Python | Get positional characters from String
Last Updated :
23 Apr, 2023
Sometimes, while working with Python strings, we can have a problem in which we need to create a substring by joining the particular index elements from a string. Let's discuss certain ways in which this task can be performed.
Method #1: Using loop This is a brute method in which this task can be performed. In this, we run a loop over the indices list and join the corresponding index characters from string.
Python3
# Python3 code to demonstrate working of
# Get positional characters from String
# using loop
# initializing string
test_str = "gfgisbest"
# printing original string
print("The original string is : " + test_str)
# initializing index list
indx_list = [1, 3, 4, 5, 7]
# Get positional characters from String
# using loop
res = ''
for ele in indx_list:
res = res + test_str[ele]
# printing result
print("Substring of selective characters : " + res)
Output : The original string is : gfgisbest
Substring of selective characters : fisbs
Time Complexity: O(n)
Auxiliary Space: O(n)
Method #2: Using generator expression + enumerate() The combination of above functionalities can be used to perform this task. In this, we run a loop using generator expression and indices extraction is done with help of enumerate().
Python3
# Python3 code to demonstrate working of
# Get positional characters from String
# using generator expression + enumerate()
# initializing string
test_str = "gfgisbest"
# printing original string
print("The original string is : " + test_str)
# initializing index list
indx_list = [1, 3, 4, 5, 7]
# Get positional characters from String
# using generator expression + enumerate()
res = ''.join((char for idx, char in enumerate(test_str) if idx in indx_list))
# printing result
print("Substring of selective characters : " + res)
Output : The original string is : gfgisbest
Substring of selective characters : fisbs
Time Complexity: O(n)
Auxiliary Space: O(n)
Method #3: Using for loop
Python3
# Python3 code to demonstrate working of
# Get positional characters from String
# using loop
# initializing string
test_str = "gfgisbest"
# printing original string
print("The original string is : " + test_str)
# initializing index list
indx_list = [1, 3, 4, 5, 7]
# Get positional characters from String
# using loop
res = ''
for i in range(0, len(test_str)):
if i in indx_list:
res += test_str[i]
# printing result
print("Substring of selective characters : " + res)
OutputThe original string is : gfgisbest
Substring of selective characters : fisbs
Time Complexity: O(n)
Auxiliary Space: O(n)
Method #4: Using for list comprehension
Python3
# Python3 code to demonstrate working of
# Get positional characters from String
# initializing string
test_str = "gfgisbest"
# printing original string
print("The original string is : " + test_str)
# initializing index list
indx_list = [1, 3, 4, 5, 7]
# printing result
print("Substring of selective characters : ", end=' ')
resultstring = ""
reslist = [resultstring+test_str[i] for i in indx_list]
print(''.join(reslist))
OutputThe original string is : gfgisbest
Substring of selective characters : fisbs
Time Complexity: O(n)
Auxiliary Space: O(n)
Method #5: Using map() and lambda function
This method uses the map() function and a lambda function to extract the characters at the specified indices and join them to create the substring.
Python3
# Python3 code to demonstrate working of
# Get positional characters from String
# using map() and lambda function
# initializing string
test_str = "gfgisbest"
# printing original string
print("The original string is : " + test_str)
# initializing index list
indx_list = [1, 3, 4, 5, 7]
# Get positional characters from String
# using map() and lambda function
res = ''.join(list(map(lambda x: test_str[x], indx_list)))
# printing result
print("Substring of selective characters : " + res)
#This code is contributed by Edula Vinay Kumar Reddy
OutputThe original string is : gfgisbest
Substring of selective characters : fisbs
Time Complexity: O(n) where n is the number of elements in the index list
Auxiliary Space: O(n) as it uses a list to store the characters at the specified indices.
Method 6 : using the reduce() function from the functools module.
step by step:
- Import the reduce() function from the functools module.
- Initialize the test_str and indx_list variables as before.
- Define a lambda function that takes two arguments: a string and an index. The lambda function returns
- the concatenation of the string with the character at the given index.
- Use the reduce() function to apply the lambda function to each element of the indx_list, starting with an empty string.
- Print the result:
Python3
from functools import reduce
# initializing string
test_str = "gfgisbest"
# initializing index list
indx_list = [1, 3, 4, 5, 7]
# define lambda function
f = lambda x, i: x + test_str[i]
# apply lambda function using reduce
res = reduce(f, indx_list, "")
# printing result
print("Substring of selective characters : " + res)
OutputSubstring of selective characters : fisbs
The time complexity of this approach is O(n), where n is the length of the indx_list.
The auxiliary space complexity is O(1), as we are not using any extra space other than the input and output variables.
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