Update pandas dataframe based on slice?

I have seen Update pandas dataframe based on slice – but I couldn’t quite find an answer for my use case.

Consider this code, where I have a starting table with "channel" and "value" columns:

import sys
if sys.version_info[0] < 3:
    from StringIO import StringIO
else:
    from io import StringIO

import pandas as pd

TESTDATA = StringIO("""channel,value
A,10
A,11
A,12
A,13
B,20
B,22
B,24
B,26
B,28
C,100
C,105
C,110
C,115
C,120
C,125
C,130
""")

mychans = ["A", "B", "C"]

df = pd.read_csv(TESTDATA)
df.insert (2, "value_rel", df["value"] - df["value"][0])

print("Starting:")
print(df.head())

for tchan in mychans:
  this_ch_data = df[df["channel"]==tchan]
  df.loc[this_ch_data.index, "value_rel"] = this_ch_data["value"] - this_ch_data["value"][0]

In the end, I want to obtain the same table with an additional "value_rel" column, which would show the values relative to the first value in that channel (slice); that is:

A, 10, 0
A, 11, 1
A, 12, 2
A, 13, 3
B, 20, 0
B, 22, 2
B, 24, 4
B, 26, 6
B, 28, 8
C,100, 0
C,105, 5
...

And if I just use this_ch_data["value_rel"] = this_ch_data["value"] - this_ch_data["value"][0] within the for loop, I get "A value is trying to be set on a copy of a slice from a DataFrame", which makes sense.

However, when the run the code, I get:

$ python3 test1.py
Starting:
  channel  value  value_rel
0       A     10          0
1       A     11          1
2       A     12          2
3       A     13          3
4       B     20         10
Traceback (most recent call last):
  File "C:/msys64/mingw64/lib/python3.9/site-packages/pandas/core/indexes/base.py", line 3361, in get_loc
    return self._engine.get_loc(casted_key)
  File "pandas/_libs/index.pyx", line 76, in pandas._libs.index.IndexEngine.get_loc
  File "pandas/_libs/index.pyx", line 108, in pandas._libs.index.IndexEngine.get_loc
  File "pandas/_libs/hashtable_class_helper.pxi", line 2131, in pandas._libs.hashtable.Int64HashTable.get_item
  File "pandas/_libs/hashtable_class_helper.pxi", line 2140, in pandas._libs.hashtable.Int64HashTable.get_item
KeyError: 0

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "C:\msys64\tmp\test1.py", line 38, in <module>
    df.loc[this_ch_data.index, "value_rel"] = this_ch_data["value"] - this_ch_data["value"][0]
  File "C:/msys64/mingw64/lib/python3.9/site-packages/pandas/core/series.py", line 942, in __getitem__
    return self._get_value(key)
  File "C:/msys64/mingw64/lib/python3.9/site-packages/pandas/core/series.py", line 1051, in _get_value
    loc = self.index.get_loc(label)
  File "C:/msys64/mingw64/lib/python3.9/site-packages/pandas/core/indexes/base.py", line 3363, in get_loc
    raise KeyError(key) from err
KeyError: 0

So, how can I update this DataFrame, based on calculations done on a (copied) slice of the same DataFrame?

>Solution :

You need to use iloc, because the index number 0 does not exist for all the tchan.

for tchan in mychans:
  this_ch_data = df[df["channel"]==tchan]
  df.loc[this_ch_data.index, "value_rel"] = \
         this_ch_data["value"] - this_ch_data["value"].iloc[0]

that said, it is good use case for groupby.transform with first. so no loop required, you can do

df['value_rel'] = df['value'] - df.groupby('channel')['value'].transform('first')