How to get every nth column in pandas?

Question

I have a dataframe which looks like this:

    a1    b1    c1    a2    b2    c2    a3    ...
x   1.2   1.3   1.2   ...   ...   ...   ...
y   1.4   1.2   ...   ...   ...   ...   ...
z   ...

What I want is grouping by every nth column. In other words, I want a dataframe with all the as, one with bs and one with cs

    a1     a2     a4
x   1.2    ...    ...
y
z

In another SO question I saw that is possibile to do df.iloc[::5,:], for example, to get every 5th raw. I could do of course df.iloc[:,::3] to get the c cols but it doesn't work for getting a and b.

Any ideas?

Answer 1

slice the columns:

df[df.columns[::2]]

To get every nth column

Example:

In [2]:
cols = ['a1','b1','c1','a2','b2','c2','a3']
df = pd.DataFrame(columns=cols)
df

Out[2]:
Empty DataFrame
Columns: [a1, b1, c1, a2, b2, c2, a3]
Index: []

In [3]:
df[df.columns[::3]]
Out[3]:

Empty DataFrame
Columns: [a1, a2, a3]
Index: []

You can also filter using startswith:

In [5]:
a = df.columns[df.columns.str.startswith('a')]
df[a]

Out[5]:
Empty DataFrame
Columns: [a1, a2, a3]
Index: []

and do the same for b cols and c cols etc..

You can get a set of all the unique col prefixes using the following:

In [19]:
df.columns.str.extract(r'([a-zA-Z])').unique()

Out[19]:
array(['a', 'b', 'c'], dtype=object)

You can then use these values to filter the columns using startswith