Python - Merge a Matrix by the Elements of First Column

When it is required to merge a matrix by the elements of first column, a simple iteration and list comprehension with setdefault method is used. This technique groups rows that have the same value in their first column.

Example

Below is a demonstration of the same ?

my_list = [[41, "python"], [13, "pyt"], [41, "is"], [4, "always"], [3, "fun"]]

print("The list is :")
print(my_list)

my_result = {}
for key, value in my_list:
    my_result.setdefault(key, []).append(value)

my_result = [[key] + value for key, value in my_result.items()]

print("The result is :")
print(my_result)

The list is :
[[41, 'python'], [13, 'pyt'], [41, 'is'], [4, 'always'], [3, 'fun']]
The result is :
[[41, 'python', 'is'], [13, 'pyt'], [4, 'always'], [3, 'fun']]

How It Works

The algorithm works in two main steps:

1. Group by first column: Use setdefault() to create a dictionary where keys are first column values and values are lists of corresponding second column values.
2. Reconstruct matrix: Convert the dictionary back to a matrix format where each row starts with the key followed by all its associated values.

Alternative Using defaultdict

You can also achieve this using defaultdict from the collections module ?

from collections import defaultdict

my_list = [[41, "python"], [13, "pyt"], [41, "is"], [4, "always"], [3, "fun"]]

print("The list is :")
print(my_list)

my_result = defaultdict(list)
for key, value in my_list:
    my_result[key].append(value)

my_result = [[key] + value for key, value in my_result.items()]

print("The result is :")
print(my_result)

The list is :
[[41, 'python'], [13, 'pyt'], [41, 'is'], [4, 'always'], [3, 'fun']]
The result is :
[[41, 'python', 'is'], [13, 'pyt'], [4, 'always'], [3, 'fun']]

Conclusion

Matrix merging by first column efficiently groups rows with identical first elements. Use setdefault() for simple cases or defaultdict for cleaner code when working with grouped data structures.