Python: Count of elements in each group from list based groups of elements

Last update on April 19 2025 12:58:05 (UTC/GMT +8 hours)

Group List by Function and Count

Write a Python program to group the elements of a list based on the given function and return the count of elements in each group.

Use collections.defaultdict to initialize a dictionary.
Use map() to map the values of the given list using the given function./li>
Iterate over the map and increase the element count each time it occurs.

Sample Solution:

Python Code:

# Import the 'defaultdict' class from the 'collections' module.
from collections import defaultdict

# Define a function called 'count_by' that counts occurrences in a list based on a provided function.
def count_by(lst, fn=lambda x: x):
    # Create a defaultdict to store the count of each value.
    count = defaultdict(int)
    
    # Iterate over the values in the list after applying the provided function.
    for val in map(fn, lst):
        # Increment the count for the current value.
        count[val] += 1
    
    # Convert the 'count' defaultdict into a regular dictionary and return it.
    return dict(count)

# Import the 'floor' function from the 'math' module.
from math import floor

# Print the result of counting occurrences in a list of floating-point numbers using the 'floor' function.
print(count_by([6.1, 4.2, 6.3], floor)) 

# Print the result of counting occurrences in a list of strings using the 'len' function.
print(count_by(['one', 'two', 'three'], len))

Sample Output:

{6: 2, 4: 1}
{3: 2, 5: 1}

Flowchart:

For more Practice: Solve these Related Problems:

Write a Python program to group list elements by their remainder when divided by a specified number and count the occurrences.
Write a Python program to group a list of strings by their first character and return the count for each group.
Write a Python program to group numbers by whether they are prime or composite and count the members in each group.
Write a Python program to group a list of integers into even and odd categories and return a dictionary with their counts.

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