I'm reading Python for Data Analysis by Wes Mckinney, but I was surprised by this data manipulation. You can see all the procedure here but I will try to summarize it here. Assume you have something like this:
In [133]: agg_counts = by_tz_os.size().unstack().fillna(0)
Out[133]:
a Not Windows Windows
tz 245 276
Africa/Cairo 0 3
Africa/Casablanca 0 1
Africa/Ceuta 0 2
Africa/Johannesburg 0 1
Africa/Lusaka 0 1
America/Anchorage 4 1
...
tz
means time zone and Not Windows
and Windows
are categories extracted from the User Agent in the original data, so we can see that there are 3 Windows users and 0 Non-windows users in Africa/Cairo from the data collected.
Then in order to get "the top overall time zones" we have:
In [134]: indexer = agg_counts.sum(1).argsort()
Out[134]:
tz
24
Africa/Cairo 20
Africa/Casablanca 21
Africa/Ceuta 92
Africa/Johannesburg 87
Africa/Lusaka 53
America/Anchorage 54
America/Argentina/Buenos_Aires 57
America/Argentina/Cordoba 26
America/Argentina/Mendoza 55
America/Bogota 62
...
So at that point, I would have thought that according to the documentation I was summing over columns (in sum(1)
) and then sorting according to the result showing arguments (as usual in argsort). First of all, I'm not sure what does it mean "columns" in the context of this series because sum(1)
is actually summing Not Windows
and Windows
users keeping that value in the same row as its time zone. Furthermore, I can't see a correlation between argsort values and agg_counts
. For example, Pacific/Auckland
has an "argsort value" (in In[134]
) of 0 and it only has a sum of 11 Windows
and Not Windows
users. Asia/Harbin
has an argsort value of 1 and appears with a sum of 3 Windows
and Not Windows users.
Can someone explain to me what is going on there? Obviously I'm misunderstanding something.
sum(1)
means sum over axis = 1
. The terminology comes from numpy
.
For a 2+ dimensional object, the 0-axis refers to the rows. Summing over the 0-axis means summing over the rows, which amounts to summing "vertically" (when looking at the table).
The 1-axis refers to the columns. Summing over the 1-axis means summing over the columns, which amounts to summing "horizontally".
numpy.argsort
returns an array of indices which tell you how to sort an array. For example:
In [72]: import numpy as np
In [73]: x = np.array([521, 3, 1, 2, 1, 1, 5])
In [74]: np.argsort(x)
Out[74]: array([2, 4, 5, 3, 1, 6, 0])
The 2 in the array returned by np.argsort
means the smallest value in x
is x[2]
, which equals 1
. The next smallest is x[4]
which is also 1. And so on.
If we define
totals = df.sum(1)
print(totals)
# tz 521
# Africa/Cairo 3
# Africa/Casablanca 1
# Africa/Ceuta 2
# Africa/Johannesburg 1
# Africa/Lusaka 1
# America/Anchorage 5
then totals.argsort()
is argsorting the values [521, 3, 1, 2, 1, 1, 5]
. We've seen the result; it is the same as numpy.argsort
:
[2, 4, 5, 3, 1, 6, 0]
These values are simply made into a Series
, with the same index
as totals
:
print(totals.argsort())
# tz 2
# Africa/Cairo 4
# Africa/Casablanca 5
# Africa/Ceuta 3
# Africa/Johannesburg 1
# Africa/Lusaka 6
# America/Anchorage 0
Associating the totals.index
with this argsort indices does not appear have intrinsic meaning, but if you compute totals[totals.argsort()]
you see the rows of totals
in sorted order:
print(totals[totals.argsort()])
# Africa/Casablanca 1
# Africa/Johannesburg 1
# Africa/Lusaka 1
# Africa/Ceuta 2
# Africa/Cairo 3
# America/Anchorage 5
# tz 521
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