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Python | Convert list into list of lists

  • Difficulty Level : Easy
  • Last Updated : 30 Jul, 2019

Given a list of strings, write a Python program to convert each element of the given list into a sublist. Thus, converting the whole list into a list of lists.

Examples:

Input : ['alice', 'bob', 'cara']
Output : [['alice'], ['bob'], ['cara']]

Input : [101, 202, 303, 404, 505] 
Output : [[101], [202], [303], [404], [505]]

 
Approach #1 : Naive Approach

Use another list ‘res’ and a for a loop. Using split() method of Python we extract each element from the list in the form of the list itself and append it to ‘res’. Finally, return ‘res’. One drawback of this method is that it does not work with integer list as ‘int’ object has no attribute ‘split’.




# Python3 program to convert 
# list into a list of lists
  
def extractDigits(lst):
    res = []
    for el in lst:
        sub = el.split(', ')
        res.append(sub)
      
    return(res)
                  
# Driver code
lst = ['alice', 'bob', 'cara']
print(extractDigits(lst))
Output:



[['alice'], ['bob'], ['cara']]

 
Approach #2 : List comprehension

List comprehension is an efficient approach as it doesn’t make use of extra space. For each element ‘el’ in list, it simply appends [el] to the output list.




# Python3 program to convert 
# list into a list of lists
  
def extractDigits(lst):
    return [[el] for el in lst]
                  
# Driver code
lst = ['alice', 'bob', 'cara']
print(extractDigits(lst))
Output:
[['alice'], ['bob'], ['cara']]

 
Approach #3 : Python map()

The given code maps the function el:[el] for each item of the given iterable ‘lst’. Hence outputs each element as a list itself.




# Python3 program to convert 
# list into a list of lists
  
def extractDigits(lst):
    return list(map(lambda el:[el], lst))
      
              
# Driver code
lst = ['alice', 'bob', 'cara']
print(extractDigits(lst))
Output:
[['alice'], ['bob'], ['cara']]

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