Better way to iterate through list and store output in a new list-CodePudding

I am a beginner in python and I would like your opinion on a more sophisticated way to write the code that follows. I want the code to iterate through the whole list (x) of items, count how many times each item appears and store the output in a new list (z). Here is the code (it works as it is) :

x = ["apple", "orange", "cherry", "apple"]

def new_list(a):
    z=[]
    for i in x:
        y = x.count(i)
        (z.append(y))
    return z

print(new_list(x))

CodePudding user response：

I would suggest to do a list comprehension:

>>> x = ["apple", "orange", "cherry", "apple"]
>>> [x.count(val) for val in x]
[2, 1, 1, 2]
>>>

Or even better with map:

>>> [*map(x.count, x)]
[2, 1, 1, 2]
>>>

CodePudding user response：

With using list.count() order of your programming is O(n^2) and with Counter and hashmap order of your programmer is O(n). Try this:

x = ["apple", "orange", "cherry", "apple"]

[x.count(i) for i in x]
# [2,1,1,2]

Or use Counter:

from collections import Counter
dct = Counter(x)
# Counter({'apple': 2, 'orange': 1, 'cherry': 1})

[dct[i] for i in x]
# [2, 1, 1, 2]

Check Runtime: (Counter is faster than list.count())

CodePudding user response：

You can use the Counter class in collections. So iterate over input list and then find count of every element.

from collections import Counter
x = ["apple", "orange", "cherry", "apple"]
c = Counter(x)
print([c[item] for item in x])
#[2, 1, 1, 2]

CodePudding user response：

Try using the Counter class:

From collections import Counter
x = ["foo", "bar", "foo", "foo"];
z = Counter(x)
print(z)
# Output: {"foo": 3, "bar": 1}

CodePudding user response：

You can use the Counter subclass of the collections module:

from collections import Counter

x = ["apple", "orange", "cherry", "apple"]


counter = Counter(x)
z = list(counter.values())

print(z) #[2,1,1]

I didn't use count because it may not be good for performance which is also pointed out by this answer.