Creating a dictionary from list of lists using only one for loop - python-3.x

Question

I came across this question in a test. There are two parts to this question:
Part i:
Given a list of flavors, eg. ['A','A','A','A','B','B','B','B','B','C','C','C','C'], write a function that returns a dictionary of the number of each flavor respectively.
My solution:
flavors = ['A','A','A','A','B','B','B','B','B','C','C','C','C']
def count_flavors(l):
dict_flavors={}
for i in l:
dict_flavors[i] = l.count(i)
return dict_flavors
print(count_flavors(flavors))
Part ii:
Using not more than ONE for loop write a function that accepts a list of lists of flavors eg. [['A', 'A', 'B', 'B', 'B', 'C', 'C'], ['A', 'A', 'B', 'B', 'B', 'B', 'C'], ['A', 'B', 'C', 'C']] and returns a dictionary for the total number of each flavor. You must include the function that you defined in part one in this solution.
(To clarify, essentially there should only be two for loops; one from part one and one from part two)
So far my solution is the following:
batches = [['A','A','A','A','B','B','B','B','B','C','C','C','C'], ['A', 'A', 'B', 'B' ,'B','B','C'], ['A','B','C','C']]
def batch_count(b):
batch_dict = []
result = {}
for j in b:
batch_dict.append(count_flavors(j))
print(batch_dict)
for i in batch_dict:
for k in i.keys():
result[k] = result.get(k,0) + i[k]
return result
print('batch count 1:' + str(batch_count(batches)))
I am struggling to find a solution that only uses one for loop for this part. I am aware that there are modules that exist for this sort of thing like collections.Counter(). Is a naive solution that does not include any modules possible for this problem?
Thanks!

Answer 1

Here is the best naive solution which I can think of in order to achieve what you want
Benefits of using the solution
No need to create extra variable like batch_dict = [], which takes unnecessary space in your system
No need to carry out multiple computations using different methods, like you did above using count_flavors()
Straight forward and easy to understand
FINAL SOLUTION
batches = [['A','A','A','A','B','B','B','B','B','C','C','C','C'], ['A', 'A', 'B', 'B' ,'B','B','C'], ['A','B','C','C']]
def batch_count(b):
result = {} # for storing final count results
# two loops are required to get into the arrays of array, not other option is there
for items in b:
# Getting the nested array item here
for item in items:
# final computation, if the item is there in the result dict, then increment
# else simply assign 1 to the item as a key which eventually gives you the total number
# of counts of each item throughout the batches array items
if item in result:
result[item] += 1
else:
result[item] = 1
return result
print('batch count 1:' + str(batch_count(batches)))
# OUTPUT
# >>> batch count 1:{'A': 7, 'C': 7, 'B': 10}
Feel free to test this out for some other batches too, and let me know. This is by far the naive solution which is possible to give out what you want to achieve. Keep learning :)
ANOTHER SOLUTION [MAKING USE OF FIRST METHOD COUNT_FLAVORS]
Hey, if you really want to use the first method, then there is a work around, but you need to compromise with one thing now, that is Counter has to be imported, but I assure you, it will be as simple as that, and will give you straight forward answer
Your count_flavors works fine, so we take the count_falvors() as is.
We will be making changes to the batch_count method now
FINAL SOLUTION
from collections import Counter
# Taking your method as is, to get the dictionary which counts
# the items occurence from your array
def count_flavors(l):
dict_flavors={}
for i in l:
dict_flavors[i] = l.count(i)
return dict_flavors
# This method will do your stuffs
def batch_count(b):
result = {} #this will be used to return the final result
# now just one loop, since we will passing the array
# to our method for computation count_flavors()
for items in b: # this will give out single array
'''
now we will call your count_flavor method
we will use Counter() to merge the dictionary data
coming from the count_flavor and then add it to the result
Counter() keep track of same item, if present in multiple
dict, ADDS +1 to the same item, doesn't duplicate value
Hence counter required
'''
if len(result) != 0:
# if the result is not empty, then result = result + data
result += Counter(count_flavors(items)) # no more extra for loop
else:
# else first fill the data by assigning it
result = Counter(count_flavors(items))
# this will give out the output in {}
# else the output will come in Counter({}) format
return dict(result)
# our test array of arrays
batches = [['A','A','A','A','B','B','B','B','B','C','C','C','C'], ['A', 'A', 'B', 'B' ,'B','B','C'], ['A','B','C','C']]
print('batch count 1:' + str(batch_count(batches)))
# OUTPUT
# >>> batch count 1:{'A': 7, 'B': 10, 'C': 7}
In this way you achieve the output with the usage of your count_flavors() method too, that too with no multiple loops in the batch_count(). Hope that will give you more clarity :). If this works out for you, you may accept the answer, for the the people who will come looking for answer to this question :)

Answer 2

The first function can become much faster by modifying your approach in this way:
def count_flavors(lst):
dict_flavors = {}
for item in lst:
if item in dict_flavors:
dict_flavors[item] += 1
else:
dict_flavors[item] = 1
return dict_flavors
You could also use Counter to simplify your code:
from collections import Counter
def count_flavors(lst):
return dict(Counter(lst))
The second function can use itertools.chain:
from collections import Counter
from itertools import chain
def batch_count(b):
return dict(Counter(chain(*b)))