How to merge columns in Pandas?

Question

I have a dataframe index_crisis and and want to create a new column that contains a 1 when the index reached a local peak and zero else. I don't know how to go on in my code. The list peak locations is: [ 2 7 9 13 16 18 21] but with month[peak_locations] I get the month of the peaks.

    Date           Index 
38  2007-06-01  -0.56
39  2007-07-01  -0.36
40  2007-08-01  0.68
41  2007-09-01  0.24
42  2007-10-01  0.22
43  2007-11-01  0.89
44  2007-12-01  0.95
45  2008-01-01  1.53
46  2008-02-01  1.01
47  2008-03-01  1.73
48  2008-04-01  1.39
49  2008-05-01  0.96
50  2008-06-01  1.26
51  2008-07-01  2.37
52  2008-08-01  1.57
53  2008-09-01  2.95
54  2008-10-01  5.7
55  2008-11-01  5.29
56  2008-12-01  5.42
57  2009-01-01  4.99
58  2009-02-01  4.45
59  2009-03-01  4.59
60  2009-04-01  4.2
61  2009-05-01  3.12
62  2009-06-01  1.85

My expected output is a column dummy that looks like:

df = pd.read_csv("index_crisis.csv", parse_dates=True)

df['Date'] = pd.to_datetime(df['Date'])
df['Date'] = pd.PeriodIndex(df.Date, freq='M').strftime("%b %Y")


data = df['Index'].values
doublediff = np.diff(np.sign(np.diff(data)))
peak_locations = np.where(doublediff == -2)[0] + 1

Possible duplicate of [How to join (merge) data frames (inner, outer, left, right)](https://stackoverflow.com/questions/1299871/how-to-join-merge-data-frames-inner-outer-left-right) — PV8, Aug 13 '19 at 07:44
I found this before but don't know how to applied it to my case.. I don't know how to say, if it merge than write a one and else zero — Dennis, Aug 13 '19 at 07:48
My local peak is my local maximum of the Index. The positions can be seen in my expected output. Or what do you mean? — Dennis, Aug 13 '19 at 07:53

score 1 · Accepted Answer · answered Aug 13 '19 at 08:08

idx = df.iloc[peak_locations].index
df['dummy'] = np.where(df.index.isin(idx), 1, 0)

    Date    Index   dummy
38  Jun 2007    -0.56   0
39  Jul 2007    -0.36   0
40  Aug 2007    0.68    1
41  Sep 2007    0.24    0
42  Oct 2007    0.22    0
43  Nov 2007    0.89    0
44  Dec 2007    0.95    0
45  Jan 2008    1.53    1
46  Feb 2008    1.01    0
47  Mar 2008    1.73    1
48  Apr 2008    1.39    0
49  May 2008    0.96    0
50  Jun 2008    1.26    0
51  Jul 2008    2.37    1
52  Aug 2008    1.57    0
53  Sep 2008    2.95    0
54  Oct 2008    5.7     1
55  Nov 2008    5.29    0
56  Dec 2008    5.42    1
57  Jan 2009    4.99    0
58  Feb 2009    4.45    0
59  Mar 2009    4.59    1
60  Apr 2009    4.2     0
61  May 2009    3.12    0
62  Jun 2009    1.85    0

score 0 · Answer 2 · answered Aug 13 '19 at 07:55

0

Find the local maximum by: the value is larger than the next AND the next value is not larger than the next:

series = df['Index'].values
s = series > series.shift(1)
df[s & (s != s.shift(-1))]

answered Aug 13 '19 at 07:55

Joost Döbken

3,450
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But I already have the peaks detected with my code. I need the part to become my expected output – Dennis Aug 13 '19 at 07:57

score 0 · Answer 3 · answered Aug 13 '19 at 08:20

I think that you want to use Numpy's fancy indexing to build your array of ones and zeros. A sequence object can be used as an indexer for a Numpy array.

Following your example, suppose that your DataFrame is 62 rows long. Then:

>>> peak_locations = [2, 7, 9, 13, 16, 18, 21]  # You generated this
>>> dummy = np.zeros(len(df), dtype=int)        # I assume length 62 in this example
>>> print(dummy)

array([0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0])

>>> dummy[peak_locations] = 1                   # This is the fancy indexing hotness 
>>> dummy

array([0, 0, 1, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 0, 1, 0, 1, 0, 0, 1,
       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0])

>>> df["dummy"] = dummy                         # Adds the new column

How to merge columns in Pandas?

3 Answers3