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Copy a column from one dataframe to another dataframe - Printable Version

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Copy a column from one dataframe to another dataframe - Led_Zeppelin - Jul-07-2022

I would like to copy a column from one dataframe and add it to a second dataframe as the last column. I know it can be done, but I am looking for a
simple way to do it. Is there a one, two or three code line way to do it? This is all using Panda and Python, of course.

Any help appreciated.

Thanks in advance.

Respectfully,

LZ

RE: Copy a column from one dataframe to another dataframe - deanhystad - Jul-07-2022

dataframe_a['New Column'] = dataframe_b['Existing Column']

RE: Copy a column from one dataframe to another dataframe - Led_Zeppelin - Jul-07-2022

Thanks, I will give it a try.

Respectfully,

LZ

RE: Copy a column from one dataframe to another dataframe - Led_Zeppelin - Jul-07-2022

I tried the simple command that you gave me:

df['machine_status'] = df1['machine_status']
Now df is the new dataframe and df1 is the old copied dataframe. I am trying to move the column named 'machine_status', from df1 to df and put it in the last column. The column is named 'machine_status' in both cases.

I got the following error:

Error:
KeyError                                  Traceback (most recent call last)
File ~\miniconda3\envs\pump-failure-pred\lib\site-packages\pandas\core\indexes\base.py:3621, in Index.get_loc(self, key, method, tolerance)
   3620 try:
-> 3621     return self._engine.get_loc(casted_key)
   3622 except KeyError as err:

File ~\miniconda3\envs\pump-failure-pred\lib\site-packages\pandas\_libs\index.pyx:136, in pandas._libs.index.IndexEngine.get_loc()

File ~\miniconda3\envs\pump-failure-pred\lib\site-packages\pandas\_libs\index.pyx:163, in pandas._libs.index.IndexEngine.get_loc()

File pandas\_libs\hashtable_class_helper.pxi:5198, in pandas._libs.hashtable.PyObjectHashTable.get_item()

File pandas\_libs\hashtable_class_helper.pxi:5206, in pandas._libs.hashtable.PyObjectHashTable.get_item()

KeyError: 'machine_status'

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

KeyError                                  Traceback (most recent call last)
Input In [25], in <cell line: 1>()
----> 1 df['machine_status'] = df1['machine_status']

File ~\miniconda3\envs\pump-failure-pred\lib\site-packages\pandas\core\frame.py:3505, in DataFrame.__getitem__(self, key)
   3503 if self.columns.nlevels > 1:
   3504     return self._getitem_multilevel(key)
-> 3505 indexer = self.columns.get_loc(key)
   3506 if is_integer(indexer):
   3507     indexer = [indexer]

File ~\miniconda3\envs\pump-failure-pred\lib\site-packages\pandas\core\indexes\base.py:3623, in Index.get_loc(self, key, method, tolerance)
   3621     return self._engine.get_loc(casted_key)
   3622 except KeyError as err:
-> 3623     raise KeyError(key) from err
   3624 except TypeError:
   3625     # If we have a listlike key, _check_indexing_error will raise
   3626     #  InvalidIndexError. Otherwise we fall through and re-raise
   3627     #  the TypeError.
   3628     self._check_indexing_error(key)

KeyError: 'machine_status'
Where is the error? I think that I entered the python code correctly. The python interpreter says that I did not.

What is a key error?

Thanks in advance.

Respectfully,

LZ
.

RE: Copy a column from one dataframe to another dataframe - deanhystad - Jul-07-2022

Is "machine_status" a key (column header) in df1?

This is how things work in theory
import pandas as pd

df1 = pd.DataFrame({"Letters": ["A", "B", "C", "D"]})
df2 = pd.DataFrame({"Numbers": [1, 2, 3, 4]})

df1["Integers"] = df2["Numbers"]  # df1 can be any name.  df2 has to be existing column
print(df1)
Output:
  Letters  Integers
0       A         1
1       B         2
2       C         3
3       D         4


RE: Copy a column from one dataframe to another dataframe - Led_Zeppelin - Jul-07-2022

Yes, it is. I took three columns off df dataframe, but before I did that, I made a copy of df dataframe and called it df1.

I am trying to put machine status back on to the slimmed down dataframe, df. I have done all of my scaling and normalizing on df, so now I am trying to put it back together. I want to reverse the dropping of three columns, but only for machine_statis column now.

That is where it fails. df is only a slimmed down column of its former self.

But "machine_statis" is the name of the column I have been discussing.

I hope this is informative.

Respectfully,

LZ

RE: Copy a column from one dataframe to another dataframe - Led_Zeppelin - Jul-07-2022

I like your example. But please understand, doing it that way would be very hard.

My machine_status column has over 220,000 values. It is not practical. I really want to use the short cut method, but I keep getting this "key_error".

That is why I chose the shorthand way.
I will keep trying.

R,

LZ

RE: Copy a column from one dataframe to another dataframe - deanhystad - Jul-07-2022

It doesn't make any difference how many "values" there are. The dataframe operations shown are working with series (columns). A column containint 20,000 values works exactly as one containing 4.

This part is just so I have two dataframes to work with.
df1 = pd.DataFrame({"Letters": ["A", "B", "C", "D"]})
df2 = pd.DataFrame({"Numbers": [1, 2, 3, 4]})
If you prefer they could each have 1,000,000 values.

I get an error when I do this:
import pandas as pd

df1 = pd.DataFrame({"Letters": ["A", "B", "C", "D"]})
df2 = pd.DataFrame({"Numbers": [1, 2, 3, 4, 5]})

df1["Integers"] = df2["Number"]
print(df1)
Error:
Traceback (most recent call last):
  File "...\lib\site-packages\pandas\core\indexes\base.py", line 3621, in get_loc
    return self._engine.get_loc(casted_key)
  File "pandas\_libs\index.pyx", line 136, in pandas._libs.index.IndexEngine.get_loc
  File "pandas\_libs\index.pyx", line 163, in pandas._libs.index.IndexEngine.get_loc
  File "pandas\_libs\hashtable_class_helper.pxi", line 5198, in pandas._libs.hashtable.PyObjectHashTable.get_item
  File "pandas\_libs\hashtable_class_helper.pxi", line 5206, in pandas._libs.hashtable.PyObjectHashTable.get_item
KeyError: 'Number'

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

Traceback (most recent call last):
  File ...test.py", line 6, in <module>
    df1["Integers"] = df2["Number"]
  File "...\lib\site-packages\pandas\core\frame.py", line 3505, in __getitem__
    indexer = self.columns.get_loc(key)
  File "...\lib\site-packages\pandas\core\indexes\base.py", line 3623, in get_loc
    raise KeyError(key) from err
KeyError: 'Number'
That looks a lot like your error. I think (am quite sure) there is no "machine_status" column in df1.

RE: Copy a column from one dataframe to another dataframe - Led_Zeppelin - Jul-07-2022

You know that is what I thought also. I will check it and see.

Thanks for your input.

Respectfully,

LZ

RE: Copy a column from one dataframe to another dataframe - Led_Zeppelin - Jul-07-2022

You are correct. But it should be there.

In an earlier line in the program, I used the command.

df1=df

Now I did this before I dropped the three columns from df.

It ("machine_status") should be there, but it is not.

How can I work a round this.

I wanted to keep a dataframe from my initial uploading.
The I can work on the initial dataframe to scale and normalize so I
can then "attach" the dropped three columns at a later time.

However, this did not work out and the error is in

ddf1 = df

Any help appreciated.

Respectfully,

LZ