Python | Custom length Matrix

Sometimes, we need to initialize a matrix in Python of variable length from the list containing elements. In this article, we will discuss the variable length method initialization and certain shorthands to do so. Let’s discuss certain ways to perform this. Method #1 : Using zip() + list comprehension The zip function combined with the list comprehension can help to achieve this particular task. The zip function can help to zip the counter list with the element list and list comprehension does the work of construction of matrix. 

Python3

# Python3 code to demonstrate
# Custom length Matrix
# using zip() + list comprehension

# initializing list
test_list = ['a', 'b', 'c']

# initializing counter list
counter_list = [1, 4, 2]

# printing original list
print ("The original list is : " + str(test_list))

# printing counter list
print ("The counter list is : " + str(counter_list))

# using zip() + list comprehension
# Custom length Matrix
res = [[i] * j for i, j in zip(test_list, counter_list)]

# printing result
print ("The custom length matrix is : " + str(res))

Output : 

The original list is : ['a', 'b', 'c']
The counter list is : [1, 4, 2]
The custom length matrix is : [['a'], ['b', 'b', 'b', 'b'], ['c', 'c']]

Method #2 : Using map() + mul operator This particular problem can also be solved using the inbuilt mul operator which performs multiplication of liked index elements and map function performs the task of formation of matrix. 

Python3

# Python3 code to demonstrate
# Custom length Matrix
# using map() + mul operator
from operator import mul

# initializing list
test_list = ['a', 'b', 'c']

# initializing counter list
counter_list = [1, 4, 2]

# printing original list
print ("The original list is : " + str(test_list))

# printing counter list
print ("The counter list is : " + str(counter_list))

# using map() + mul operator
# Custom length Matrix
res = list(map(mul, [['a'], ['b'], ['c']], counter_list))

# printing result
print ("The custom length matrix is : " + str(res))

Output : 

The original list is : ['a', 'b', 'c']
The counter list is : [1, 4, 2]
The custom length matrix is : [['a'], ['b', 'b', 'b', 'b'], ['c', 'c']]

Method #3 : Using looping + *:  Here is another approach using a for loop and the * operator:

Python3

# Initialize the lists
test_list = ['a', 'b', 'c']
counter_list = [1, 4, 2]

# Initialize the result list
result = []

# Iterate through the lists
for i, j in zip(test_list, counter_list):
    # Append the element from test_list repeated j times to the result list
    result.append([i] * j)

# Print the original lists
print("The original list is :", test_list)
print("The counter list is :", counter_list)

# Print the result
print("The custom length matrix is :", result)
#This code is contributed by Edula Vinay Kumar Reddy

Output

The original list is : ['a', 'b', 'c']
The counter list is : [1, 4, 2]
The custom length matrix is : [['a'], ['b', 'b', 'b', 'b'], ['c', 'c']]

This approach iterates through the test_list and counter_list using the zip() function, and for each element and its corresponding count, it appends a list containing the element repeated j times to the result list using the * operator.

For example, given the lists [‘a’, ‘b’, ‘c’] and [1, 4, 2], the resulting list would be [[‘a’], [‘b’, ‘b’, ‘b’, ‘b’], [‘c’, ‘c’]].