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Python – Group list by first character of string

  • Last Updated : 30 Jan, 2020

Sometimes, we have a use case in which we need to perform the grouping of strings by various factors, like first letter or any other factor. These type of problems are typical to database queries and hence can occur in web development while programming. This article focuses on one such grouping by first letter of string. Let’s discuss certain ways in which this can be performed.

Method #1 : Using next() + lambda + loop
The combination of above 3 functions is used to solve this particular problem by the naive method. The lambda function performs the task of finding like initial character, and next function helps in forward iteration.




# Python3 code to demonstrate
# Initial Character Case Categorization
# using next() + lambda + loop
  
# initializing list
test_list = ['an', 'a', 'geek', 'for', 'g', 'free']
  
# printing original list
print("The original list : " + str(test_list))
  
# using next() + lambda + loop
# Initial Character Case Categorization
util_func = lambda x, y: x[0] == y[0]
res = []
for sub in test_list:
    ele = next((x for x in res if util_func(sub, x[0])), [])
    if ele == []:
        res.append(ele)
    ele.append(sub)
  
# print result
print("The list after Categorization : " + str(res))
Output :
The original list : ['an', 'a', 'geek', 'for', 'g', 'free']
The list after Categorization : [['an', 'a'], ['geek', 'g'], ['for', 'free']]

 

Method #2 : Using sorted() + groupby()
This particular task can also be solved using the groupby function which offers a conventional method to solve this problem. The sorted function sorts the elements by initial character to be feed to groupby for the relevant grouping.






# Python3 code to demonstrate
# Initial Character Case Categorization
# using sorted() + groupby()
from itertools import groupby
  
# initializing list
test_list = ['an', 'a', 'geek', 'for', 'g', 'free']
  
# printing original list
print("The original list : " + str(test_list))
  
# using sorted() + groupby()
# Initial Character Case Categorization
util_func = lambda x: x[0]
temp = sorted(test_list, key = util_func)
res = [list(ele) for i, ele in groupby(temp, util_func)]
  
# print result
print("The list after Categorization : " + str(res))
Output :
The original list : ['an', 'a', 'geek', 'for', 'g', 'free']
The list after Categorization : [['an', 'a'], ['geek', 'g'], ['for', 'free']]

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