get the value from another values if value is nan [duplicate] - python-3.x

Question

I am trying to create a column which contains only the minimum of the one row and a few columns, for example:
A0 A1 A2 B0 B1 B2 C0 C1
0 0.84 0.47 0.55 0.46 0.76 0.42 0.24 0.75
1 0.43 0.47 0.93 0.39 0.58 0.83 0.35 0.39
2 0.12 0.17 0.35 0.00 0.19 0.22 0.93 0.73
3 0.95 0.56 0.84 0.74 0.52 0.51 0.28 0.03
4 0.73 0.19 0.88 0.51 0.73 0.69 0.74 0.61
5 0.18 0.46 0.62 0.84 0.68 0.17 0.02 0.53
6 0.38 0.55 0.80 0.87 0.01 0.88 0.56 0.72
Here I am trying to create a column which contains the minimum for each row of columns B0, B1, B2.
The output would look like this:
A0 A1 A2 B0 B1 B2 C0 C1 Minimum
0 0.84 0.47 0.55 0.46 0.76 0.42 0.24 0.75 0.42
1 0.43 0.47 0.93 0.39 0.58 0.83 0.35 0.39 0.39
2 0.12 0.17 0.35 0.00 0.19 0.22 0.93 0.73 0.00
3 0.95 0.56 0.84 0.74 0.52 0.51 0.28 0.03 0.51
4 0.73 0.19 0.88 0.51 0.73 0.69 0.74 0.61 0.51
5 0.18 0.46 0.62 0.84 0.68 0.17 0.02 0.53 0.17
6 0.38 0.55 0.80 0.87 0.01 0.88 0.56 0.72 0.01
Here is part of the code, but it is not doing what I want it to do:
for i in range(0,2):
df['Minimum'] = df.loc[0,'B'+str(i)].min()

Answer 1

This is a one-liner, you just need to use the axis argument for min to tell it to work across the columns rather than down:
df['Minimum'] = df.loc[:, ['B0', 'B1', 'B2']].min(axis=1)
If you need to use this solution for different numbers of columns, you can use a for loop or list comprehension to construct the list of columns:
n_columns = 2
cols_to_use = ['B' + str(i) for i in range(n_columns)]
df['Minimum'] = df.loc[:, cols_to_use].min(axis=1)

Answer 2

For my tasks a universal and flexible approach is the following example:
df['Minimum'] = df[['B0', 'B1', 'B2']].apply(lambda x: min(x[0],x[1],x[2]), axis=1)
The target column 'Minimum' is assigned the result of the lambda function based on the selected DF columns['B0', 'B1', 'B2']. Access elements in a function through the function alias and his new Index(if count of elements is more then one). Be sure to specify axis=1, which indicates line-by-line calculations.
This is very convenient when you need to make complex calculations.
However, I assume that such a solution may be inferior in speed.
As for the selection of columns, in addition to the 'for' method, I can suggest using a filter like this:
calls_to_use = list(filter(lambda f:'B' in f, df.columns))
literally, a filter is applied to the list of DF columns through a lambda function that checks for the occurrence of the letter 'B'.
after that the first example can be written as follows:
calls_to_use = list(filter(lambda f:'B' in f, df.columns))
df['Minimum'] = df[calls_to_use].apply(lambda x: min(x), axis=1)
although after pre-selecting the columns, it would be preferable:
df['Minimum'] = df[calls_to_use].min(axis=1)