I have arrays like this:
['[camera_positive,3]', '[lens_positive,1]', '[camera_positive,2]', '[lens_positive,1]', '[lens_positive,1]', '[camera_positive,1]']
How to sum all value on index [1] with same string on index [0]?
Example:
camera_positive = 3 + 2 + 1 = 6
lens_positive = 1 + 1 + 1 = 3
You could use set in order to extract the unique keys and then use list comprehension to compute the sum for each key:
data = [['camera_positive', 3],
['lens_positive', 1],
['camera_positive', 2],
['lens_positive', 1],
['lens_positive', 1],
['camera_positive', 1]]
keys = set(key for key, value in data)
for key1 in keys:
total = sum(value for key2, value in data if key1 == key2)
print("key='{}', sum={}".format(key1, total))
this gives:
key='camera_positive', sum=6
key='lens_positive', sum=3
I'm assuming that you have a list of list, not a list of strings as shown in the question. Otherwise you'll have to do some parsing. That said, I would solve this problem by creating a dictionary, and then iterating over the values and adding them to the dictionary as you go.
The default dict allows this program to work without getting a key error, as it'll assume 0 if the key does not exist yet. You can read up on defaultdict here: https://docs.python.org/3.3/library/collections.html#collections.defaultdict
lmk if that helps!
>>> from collections import defaultdict
>>> d = defaultdict(int)
>>> d
defaultdict(<class 'int'>, {})
>>> lst=[['a',1], ['b', 2], ['a',4]]
>>> for k, v in lst:
... d[k] += v
...
>>> d
defaultdict(<class 'int'>, {'a': 5, 'b': 2})
You could group the entries by their first index using groupby with lambda x: x[0] or operator.itemgetter(0) as key.
This is maybe a bit less code than what Nick Brady showed. However you would need to sort the list first (for the same key), so it might be slower than his approach.
Collectionspackage at all? These are useful tools for the problem.