[solved] iteration + df.loc

[ju21878436312]

I'd like to iterate through the lines by a for loop.
And when the condition "df['col2'] == "second"is true, I'd like to do some calculations in this line in column df["col3"], e.g. df.loc[i, "col3"] = i.
Could someone give me a hint?

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import pandas as pd import numpy as np   d = {'col1': [95, 82, 71, 93, 91, 16, 78, 81, 90, 89], 'col2': ["first", "first", "first", "first", "second", "second", "second", "second", "second", "second"]}   df = pd.DataFrame(data=d) df     df["col3"]=""   for i in range(len(df["col1"])):     df.loc[(df['col2']  == "second"),"col3"]=i       df

what I get is:

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col1    col2    col3 95  first   82  first   71  first   93  first   91  second  9 16  second  9 78  second  9 81  second  9 90  second  9 89  second  9

what I want to get is:

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col1    col2    col3 95  first   82  first   71  first   93  first   91  second  5 16  second  6 78  second  7 81  second  8 90  second  9 89  second  10

[perfringo]

I'd like to iterate through the lines by a for loop.

Why? This is pandas and usually it's not good idea to iterate over rows or columns of dataframe. Instead use pandas and numpy built-in methods.

Its unclear what values must be in col3, based on code provided I assume that row index:

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import pandas as pd import numpy as np   data = {'col1': [95, 82, 71, 93, 91, 16, 78, 81, 90, 89],         'col2': ["first", "first", "first", "first", "second", "second", "second", "second", "second", "second"]}   df = pd.DataFrame(data)   df['col3'] = np.where(df['col2']=='second', df.index, "")

Output:
   col1    col2 col3
0    95   first
1    82   first
2    71   first
3    93   first
4    91  second    4
5    16  second    5
6    78  second    6
7    81  second    7
8    90  second    8
9    89  second    9

[ju21878436312]

Thank you, I'll use your method!