Iterating over a list of lists - loops

Question

My list looks like this:
['"date","supermarket","categoryA",10',
'"date","candy store","categoryB",5',
'"date","drugstore","categoryC",6',
'"date","supermarket","categoryA",20',
'"date","candy store","categoryB",2',
'"date","drugstore","categoryC",90']
etc
I'm trying to aggregate the numbers per category -- categoryA B C etc
So far, It's been three days of mostly sideways action. I really should get a book on Python as I've just jumped in and now here I'm asking you guys.
I know how to do this in mysql but that logic is not helping me here.
My code:
for x in range(0 , len(list)):
for y in list[x][2]:
value += list[x][3]
Tearing my hairs out, and I don't have many of those left...

Answer 1

Use dictionary to hold the aggregation and iterate list using in:
aggregate = {}
for x in list:
if (x[2] not in aggregate):
aggregate[x[2]] = 0
aggregate[x[2]] += x[3]
The above assumes your list of lists looks like this:
[
["date","supermarket","categoryA",10],
["date","candy store","categoryB",5]
]

Answer 2

Using python dictionaries, simplifies lot of things. This would work:
category_aggregate_dictionary = {}
for x in range(0 , len(list)):
for y in list[x][2]:
value = list[x][3]
category_aggregate_dictionary[y] = 0 if category_aggregate_dictionary.get(y, None) == None
category_aggregate_dictionary[y] += float(value)
Finally, category_aggregate_dictionary["categoryA"] should give you aggregate number of categoryA.
Hope it Helps : )

Answer 3

Here I've assumed you actually have a list of lists. (See my value for "entries" below.)
from collections import Counter
entries = [
["date", "supermarket", "categoryA", 10],
["date", "candy store", "categoryB", 5],
["date", "drugstore", "categoryC", 6],
["date", "supermarket", "categoryA", 20],
["date", "candy store", "categoryB", 2],
["date", "drugstore", "categoryC", 90]
]
# A Counter is much like a dictionary with a default value of 0
category_counts = Counter()
for entry in entries:
category = entry[2]
count = entry[3]
category_counts[category] += count
# You have the counts already at this point. This loop will
# just print them out in sorted order (by category name).
for category in sorted(category_counts.keys()):
print('{}: {}'.format(category, category_counts[category]))
# Output:
# categoryA: 30
# categoryB: 7
# categoryC: 96

Answer 4

If you are dealing with a list of string like that you can use a ast.literal_eval() function in order to evaluate your strings as tuples then use defaultdict() for aggregating the numbers:
>>> from collections import defaultdict
>>> from ast import literal_eval
>>> d = defaultdict(int)
>>> for item in lst:
... *_, cat, num = literal_eval(item)
... d[cat]+=num
...
>>> d
defaultdict(<class 'int'>, {'9': 0, 'categoryA': 30, 'categoryC': 96, 'categoryB': 7})