Python – Index Value repetition in List

Given a list of elements, The task is to write a Python program to repeat each index value as per the value in that index.

Input : test_list = [3, 0, 4, 2] 
Output : [0, 0, 0, 2, 2, 2, 2, 3, 3] 
Explanation : 0 is repeated 3 times as its index value is 3.

Input : test_list = [3, 4, 2] 
Output : [0, 0, 0, 1, 1, 1, 1, 2, 2] 
Explanation : 1 is repeated 4 times as its value is 4. 

Method #1: Using list comprehension + enumerate()

In this, we perform task of repetition using * operator, and enumerate() is used to get indices along with values for repetition. List comprehension is used to iteration all the elements.

Python3

# Python3 code to demonstrate working of
# Index Value repetition in List
# Using list comprehension + enumerate()

# initializing Matrix
test_list = [3, 0, 4, 2]

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

# enumerate() gets index and value of similar index element
res = [ele for sub in ([idx] * ele for idx,
                       ele in enumerate(test_list)) for ele in sub]

# printing result
print("Constructed List : " + str(res))

Output

The original list is : [3, 0, 4, 2]
Constructed List : [0, 0, 0, 2, 2, 2, 2, 3, 3]

Method #2: Using chain.from_iterable() + list comprehension

In this, we perform the last step of flattening of list using chain.from_iterable(). List comprehension performs the task of iteration of all the elements. 

Python3

# Python3 code to demonstrate working of
# Index Value repetition in List
# Using chain.from_iterable() + list comprehension
import itertools

# initializing Matrix
test_list = [3, 0, 4, 2]

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

# enumerate() gets index and
# value of similar index element
# from_iterable() used to flatten
res = list(itertools.chain(*([idx] * ele for idx,
                             ele in enumerate(test_list))))

# printing result
print("Constructed List : " + str(res))

Output

The original list is : [3, 0, 4, 2]
Constructed List : [0, 0, 0, 2, 2, 2, 2, 3, 3]

Method #3: Using extend() method

In this, we perform task of repetition using * operator, and add the elements using the extend() method in the new list.

Python3

# Python3 code to demonstrate working of
# Index Value repetition in List

# initializing Matrix
test_list = [3, 0, 4, 2]

# printing original list
print("The original list is : " + str(test_list))
res = []
for i in range(0, len(test_list)):
    res.extend([i]*test_list[i])

# printing result
print("Constructed List : " + str(res))

Output

The original list is : [3, 0, 4, 2]
Constructed List : [0, 0, 0, 2, 2, 2, 2, 3, 3]