Python | Filter String with substring at specific position

Sometimes, while working with Python string lists, we can have a problem in which we need to extract only those lists that have a specific substring at a specific position. This kind of problem can come in data processing and web development domains. Let us discuss certain ways in which this task can be performed. 

Method #1: Using list comprehension + list slicing The combination of the above functionalities can be used to perform this particular task. In this, we check for substring range using list slicing, and the task of extraction list is compiled in list comprehension. 

Python3

# Python3 code to demonstrate
# Filter String with substring at specific position
# using list comprehension + list slicing

# Initializing list
test_list = ['geeksforgeeks', 'is', 'best', 'for', 'geeks']

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

# Initializing substring
sub_str = 'geeks'

# Initializing range
i, j = 0, 5

# Filter String with substring at specific position
# using list comprehension + list slicing
res = [ele for ele in test_list if ele[i: j] == sub_str]

# printing result
print ("Filtered list : " + str(res))

Output : 

The original list is : ['geeksforgeeks', 'is', 'best', 'for', 'geeks']
Filtered list : ['geeksforgeeks', 'geeks']

Time Complexity: O(n), where n is the length of the input list. This is because we’re using the list comprehension + list slicing which has a time complexity of O(n) in the worst case.
Auxiliary Space: O(n), as we’re using additional space res other than the input list itself with the same size of input list. 

Method #2 : Using filter() + lambda The combination of above methods can be used to perform this task. In this, we filter the elements using logic compiled using lambda using filter(). 

Python3

# Python3 code to demonstrate
# Filter String with substring at specific position
# using filter() + lambda

# Initializing list
test_list = ['geeksforgeeks', 'is', 'best', 'for', 'geeks']

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

# Initializing substring
sub_str = 'geeks'

# Initializing range
i, j = 0, 5

# Filter String with substring at specific position
# using filter() + lambda
res = list(filter(lambda ele: ele[i: j] == sub_str, test_list))

# printing result
print ("Filtered list : " + str(res))

Output : 

The original list is : ['geeksforgeeks', 'is', 'best', 'for', 'geeks']
Filtered list : ['geeksforgeeks', 'geeks']

Time Complexity: O(n*m), where n is the length of the input list and m is the length of the substring

Auxiliary Space: O(k), where k is the number of elements that meet

Method #3: Using find() method

Python3

# Python3 code to demonstrate
# Filter String with substring at specific position

# Initializing list
test_list = ['geeksforgeeks', 'is', 'best', 'for', 'geeks']

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

# Initializing substring
sub_str = 'geeks'

# Initializing range
i, j = 0, 5

# Filter String with substring at specific position
res=[]
for k in test_list:
    if(k.find(sub_str)==i):
        res.append(k)


# printing result
print ("Filtered list : " + str(res))

Output

The original list is : ['geeksforgeeks', 'is', 'best', 'for', 'geeks']
Filtered list : ['geeksforgeeks', 'geeks']

Method 4: Using RegEx

Python3

import re

#Initializing list
test_list = ['geeksforgeeks', 'is', 'best', 'for', 'geeks']

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

#Initializing substring
sub_str = 'geeks'

#Initializing range
i, j = 0, 5

#Filter String with substring at specific position
#using re.match()
res = [ele for ele in test_list if re.match(r"^{}.*".format(sub_str), ele[i: j])]

#printing result
print ("Filtered list : " + str(res))
#This code is contributed by Edula Vinay Kumar Reddy

Output

The original list is : ['geeksforgeeks', 'is', 'best', 'for', 'geeks']
Filtered list : ['geeksforgeeks', 'geeks']

Time complexity: O(n) where n is the length of the list.
Auxiliary Space: O(m) where m is the length of the longest string in the list.

Method 5: Using itertools.islice() and str.startswith():

Algorithm :

1. Initialize a list of strings.
2. Initialize the substring and the range for the substring to check.
3. Use a for loop to iterate through each element in the input list.
4. For each element, use the startswith() method to check if the substring is present at the specified range.
5. If the substring is present at the specified range, add the element to a new list.
6. Return the new list containing the filtered elements.

Python3

import itertools

#Initializing list
test_list = ['geeksforgeeks', 'is', 'best', 'for', 'geeks']

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

#Initializing substring
sub_str = 'geeks'

#Initializing range
i, j = 0, 5

#Filter String with substring at specific position
res = [ele for ele in test_list if ele.startswith(sub_str, i, j)]

#printing result
print ("Filtered list : " + str(res))

#This code is contributed by Jyothi pinjala.

Output

The original list is : ['geeksforgeeks', 'is', 'best', 'for', 'geeks']
Filtered list : ['geeksforgeeks', 'geeks']

The time complexity : O(n * k), where n is the length of the input list and k is the length of the substring. This is because for each element in the list, we need to check if the substring is present at a specific position, which takes O(k) time.

The auxiliary space : O(m), where m is the length of the resulting filtered list. This is because we are creating a new list to store the filtered elements. The space complexity of the itertools.islice() method is also O(m), as it returns an iterator object that generates the filtered elements on-the-fly without creating a new list.