Pandas - Adding Column to DataFrame, but Items Get Reordered-CodePudding

I'm trying to simply add a column from another dataframe of the same length using df['columnname'] = df2['columnname'], but the column appears in df with the lines in a different order.

Is there any rhyme or reason to this? Am I doing pandas wrong? Would love any help for this low level data transformation.

CodePudding user response：

Problem is different indices in both Dataframes, you can assign array or list for avoid it, only necessary same length of both DataFrames:

df = pd.DataFrame({'col': [0,1,2]})
df2 = pd.DataFrame({'columnname': [5,6,7]}, index=[1,0,2])

df['columnname'] = df2['columnname']
df['columnname1'] = df2['columnname'].to_numpy()
df['columnname2'] = df2['columnname'].to_list()

print (df)
   col  columnname  columnname1  columnname2
0    0           6            5            5
1    1           5            6            6
2    2           7            7            7

CodePudding user response：

You can use reindex to avoid different indexes:

df['columnname'] = df2.reindex(df.index)['columnname']