How to sum column values with python

Question

I have a rowset that looks like this:

defaultdict(<type 'dict'>, 
{
   u'row1': {u'column1': 33, u'column2': 55, u'column3': 23}, 
   u'row2': {u'column1': 32, u'column2': 32, u'column3': 17}, 
   u'row3': {u'column1': 31, u'column2': 87, u'column3': 18}
})

I want to be able to easily get the sum of column1, column2, column3. It would be great if I could do this for any number of columns, receiving the result in a hash map that looks like columnName => columnSum. As you might guess, its not possible for me to obtain the summed values from the database in first place, thus the reason to ask the question.

jamylak · Accepted Answer · 2012-08-08 14:42:15Z

7

>>> from collections import defaultdict
>>> x = defaultdict(dict, 
    {
        u'row1': {u'column1': 33, u'column2': 55, u'column3': 23}, 
        u'row2': {u'column1': 32, u'column2': 32, u'column3': 17}, 
        u'row3': {u'column1': 31, u'column2': 87, u'column3': 18}
    }) 

>>> sums = defaultdict(int)
>>> for row in x.itervalues():
        for column, val in row.iteritems():
            sums[column] += val


>>> sums
defaultdict(<type 'int'>, {u'column1': 96, u'column3': 58, u'column2': 174})

Ooh a much better way!

>>> from collections import Counter
>>> sums = Counter()
>>> for row in x.values():
        sums.update(row)


>>> sums
Counter({u'column2': 174, u'column1': 96, u'column3': 58})

edited Aug 8, 2012 at 14:42

answered Mar 28, 2012 at 8:42

jamylak

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1 Comment

Burhan Khalid Over a year ago

+1 for explicit better than implicit and output from console.

Kimvais · Accepted Answer · 2012-03-28 09:08:03Z

2

Nested generators + list comprehension does the trick:

>>> foo
defaultdict(<type 'dict'>, {u'row1': {u'column1': 33, u'column3': 23, u'column2': 55}, u'row2': {u'column1': 32, u'column3': 17, u'column2': 32}, u'row3': {u'column1': 31, u'column3': 18, u'column2': 87}})
>>> dict(zip(foo.values()[0].keys(), [sum(j[k] for j in (i.values() for _,i in foo.items())) for k in range(3)]))
{u'column1': 96, u'column3': 58, u'column2': 174}

edited Mar 28, 2012 at 9:08

answered Mar 28, 2012 at 8:54

Kimvais

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1 Comment

jamylak Over a year ago

Return the result in a hash map?

WeaselFox · Accepted Answer · 2012-03-28 08:50:01Z

0

not the most pythonic but here:

for val in defaultdict.values() :
   sum1 += val['column1']
   sum2 += val['column2']
   sum3 += val['column3']
final_dict = {'column1' : sum1,'column2' : sum2,'column3' : sum3 }

edited Mar 28, 2012 at 8:50

answered Mar 28, 2012 at 8:43

WeaselFox

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Comments

moldovean · Accepted Answer · 2014-08-10 08:45:00Z

0

Here's another answer, if I may suggest a solution. First put your data in a Matrix. Then, multiply the matrix by a vector of ones.

import numpy as np
A = np.random.normal(size = (3,3))

Now to get the sum of columns, simply use the dot product.

np.dot(A, np.ones(3))

To stack the rows rather than the columns, simply transpose the matrix.

np.dot(A.T, np.ones(3))

answered Aug 10, 2014 at 8:45

moldovean

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