Here I have a list of list:
[['a', 3], ['b', 2], ['b', 1.1], ['c', 5],['c', 1.2]]
How can i get a list of:
[['a', 3], ['b', 3.1],['c', 6.2],]
I could only think of some very clumpsy way. looping the list with set() but is there a more efficient way?
You can use a dictionary to store the values for each key. Then, make a list of lists from the dictionary elements.
data = [['a', 3], ['b', 2], ['b', 1.1], ['c', 5],['c', 1.2]]
mapper = {}
for pair in data:
key, val = pair
mapper[key] = mapper.get(key, 0) + val
data_merged = list(map(list, mapper.items()))
print(data_merged)
Output:
[['a', 3], ['b', 3.1], ['c', 6.2]]
Explanation:
mapper).
list(map(list, mapper.items())) is equivalent to list(mapper.items()).
list(mapper.items()) returns a list of tuples of the elements. The OP wants a list of lists that is the reason to use list(map(list, mapper.items())).If that list is sorted by its first elements as seen, then itertools.groupby does the job:
from itertools import groupby
[[key, sum(lst[1] for lst in group)] for key, group in groupby(a, key=lambda e: e[0])]
We group by the first entry of the sub lists. Then for each group, we sum the second elements and put them in a list along with the corresponding key which is the first element i.e. the letter we grouped on. The list comprehension gives the total answer thereby.
This is probably what you want but it's not very pretty:
my_list = [['a', 3], ['b', 2], ['b', 1.1], ['c', 5],['c', 1.2]]
[[key, sum(l[1] for l in my_list if l[0]==key)] for key in set([l[0] for l in my_list])]
set and the inner list comprehension would help fasten the process of understanding it.
This is how you can solve it using pandas:
import pandas as pd
df = pd.DataFrame([['a', 3], ['b', 2], ['b', 1.1], ['c', 5],['c', 1.2]],columns = ['Letter','Value'])
df.groupby(['Letter']).sum().reset_index().values.tolist()
You can also use a defaultdict with a default value of 0:
from collections import defaultdict
data = [['a', 3], ['b', 2], ['b', 1.1], ['c', 5],['c', 1.2]]
sums = defaultdict(int)
for key, value in data:
sums[key] += value
out = [[key, value] for key, value in sums.items()]
print(out)
# [['a', 3], ['b', 3.1], ['c', 6.2]]
