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 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))

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']]

Time Complexity: O(n*n), where n is the length of the list test_list 
Auxiliary Space: O(n) additional space of size n is created where n is the number of elements in the res list 

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 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))

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:

# 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

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']].

Method #4 : Using for loop+while loop

1. Initiate a for loop and a while loop inside it, append each element of test_list to an empty list by each corresponding element of counter_list times and append this list to output list
2. Display output list

# Python3 code to demonstrate
# Custom length Matrix

# 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))

# Custom length Matrix
res = []
for i in range(0,len(test_list)):
    x=[]
    j=1
    while(j<=counter_list[i]):
        x.append(test_list[i])
        j+=1
    res.append(x)
        
# printing result
print ("The custom length matrix is : " + str(res))

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']]

Time Complexity : O(M*N) M - length of test_list N - length of counter_list
Auxiliary Space : O(M*N) M - length of test_list N - length of counter_list

Method #5: Using itertools.repeat() 

In this approach, we will use the itertools.repeat() to achieve this particular task. The itertools.repeat() function is used to repeat a particular element n times, where n is specified by the counter_list. 

Algorithm:

Initialize two empty lists, result and temp.
Iterate through each element in test_list.
Use itertools.repeat() to repeat the current element i in test_list j times, where j is the corresponding element in counter_list.
Convert the output of itertools.repeat() to a list using the list() function and append it to temp.
Append temp to result.
Print the original lists and the resulting matrix.

# Importing required module
import itertools

# 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))

# Initializing empty result list
result = []

# Iterate through each element in test_list
for i in test_list:
    # Repeat the current element i in test_list j times, where j is the corresponding element in counter_list
    temp = list(itertools.repeat(i, counter_list[test_list.index(i)]))
    # Append temp to result
    result.append(temp)
print("The custom length matrix is :", result)

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']]

Time Complexity: O(n^2), where n is the length of the list test_list. This is because we use itertools.repeat() and the list() function to repeat each element i in test_list j times, where j is the corresponding element in counter_list, resulting in a list of length j. Thus, the overall time complexity is O(n*n).

Auxiliary Space: O(n^2), where n is the length of the list test_list. This is because we create a list of lists containing the repeated elements, with a maximum length of n, resulting in a space complexity of O(n*n).

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