python pandas operations on columns

Question

Hi I would like to know the best way to do operations on columns in python using pandas.

I have a classical database which I have loaded as a dataframe, and I often have

Answer 1

You can just use a boolean mask with either the .loc or .ix attributes of the DataFrame.

mask = df['A'] > 2
df.ix[mask, 'A'] = df.ix[mask, 'C'] - df.ix[mask, 'D']

If you have a lot of branching things then you can do:

def func(row):
    if row['A'] > 0:
        return row['B'] + row['C']
    elif row['B'] < 0:
        return row['D'] + row['A']
    else:
        return row['A']

df['A'] = df.apply(func, axis=1)

apply should generally be much faster than a for loop.