How to loop and make array object in python?

Question

I want to make array object like this:

{'book1':3, 'book2':4, 'book3':5}

from an array like this

['book1', 'book1', 'book1', 'book2', 'book2', 'book2', 
'book2', 'book3', 'book3', 'book3', 'book3', 'book3']

How to do that? my idea is looping, but dont know to count the same value

*sorry for bad explanation

In Python that first data structure is called a [dictionary](https://docs.python.org/3/tutorial/datastructures.html#dictionaries). Looping is certainly a valid way to accomplish what you want to do. I'd suggest reading up on dictionaries, trying it out, and updating this with your code if you have a specific question or problem. Note that it's not necessary but using [`defaultdict`](https://docs.python.org/3/library/collections.html#collections.defaultdict) can provide a concise solution. — kungphu, Apr 20 '19 at 04:39

score 2 · Answer 1 · answered Apr 20 '19 at 04:46

collections.Counter is a handy builtin for this exact task:

from collections import Counter

lst = ['book1', 'book1', 'book1', 'book2', 'book2', 'book2', 'book2', 'book3', 'book3', 'book3', 'book3', 'book3']
print(dict(Counter(lst)))

Output:

{'book1': 3, 'book2': 4, 'book3': 5}

score 0 · Answer 2 · answered Apr 20 '19 at 04:40

You could do something like this:

arr = ['book1', 'book1', 'book1', 'book2', 'book2', 'book2', 'book2', 'book3', 'book3', 'book3', 'book3', 'book3']

final = {}

# loop through each item in the array
# if it is NOT found in the dictionary, put it
# in the dictionary and give it a count of 1
# if it IS found in the dictionary, increment its value
for x in arr:
    if x in final:
        final[x] += 1
    else:
        final[x] = 1

print(final)

score 0 · Answer 3 · answered Apr 20 '19 at 04:47

You can also use collections.Counter, which is made for exactly such thing.

from collections import Counter

li =['book1', 'book1', 'book1', 'book2', 'book2', 'book2', 'book2', 'book3', 'book3', 'book3', 'book3', 'book3']
print(dict(Counter(li)))
#{'book1': 3, 'book2': 4, 'book3': 5}

How to loop and make array object in python?

3 Answers3