在Pandas Dataframe上滑动窗口

Question

I have a large pandas dataframe of time-series data.

I currently manipulate this dataframe to create a new, smaller dataframe that is rolling average of every 10 rows. ie a rolling window technique. Like this:

def create_new_df(df):
    features = []
    x = df['X'].astype(float)
    i = x.index.values
    time_sequence = [i] * 10
    idx = np.array(time_sequence).T.flatten()[:len(x)]
    x = x.groupby(idx).mean()
    x.name = 'X'
    features.append(x)
    new_df = pd.concat(features, axis=1)
    return new_df

Code to test:

columns = ['X']
df_ = pd.DataFrame(columns=columns)
df_ = df_.fillna(0) # with 0s rather than NaNs
data = np.array([np.arange(20)]*1).T
df = pd.DataFrame(data, columns=columns)

test = create_new_df(df)
print test

Output:

      X
0   4.5
1  14.5

However, I want the function to make the new dataframe using a sliding window with a 50% overlap

So the output would look like this:

      X
0   4.5
1   9.5
2  14.5

How can I do this?

Here's what I've tried:

from itertools import tee, izip

def window(iterable, size):
    iters = tee(iterable, size)
    for i in xrange(1, size):
        for each in iters[i:]:
            next(each, None)
    return izip(*iters)

for each in window(df, 20):
    print list(each) # doesn't have the desired sliding window effect

Some might also suggest using the pandas rolling_mean() methods, but if so, I can't see how to use this function with window overlap.

Any help would be much appreciated.

Answer 1

I think pandas rolling techniques are fine here. Note that starting with version 0.18.0 of pandas, you would use rolling().mean() instead of rolling_mean() .

>>> df=pd.DataFrame({ 'x':range(30) })
>>> df = df.rolling(10).mean()           # version 0.18.0 syntax
>>> df[4::5]                             # take every 5th row

       x
4    NaN
9    4.5
14   9.5
19  14.5
24  19.5
29  24.5