Numpy count items in array and store in a dictionary

Question

Lets say i have multiple numpy arrays like this: [1, 2, 4, 7, 1, 4, 6, 8, 1, 8, 2, 5]

I want to count the number of times each item appears in the array, and store the result in a dictionary:

{1: 3, 2: 2, 3: 0, 4: 2, 5: 1, 6: 1, 7: 1, 8: 2}

Is there a faster way to do this than to simply loop over the array and counting the items and storing them in dictionary?

Or maybe [this](https://stackoverflow.com/questions/2600191/how-can-i-count-the-occurrences-of-a-list-item)? — David Wierichs, Jun 22 '20 at 18:28
Does this answer your question? [How can I count the occurrences of a list item?](https://stackoverflow.com/questions/2600191/how-can-i-count-the-occurrences-of-a-list-item) — David Wierichs, Jun 22 '20 at 18:30

score 2 · Accepted Answer · answered Jun 22 '20 at 18:29

2

Numpy, in its wisdom, has functionality for this:

np.unique(x, return_counts=True)

It does not return a dictionary, but you can easily convert the result into one.

answered Jun 22 '20 at 18:29

Igor Rivin

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At least with this small example, `collections.Counter` is faster. – hpaulj Jun 22 '20 at 19:22

Pavan kumar · Answer 2 · 2020-06-22T18:45:17.740

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You can do that using count:

MyList = [1, 2, 4, 7, 1, 4, 6, 8, 1, 8, 2, 5]
my_dict = {i:MyList.count(i) for i in MyList}
print(my_dict)

this will definetely work you will find more information here

I think the below one is the easiest way

from collections import Counter
MyList = [1, 2, 4, 7, 1, 4, 6, 8, 1, 8, 2, 5]
print(Counter(MyList))

output would be Counter({1: 3, 2: 2, 4: 2, 8: 2, 7: 1, 6: 1, 5: 1})

edited Jun 22 '20 at 18:45

answered Jun 22 '20 at 18:33

Pavan kumar

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this does work. it is also _O(N^2)_ . So do not use this for sizeable lists – WestCoastProjects Jun 18 '21 at 12:29

score 0 · Answer 3 · answered Jun 22 '20 at 18:46

0

You can do this in pandas

>>> import pandas as pd
>>> a = pd.Series([1, 2, 4, 7, 1, 4, 6, 8, 1, 8, 2, 5])
>>> a.value_counts().to_dict()
{1: 3, 8: 2, 4: 2, 2: 2, 7: 1, 6: 1, 5: 1}

answered Jun 22 '20 at 18:46

Balaji Ambresh

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Numpy count items in array and store in a dictionary

3 Answers3