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Python | Get positional characters from String

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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)

                    

Output
The 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))

                    

Output
The 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

                    

Output
The 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:

  1. Import the reduce() function from the functools module.
  2. Initialize the test_str and indx_list variables as before.
  3. Define a lambda function that takes two arguments: a string and an index. The lambda function returns
  4. the concatenation of the string with the character at the given index.
  5. Use the reduce() function to apply the lambda function to each element of the indx_list, starting with an empty string.
  6. 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)

                    

Output
Substring 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.



Last Updated : 23 Apr, 2023
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