	200
bar	1	500
bar	1	500

Within Pandas, I can set client and client case to be indexes, but I would like to number these indexes within groups to have an additional column that becomes something like below.

client	client case	client payment	payment number
foo	1	200	1
foo	1	250	2
foo	1	225	3
foo	2	200	1
bar	1	500	1
bar	1	500	2

Essentially just creating some kind of payment numbering system within each client case.

I could theoretically do this by creating some kind of list by iterating over the rows but I know that this isn’t the optimal or suggested way of using Pandas.

Asked By: Ryan Honea

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Source

Answer 1

this would do

df['payment number'] = 1 + df.groupby(['client', 'client case']).cumcount()

output

  client  client case  client payment  payment number
0    foo            1             200               1
1    foo            1             250               2
2    foo            1             225               3
3    foo            2             200               1
4    bar            1             500               1
5    bar            1             500               2

Answered By: Yuca

Adding a numerical index to a categorical index in Pandas

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