CODEWITHSUNDEEP
python
Rachid O
Rachid O

Reputation: 14002

python count list item occurences and put result in list

I have a list like this 

L=['d','f','d','c','c','f','d','f']

and i would like to count how many d,f and c occurences are in L and store the result like:

R=[['d',3],['f',3],['c',2]]

What is the best approch (algorithm) ?

Upvotes: 2

Views: 147

Answers (4)

Abhijit
Abhijit

Reputation: 63737

A possible solution using itertools.groupby on a sorted data

Implementation

from itertools import groupby
[[k,  len(list(v))] for k, v in groupby(sorted(L))]

Output

[['c', 2], ['d', 3], ['f', 3]]

Performance Comparison

    In [9]: L = [choice(ascii_letters) for _ in range(1000)]

    In [10]: %timeit [[k,  len(list(v))] for k, v in groupby(sorted(L))]
    1000 loops, best of 3: 271 us per loop

    In [11]: %timeit Counter(L).items()
    1000 loops, best of 3: 306 us per loop

Note

It should be noted that the overhead in the Counter Solution in hashing the data, overshoots the Sorting Complexity in Tim's Sort

Upvotes: 1

thefourtheye
thefourtheye

Reputation: 239473

L=['d','f','d','c','c','f','d','f']
from collections import Counter
print Counter(L)

Output

Counter({'d': 3, 'f': 3, 'c': 2})

You can use Counter.most_common method to get the result like this

print Counter(L).most_common()

Output

[('d', 3), ('f', 3), ('c', 2)]

Upvotes: 1

Nafiul Islam
Nafiul Islam

Reputation: 82470

I feel a dictionary would be better for this:

>>> from collections import Counter
>>> L = ['d','f','d','c','c','f','d','f']
>>> Counter(L)
Counter({'d': 3, 'f': 3, 'c': 2})

However, if you're adamant about a list of lists:

>>> L=['d','f','d','c','c','f','d','f']
>>> from collections import Counter
>>> var = Counter(L)
>>> [[key, value] for key, value in var.items()]
[['c', 2], ['d', 3], ['f', 3]]

Upvotes: 2

mhlester
mhlester

Reputation: 23231

The best approach (algorithm), is to not do it yourself!

>>> from collections import Counter
>>> L=['d','f','d','c','c','f','d','f']
>>> Counter(L)
Counter({'d': 3, 'f': 3, 'c': 2})

If you insist on a list:

>>> Counter(L).items()
[('c', 2), ('d', 3), ('f', 3)]

Upvotes: 5

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