counting element occurrences
#import the Counter class from collections module
from collections import Counter
#An iterable with elements to count
data = 'aabbbccccdeefff'
#create the Counter object
c = Counter(data)
print(c)
#get the count of a specific element
print(c['f'])
The collections module in the standard library provide the Counter class that is a container which stores elements as dictionary keys, and their counts as dictionary values. The class is a subclass of the dict class and its objects shares a lot of functionality with the standard dictionaries.
from collections import Counter
print(issubclass(Counter, dict))Counter objects are typically used in situations where you need to keep track of how many of each element appears in an iterable object such as a list, set, tuple, e.t.c This can be the case for example when performing analysis on a dataset.
Instantiating Counter objects
Counter objects can be initialized by either passing in a sequence of data such as a list or tuple, or by manually setting the initial values.
Counter(iterable = None, **kwargs)The class only works with hashable elements, if the iterable contains any unhashable element, a TypeError will be raised.
Instantiate a Counter object from an iterable object.
from collections import Counter
data = ['Java', 'Python', 'Java', 'C++', 'Python', 'PHP', 'C++', 'Java', 'PHP', 'Java', 'Python']
c = Counter(data)
print(c)Alternatively we can populate the initial values at instantiation as shown below.
from collections import Counter
c = Counter({'Python': 10, 'Java': 8, 'C++': 12, 'PHP': 9})
print(c)Updating the dataset
The update method changes the dataset after the counter have been instantiated
Update the sequence after instantiation
from collections import Counter
c = Counter()
data = ['Python', 'C++', 'Java', 'Python', 'C++', 'Python']
#update the counter
c.update(data)
print(c)Counter methods
Since the Counter class is a subclass of the dict class, it inherits all of the methods of the dict class. It also defines other useful methods which includes:
| method | description |
|---|---|
most_common(n) |
Returns a list of tuples showing the n most commonly encountered elements and their respective counts. |
elements() |
Returns an iterator over the elements of the |
subtract(iterable = None, **kwargs) |
Subtracts counts of elements in the given iterable from the current counts. |
update(iterable = None, **kwargs) |
Updates the Counter with the elements from another iterable. |
Get the n most common elements
The most_common() method returns the most common elements as per the given integer argument. It returns a list containing tuples of the form (item, frequency) in descending order of frequency.
most_common(n)from collections import Counter
data = ['Java', 'Python', 'Java', 'C++', 'Python', 'PHP', 'C++', 'Java', 'PHP', 'Java', 'Python']
c = Counter(data)
print(c.most_common(3))In the above example, the most_common() method is used to get the 3 most common elements from the list.
Accessing Counts
Since Counter objects are a dictionaries we can use the dictionary way of accessing elements to access the count of a particular element, i.e counter['x'].
from collections import Counter
data = ['Java', 'Python', 'Java', 'C++', 'Python', 'PHP', 'C++', 'Java', 'PHP', 'Java', 'Python']
c = Counter(data)
print(c['Java'])We can also use the get() method for the same purpose.
from collections import Counter
data = ['Java', 'Python', 'Java', 'C++', 'Python', 'PHP', 'C++', 'Java', 'PHP', 'Java', 'Python']
c = Counter(data)
print(c.get('Python'))Operations on Counter objects
Counter objects support various arithmetic as well as set operations. Some of these operations are.
| operation | Description |
|---|---|
c1 + c2 |
You can add two Counter objects by using the + operator, resulting in a new Counter with the sum of the count of each element. |
c1 - c2 |
You can subtract two Counter objects by using the - operator. This will remove all elements found in c2 from c1. |
c1 & c2 |
The union of two Counters results in another Counter object with all elements of both c1 and c2, with the value/ count of each element being the sum ots count in c1 and c2. |
c1 | c2 |
Intersecting two Counters keeps only the elements that appear in both Counters. The resulting Counter will be the minimum of the two Counters. |
Addition
from collections import Counter
data1 = ['Java', 'Python', 'Java', 'C++', 'Python', 'PHP', 'C++', 'Java', 'PHP', 'Java', 'Python']
data2 = ['PHP', 'Python', 'Java', 'Python']
c1 = Counter(data1)
c2 = Counter(data2)
c3 = c1 + c2
print(c3)Intersection
from collections import Counter
data1 = ['Java', 'Python', 'Java', 'C++', 'Python', 'PHP', 'C++', 'Java', 'PHP', 'Java', 'Python']
data2 = ['PHP', 'Python', 'Java', 'Python']
c1 = Counter(data1)
c2 = Counter(data2)
c3 = c1 | c2
print(c3)