pandas Indexing and selecting data Path Dependent Slicing


Example

It may become necessary to traverse the elements of a series or the rows of a dataframe in a way that the next element or next row is dependent on the previously selected element or row. This is called path dependency.

Consider the following time series s with irregular frequency.

#starting python community conventions
import numpy    as np
import pandas   as pd

# n is number of observations
n = 5000

day = pd.to_datetime(['2013-02-06'])
# irregular seconds spanning 28800 seconds (8 hours)
seconds = np.random.rand(n) * 28800 * pd.Timedelta(1, 's')
# start at 8 am
start = pd.offsets.Hour(8)
# irregular timeseries
tidx = day + start + seconds
tidx = tidx.sort_values()

s = pd.Series(np.random.randn(n), tidx, name='A').cumsum()
s.plot();

enter image description here

Let's assume a path dependent condition. Starting with the first member of the series, I want to grab each subsequent element such that the absolute difference between that element and the current element is greater than or equal to x.

We'll solve this problem using python generators.

Generator function

def mover(s, move_size=10):
    """Given a reference, find next value with
    an absolute difference >= move_size"""
    ref = None
    for i, v in s.iteritems():
        if ref is None or (abs(ref - v) >= move_size):
            yield i, v
            ref = v

Then we can define a new series moves like so

moves = pd.Series({i:v for i, v in mover(s, move_size=10)},
                  name='_{}_'.format(s.name))

Plotting them both

moves.plot(legend=True)
s.plot(legend=True)

enter image description here


The analog for dataframes would be:

def mover_df(df, col, move_size=2):
    ref = None
    for i, row in df.iterrows():
        if ref is None or (abs(ref - row.loc[col]) >= move_size):
            yield row
            ref = row.loc[col]

df = s.to_frame()
moves_df = pd.concat(mover_df(df, 'A', 10), axis=1).T

moves_df.A.plot(label='_A_', legend=True)
df.A.plot(legend=True)

enter image description here