python - Get first non-null value per row

Question

I have a sample dataframe show as below. For each line, I want to check the c1 first, if it is not null, then check c2. By this way, find the first notnull column and store that value to column result.

ID  c1  c2  c3  c4  result
1   a   b           a
2       cc  dd      cc
3           ee  ff  ee
4               gg  gg

I am using this way for now. but I would like to know if there is a better method.(The column name do not have any pattern, this is just sample)

df["result"] = np.where(df["c1"].notnull(), df["c1"], None)
df["result"] = np.where(df["result"].notnull(), df["result"], df["c2"])
df["result"] = np.where(df["result"].notnull(), df["result"], df["c3"])
df["result"] = np.where(df["result"].notnull(), df["result"], df["c4"])
df["result"] = np.where(df["result"].notnull(), df["result"], "unknown)

When there are lots of columns, this method looks not good.

Answer 1

Use back filling NaNs first and then select first column by iloc:

df['result'] = df[['c1','c2','c3','c4']].bfill(axis=1).iloc[:, 0].fillna('unknown')

Or:

df['result'] = df.iloc[:, 1:].bfill(axis=1).iloc[:, 0].fillna('unknown')

---

print (df)
   ID   c1   c2  c3   c4 result
0   1    a    b   a  NaN      a
1   2  NaN   cc  dd   cc     cc
2   3  NaN   ee  ff   ee     ee
3   4  NaN  NaN  gg   gg     gg

Performance:

df = pd.concat([df] * 1000, ignore_index=True)


In [220]: %timeit df['result'] = df[['c1','c2','c3','c4']].bfill(axis=1).iloc[:, 0].fillna('unknown')
100 loops, best of 3: 2.78 ms per loop

In [221]: %timeit df['result'] = df.iloc[:, 1:].bfill(axis=1).iloc[:, 0].fillna('unknown')
100 loops, best of 3: 2.7 ms per loop

#jpp solution
In [222]: %%timeit
     ...: cols = df.iloc[:, 1:].T.apply(pd.Series.first_valid_index)
     ...: 
     ...: df['result'] = [df.loc[i, cols[i]] for i in range(len(df.index))]
     ...: 
1 loop, best of 3: 180 ms per loop

#c???s????'  s solution
In [223]: %timeit df['result'] = df.stack().groupby(level=0).first()
1 loop, best of 3: 606 ms per loop