How keep a value in a DataFrame using the values of another DataFrame as indexes and columns reference (and replace the others)?

Question

I have the following two DataFrames:

import pandas as pd

df = pd.DataFrame([[0, 0, 0, 0, 0],
                   [0, 0, 0, 0, 0],
                   [0, 0, 0, 0, 0],
                   [0, 0, 0, 0, 0],
                   [0, 0, 0, 0, 0]],
                  index = [0, 0.25, 0.50, 0.75, 1],
                  columns = [0, 0.25, 0.50, 0.75, 1])

df_cross = pd.DataFrame([[0.0, 0.25],
                         [0.0, 0.75],
                         [0.5, 1]],
                        columns = ['indexes_to_keep',
                                   'cols_to_keep'])

df:

      0.00  0.25  0.50  0.75  1.00
0.00     0     0     0     0     0
0.25     0     0     0     0     0
0.50     0     0     0     0     0
0.75     0     0     0     0     0
1.00     0     0     0     0     0

df_cross:

   indexes_to_keep  cols_to_keep
0              0.0          0.25
1              0.0          0.75
2              0.5          1.00

In the df I have my storaged data, and the df_cross contains the indexes and columns that I want to keep the values. The values in df which the index and columns do not match with any row of df_cross I want to replace by a string (for example "NaN").

The expected output is:

     0.00 0.25 0.50 0.75 1.00
0.00  NaN    0  NaN    0  NaN
0.25  NaN  NaN  NaN  NaN  NaN
0.50  NaN  NaN  NaN  NaN    0
0.75  NaN  NaN  NaN  NaN  NaN
1.00  NaN  NaN  NaN  NaN  NaN

Thanks in advance.

Answer 1

Pandas does not support setting elements with arrays of coordinates. You would need to use numpy:

# integer locs
rows = df.index.get_indexer(df_cross.indexes_to_keep)
cols = df.columns.get_indexer(df_cross.cols_to_keep)

# where we want to keep the data
mask = np.full(df.shape, False)
mask[rows, cols] = True

df[:] = df.where(mask)

Another way, with just Pandas, to create mask is:

mask = (df_cross.assign(val=True)
          .set_index(['indexes_to_keep', 'cols_to_keep'])
          ['val'].unstack(fill_value=False)
       )

Output:

      0.00  0.25  0.50  0.75  1.00
0.00   NaN   0.0   NaN   0.0   NaN
0.25   NaN   NaN   NaN   NaN   NaN
0.50   NaN   NaN   NaN   NaN   0.0
0.75   NaN   NaN   NaN   NaN   NaN
1.00   NaN   NaN   NaN   NaN   NaN

Answer 2

Let us try crosstab on df_cross, then use where to mask the values

s = pd.crosstab(*df_cross.values.T)
df.where(s == 1)

---

      0.00  0.25  0.50  0.75  1.00
0.00   NaN   0.0   NaN   0.0   NaN
0.25   NaN   NaN   NaN   NaN   NaN
0.50   NaN   NaN   NaN   NaN   0.0
0.75   NaN   NaN   NaN   NaN   NaN
1.00   NaN   NaN   NaN   NaN   NaN

PS: pd.crosstab(*df_cross.values.T) is just a syntactical shortcut and is effectively equivalent to using pd.crosstab(df.indexes_to_keep, df.cols_to_keep)