CODEWITHSUNDEEP
pythonnumpysum
Ben
Ben

Reputation: 391

Numpy Sum from a multidimensional array?

If I had data such as: 

data = [[3, 2014], [4, 2014], [6, 2013], [6,2013]] etc...

What is the best way to calculate the sum by year in python?

Upvotes: 1

Views: 1553

Answers (5)

Marcus Müller
Marcus Müller

Reputation: 36337

There's a specific python standard library class for that, Counter:

from collections import Counter
from operator import add

counters = [Counter({row[1]:row[0]}) for row in data]
result = reduce(add, counters)

your result is a dict-behaving object:

{2013: 12, 2014: 7}

Upvotes: 1

Padraic Cunningham
Padraic Cunningham

Reputation: 180391

I would use a dict if you need both the year and sum:

from collections import defaultdict

data = [[3, 2014], [4, 2014], [6, 2013], [6,2013]]
d = defaultdict(int)

for v, k in data:
    d[k] += v
print(d)

Prints:

defaultdict(<type 'int'>, {2013: 12, 2014: 7})

Upvotes: 3

head7
head7

Reputation: 1433

As reported by DSM, using pandas and grouby it seems easy:

import pandas as pd
data = [[3, 2014], [4, 2014], [6, 2013], [6,2013]]
df = pd.DataFrame(data, columns=['value', 'year'])
df.groupby(['year']).sum()

which returns:

      value
year       
2013     12
2014      7

It nice because you can easy get more information like mean, median, std etc.. 

df.groupby(['year']).mean()
df.groupby(['year']).median() 
df.groupby(['year']).std()

Upvotes: 1

user
user

Reputation: 5696

You can use counter() and +=.

import collections
data = [[3, 2014], [4, 2014], [6, 2013], [6,2013]]

c = collections.Counter()

for i, j in data:
    c += collections.Counter({j: i})

print(c)

A Counter is a dict subclass for counting hashable objects. It is an unordered collection where elements are stored as dictionary keys and their counts are stored as dictionary values.

You can add Counters, for example: 

a = collections.Counter(a=1, b=2)
b = collections.Counter(a=3, c=3)    
print(a+b)

prints Counter({'a': 4, 'c': 3, 'b': 2}).

Upvotes: 0

Julien Spronck
Julien Spronck

Reputation: 15423

Not sure if I understand the question. Here might be a simple answer without added modules.

dic = {}

for dat, year in data:
    if year not in dic:
        dic[year] = dat
    else:
        dic[year] += dat

or if you prefer

dic = {}
for dat, year in data:
    dic[year] = dat if year not in dic else dic[year] + dat

Upvotes: 1

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