CODEWITHSUNDEEP
pythonlistgrouping
DaniV
DaniV

Reputation: 489

Add values from a list of lists given a key

In the following list of lists, I am looking to extract a list of lists grouped by the first element of each subblist. Within each group, I would like to add up the respective values of the remaining elements.

ex = [['1', -104.08,  -1],
      ['1',  629.58,   5],
      ['1', 1032.51,   4],
      ['1', -308.18,  -9],
      ['3', -987.18,  34],
      ['3',-1153.71, -21],
      ['3',  -88.08,  75],
      ['3', -139.34, -85],
      ['6',   141.7,  90],
      ['6',   14.77,   4],
      ['6',    74.6,  -5]]

The result should be something like this:

list_result = [['1',  1249.83,   -1],
               ['3', -2368.31,    3],
               ['6',   231.07,   89]]

I have tried by methods like list (set ()), to set the key, and then I have summed manually, but I am sure that there are more efficient ways with fewer lines.

Upvotes: 1

Views: 109

Answers (2)

Mad Physicist
Mad Physicist

Reputation: 114518

itertools.groupby should be suitable if your list is already sorted by the first element:

from itertools import groupby
from operator import itemgetter

result = []
for k, g in groupby(ex, itemgetter(0)):
    _, a, b = zip(*g)
    result.append([k, sum(a), sum(b)])

You can write a somewhat illegible and wasteful one-liner:

[[k, sum(a), sum(b)] for k, (_, a, b) in ((k, zip(*g)) for k, g in groupby(ex, itemgetter(0)))]

Upvotes: 3

iamjing66
iamjing66

Reputation: 61

as this q for me, i will make a dict to get same value and change it to a list

ex = [['1', -104.08,   -1],
       ['1',  629.58,   5],
       ['1', 1032.51,   4],
       ['1', -308.18,  -9],
       ['3', -987.18,  34],
       ['3',-1153.71, -21],
       ['3',  -88.08,  75],
       ['3', -139.34, -85],
       ['6',   141.7,  90],
       ['6',   14.77,   4],
       ['6',    74.6,  -5]]

e = dict()
for i in ex:
    if i[0] in e.keys():
        e[i[0]][0] = e[i[0]][0]+i[1]
        e[i[0]][1] = e[i[0]][1]+i[2]
    else:
        e[i[0]] = [i[1], i[2]]
e1 = [[k,v[0],v[1]] for k,v in e.items()]
print(e1)
# output: 
# [['1', 1249.83, -1], ['3', -2368.31, 3], ['6', 231.07, 89]]

Upvotes: 1

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