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POCompute the nth day from the first event in Pandas
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13156660
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2012-10-31T11:16:57.187
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2012-11-01T10:35:43.893
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2017-05-23T12:27:05.493
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I have the following data frame, subsetted from my original data frame, with columns event, unixtime, and day, and I want to add another column arbday which is the nth day since the first event (with the first visit being day 1): <pre><code>import numpy as np import datetime as dt >>> testdf = pd.DataFrame({'event': range(1,4), 'unixtime': [1346617885925, 1346961625305,1347214217566]},index=[343352,343353,343354]) >>> testdf['day'] = testdf['unixtime'].apply(lambda x: dt.datetime.utcfromtimestamp(x/1000).date()) event unixtime day arbday 343352 1 1346617885925 2012-09-02 1 343353 2 1346961625305 2012-09-06 5 343354 3 1347214217566 2012-09-09 8 </code></pre> After looking around, I tried to do this by: <pre><code>>>> testdf2['arbday'] = np.where(testdf2['event']==1, 1, testdf2.day.apply(lambda x: x-x[:1])) event unixtime day arbday 343352 1 1346617885925 2012-09-02 1 343353 2 1346961625305 2012-09-06 NaN 343354 3 1347214217566 2012-09-09 NaN or >>> testdf2['arbday'] = np.where(testdf2['event']==1, 1, testdf2.day.apply(lambda x: dt.timedelta(x-x[:1]))) TypeError: 'datetime.date' object is not subscriptable </code></pre> What is the correct way to do this? Any pointer is much appreciated! EDIT: A follow-up question regarding to applying this over groups is <a href="https://stackoverflow.com/questions/13175251/compute-the-nth-day-from-the-first-event-within-each-group-in-pandas">here</a>.
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<python><datetime><pandas>
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Compute the nth day from the first event in Pandas
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