Python | Find the list elements starting with specific letter

Sometimes, we require to get the words that start with a specific letter. This kind of use case is quite common in places of common programming projects or competitive programming. Let’s discuss certain shorthands to deal with this problem in Python. 

Method #1: Using list comprehension + lower() This problem can be solved using the combination of the above two functions, list comprehension performs the task of extending the logic to whole list and lower function checks for case insensitivity with the target word of argument letter. 

Python3

# Python3 code to demonstrate
# Words starting with specific letter
# using list comprehension + lower()

# initializing list
test_list = ['Akash', 'Nikhil', 'Manjeet', 'akshat']

# initializing check letter
check = 'A'

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

# using list comprehension + lower()
# Words starting with specific letter
res = [idx for idx in test_list if idx[0].lower() == check.lower()]

# print result
print("The list of matching first letter : " + str(res))

Output

The original list : ['Akash', 'Nikhil', 'Manjeet', 'akshat']
The list of matching first letter : ['Akash', 'akshat']

Time complexity: O(n), where n is the length of the test_list. The  list comprehension + lower() takes O(n) time
Auxiliary Space: O(n), where n is the number of elements in the list test_list

Method #2: Using list comprehension + startswith() + lower() This method is similar to the above method but rather than checking for equality with operator, it checks using the startswith function which is inbuilt provided by python inbuilt library. 

Python3

# Python3 code to demonstrate
# Words starting with specific letter
# using list comprehension + startswith() + lower()

# initializing list
test_list = ['Akash', 'Nikhil', 'Manjeet', 'akshat']

# initializing check letter
check = 'A'

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

# using list comprehension + startswith() + lower()
# Words starting with specific letter
res = [idx for idx in test_list if idx.lower().startswith(check.lower())]

# print result
print("The list of matching first letter : " + str(res))

Output

The original list : ['Akash', 'Nikhil', 'Manjeet', 'akshat']
The list of matching first letter : ['Akash', 'akshat']

Time complexity: O(n), where n is the length of the test_list.
Auxiliary Space: O(n), where n is the number of elements in list res.

Method #3 : Using find() and lower() methods

Python3

# Python3 code to demonstrate
# Words starting with specific letter

# initializing list
test_list = ['Akash', 'Nikhil', 'Manjeet', 'akshat']

# initializing check letter
check = 'A'

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

# Words starting with specific letter
res = []
for i in test_list:
    if(i.find(check) == 0 or i.find(check.lower()) == 0):
        res.append(i)
# print result
print("The list of matching first letter : " + str(res))

Output

The original list : ['Akash', 'Nikhil', 'Manjeet', 'akshat']
The list of matching first letter : ['Akash', 'akshat']

Method #4 : Using filter(),lambda function and lower() methods

Python3

# Python3 code to demonstrate
# Words starting with specific letter
# using filter + lower()

# initializing list
test_list = ['Akash', 'Nikhil', 'Manjeet', 'akshat']

# initializing check letter
check = 'A'

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

# using filter + lambda function + lower()
# Words starting with specific letter
res = list(filter(lambda x: x[0].lower() == check.lower(), test_list))
# print result
print("The list of matching first letter : " + str(res))

Output

The original list : ['Akash', 'Nikhil', 'Manjeet', 'akshat']
The list of matching first letter : ['Akash', 'akshat']

Method #5: Using  re (regular expression)

The re (regular expression) approach to finding elements in a list that start with a specific letter involves using the re module in Python. This module allows you to use a special syntax called regular expressions to search for patterns in strings.

To use the re approach, you will first need to import the re module. Then, you can use the re.match() function to search for elements in the list that match a specific pattern. In this case, the pattern will be a regular expression that specifies that the element must start with the target letter.

Python3

# Python3 code to find elements in a
# list that start with a specific letter
import re

# Initialize the list of elements
test_list = ['Akash', 'Nikhil', 'Manjeet', 'akshat']

# Initialize the target letter
check = 'A'
check = check.lower()
# Print the original list
print("The original list : ", test_list)

# Use a regular expression to match elements that start with the target letter
res = [x for x in test_list if re.match("^{}".format(check), x.lower())]

# Print the result
print("The list of elements that start with '{}': {}".format(check, res))

Output

The original list :  ['Akash', 'Nikhil', 'Manjeet', 'akshat']
The list of elements that start with 'a': ['Akash', 'akshat']

Time complexity: O(n)
Auxiliary space: O(n)

Method #6: Using numpy.array() and numpy.char.startswith()

This method uses NumPy library to create an array of strings from the given list and then applies numpy.char.startswith() to find the elements starting with a specific letter.

Python3

#Python3 code to demonstrate
#Words starting with specific letter
#using numpy.array() and numpy.char.startswith()
import numpy as np

#initializing list
test_list = ['Akash', 'Nikhil', 'Manjeet', 'akshat']

#initializing check letter
check = 'A'

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

#create a numpy array from the list
np_arr = np.array(test_list)

#using numpy.char.startswith()
#Words starting with specific letter
res = list(np_arr[np.char.startswith(np_arr, check)])
res.extend(list(np_arr[np.char.startswith(np_arr, check.lower())]))
#print result
print("The list of matching first letter : " + str(res))

Output
The original list : [‘Akash’, ‘Nikhil’, ‘Manjeet’, ‘akshat’]
The list of matching first letter : [‘Akash’, ‘akshat’]

Time complexity: O(n), where n is the length of the test_list.
Auxiliary Space: O(n), where n is the length of the test_list.