pandas dataframe 从另一个 dataframe 有条件更新

Question

I have a master dataframe with two sets of values:

df1 = pd.DataFrame({'id1': [1, 1, 2, 2],
               'dir1': [True, False, True, False],
               'value1': [55, 40, 84, 31],
               'id2': [3, 3, 4, 4],
               'dir2': [True, False, False, True],
               'value2': [60, 30, 7, 15]})

   id1   dir1  value1  id2   dir2  value2
0    1   True      55    3   True      60
1    1  False      40    3  False      30
2    2   True      84    4  False       7
3    2  False      31    4   True      15

I then have an update dataframe that looks like this:

df2 = pd.DataFrame({'id': [1, 2, 3, 4],
               'value': [21, 22, 23, 24]})
   id  value
0   1     21
1   2     22
2   3     23
3   4     24

I want to update df1 with the new values of df2 but only where dirX is True. Data should then look like this:

   id1   dir1  value1  id2   dir2  value2
0    1   True     *21    3   True     *23
1    1  False      40    3  False      30
2    2   True     *22    4  False       7
3    2  False      31    4   True     *24

Any idea if something like this is even possible? I tried looking at.update but I could not get it to work. I'm fairly new to python and only coding at 23:00, so maybe I'm just not as sharp as I need to be.

Answer 1

I agree with Thales' answer. First, you merge df2 with df1 based on id1:

df = df1.merge(df2, left_on='id1', right_on='id')

Then, you replace value1 based on dir1 with value :

df.value1 = np.where(df.dir1 == True, df.value, df.value1)

Then, you drop the extra columns

df = df.drop(['id', 'value'],axis=1)

Then, you merge df2 with df1 based on id2 :

df = df.merge(df2, left_on='id2', right_on='id')

Do the same replacing, but for value2

df.value2 = np.where(df.dir2 == True, df.value, df.value2)

Then, drop the extra columns:

df = df.drop(['id', 'value'],axis=1)

The resulting dataframe will look like:

   id1   dir1  value1  id2   dir2  value2
0    1   True      21    3   True      23
1    1  False      40    3  False      30
2    2   True      22    4  False       7
3    2  False      31    4   True      24

Answer 2

Try to use np.where function from numpy.

Maybe something like this:

df_1['value1'] = np.where(df_1['dir2'] == True, df_2['value'], df_1['value1'])

Maybe you'll need some adjustments or some merges, but I think this will help you to find a solution.