df is original DataFrame, csv file.
a = df.head(3) # get part of df.
This is table a.
b = a.loc[1:3,'22':'41'] #select part of a.
c = pd.DataFrame(data=b,index=['a','b'],columns=['v','g']) # give index and columns
final
b show 2x2. I get four value.
c show 2x2 NaN. I get four NaN.
why c don't contain any number?
Try using .values, you are running into 'intrinsic data alignment'
c = pd.DataFrame(data=b.values,index=['a','b'],columns=['v','g']) # give index and columns
Pandas likes to align indexes, by converting your 'b' dataframe into a np.array, you can then use the pandas dataframe constructor to build a new dataframe with those 2x2 values assigning new indexing.
Your DataFrame b already contains row and column indices, so when you try to create DataFrame c and you pass index and columns keyword arguments, you are implicitly indexing out of the original DataFrame b.
If all you want to do is re-index b, why not do it directly?
b = b.copy()
b.index = ['a', 'b']
b.columns = ['v', 'g']
