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Python | Sort alternate numeric and alphabet list

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Sometimes, while performing sorting in list, we have a problem in which we need to perform particular type of sorting in which we need to sort in alternate ways in which we have numerics and alphabets sorted in order. Lets discuss certain ways in which this task can be performed. 

Method #1 : Using isalpha() + isnumeric() + zip_longest() The combination of above methods can be used to perform this task. In this, we separate the numeric and alphabets and then perform a sort on them separately and join using zip_longest(). 

Python3

# Python3 code to demonstrate
# Sort alternate numeric and alphabet list
# using isalpha() + isnumeric() + zip_longest()
from itertools import zip_longest
 
# Initializing list
test_list = ['3', 'B', '2', 'A', 'C', '1']
 
# printing original list
print("The original list is : " + str(test_list))
 
# Sort alternate numeric and alphabet list
# using isalpha() + isnumeric() + zip_longest()
num_list = sorted(filter(str.isnumeric, test_list),
                       key = lambda sub: int(sub))
 
chr_list = sorted(filter(str.isalpha, test_list))
res = [ele for sub in zip_longest(num_list, chr_list)
                              for ele in sub if ele]
     
# printing result
print ("List after performing sorting : " + str(res))

                    
Output : 
The original list is : ['3', 'B', '2', 'A', 'C', '1']
List after performing sorting : ['1', 'A', '2', 'B', '3', 'C']

  Method #2 : Using sorted() + key + lambda + isnumeric() The combination of above methods can be used to perform this task. In this, we perform the sorting in alternate manner using ord() and lambda function, testing using isnumeric(). 

Python3

# Python3 code to demonstrate
# Sort alternate numeric and alphabet list
# using sorted() + key + lambda + isnumeric()
from itertools import zip_longest
 
# Initializing list
test_list = ['3', 'B', '2', 'A', 'C', '1']
 
# printing original list
print("The original list is : " + str(test_list))
 
# Sort alternate numeric and alphabet list
# using sorted() + key + lambda + isnumeric()
res = sorted(test_list, key = lambda ele : (int(ele), 0)
      if ele.isnumeric()
      else ((ord(ele) - 64) % 26, 1))
 
# printing result
print ("List after performing sorting : " + str(res))

                    
Output : 
The original list is : ['3', 'B', '2', 'A', 'C', '1']
List after performing sorting : ['1', 'A', '2', 'B', '3', 'C']

Method #3: 

  • Split the list into two lists: one for digits and one for characters.
  • Sort the lists using the built-in sort() function.
  • Interleave the sorted lists and create the final result list.

Python3

# Python3 code to demonstrate
# Sort alternate numeric and alphabet list
 
# Initializing list
test_list = ['3', 'B', '2', 'A', 'C', '1']
 
# printing original list
print("The original list is : " + str(test_list))
 
# Split the list into digits and characters
digits = [x for x in test_list if x.isdigit()]
chars = [x for x in test_list if x.isalpha()]
 
# Sort the lists
digits.sort()
chars.sort()
 
# Interleave the sorted lists
res = [None] * len(test_list)
for i in range(len(test_list)):
    if i % 2 == 0:
        res[i] = digits[i//2]
    else:
        res[i] = chars[i//2]
 
# printing result
print ("List after performing sorting : " + str(res))

                    

Output
The original list is : ['3', 'B', '2', 'A', 'C', '1']
List after performing sorting : ['1', 'A', '2', 'B', '3', 'C']

Time complexity: O(nlogn)

Auxiliary Space:O(n)

Method #4: Using a custom sorting algorithm

  1. Define a function custom_sort that takes an element ele and returns a tuple (key, value) where key is a string representing the type of the element (digit or character) and value is the element itself.
  2. Use the sorted function to sort the list test_list using the custom_sort function as the key.
  3. Create two empty lists digits and chars.
  4. Iterate through the sorted list res and append the elements to either the digits list or the chars list, depending on their type.
  5. Use the zip function to combine the digits and chars lists into a single list.
  6. Use the itertools.chain.from_iterable function to flatten the combined list.
  7. Convert the flattened list back to a string using the join function.

Python3

import itertools
 
def custom_sort(ele):
    if ele.isnumeric():
        return ('digit', int(ele))
    else:
        return ('char', ele)
 
# Initializing list
test_list = ['3', 'B', '2', 'A', 'C', '1']
 
# printing original list
print("The original list is : " + str(test_list))
 
# Sort alternate numeric and alphabet list using custom sorting algorithm
res = sorted(test_list, key=custom_sort)
 
# Create two lists for digits and characters
digits = []
chars = []
 
# Append elements to either digits or chars list depending on their type
for ele in res:
    if ele.isnumeric():
        digits.append(ele)
    else:
        chars.append(ele)
 
# Combine digits and chars lists into a single list
combined = list(zip(digits, chars))
 
# Flatten the combined list
result = list(itertools.chain.from_iterable(combined))
 
# Convert the flattened list back to a string
result_str = ''.join(result)
 
# printing result
print("List after performing sorting : " + str(result))

                    

Output
The original list is : ['3', 'B', '2', 'A', 'C', '1']
List after performing sorting : ['1', 'A', '2', 'B', '3', 'C']

Time complexity: Sorting the list takes O(n log n) time using the sorted function, where n is the length of the list. Iterating through the sorted list takes O(n) time. Combining and flattening the lists takes O(n) time. Therefore, the overall time complexity is O(n log n).

Auxiliary space: We create two additional lists (digits and chars) and one additional tuple list (combined), each with a maximum size of n/2, where n is the length of the input list. Therefore, the auxiliary space complexity is O(n).



Last Updated : 22 Apr, 2023
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