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
test_str = "gfgisbest"
print ( "The original string is : " + test_str)
indx_list = [ 1 , 3 , 4 , 5 , 7 ]
res = ''
for ele in indx_list:
res = res + test_str[ele]
print ( "Substring of selective characters : " + res)
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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
test_str = "gfgisbest"
print ( "The original string is : " + test_str)
indx_list = [ 1 , 3 , 4 , 5 , 7 ]
res = ''.join((char for idx, char in enumerate (test_str) if idx in indx_list))
print ( "Substring of selective characters : " + res)
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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
test_str = "gfgisbest"
print ( "The original string is : " + test_str)
indx_list = [ 1 , 3 , 4 , 5 , 7 ]
res = ''
for i in range ( 0 , len (test_str)):
if i in indx_list:
res + = test_str[i]
print ( "Substring of selective characters : " + res)
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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
test_str = "gfgisbest"
print ( "The original string is : " + test_str)
indx_list = [ 1 , 3 , 4 , 5 , 7 ]
print ( "Substring of selective characters : " , end = ' ' )
resultstring = ""
reslist = [resultstring + test_str[i] for i in indx_list]
print (''.join(reslist))
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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
test_str = "gfgisbest"
print ( "The original string is : " + test_str)
indx_list = [ 1 , 3 , 4 , 5 , 7 ]
res = ''.join( list ( map ( lambda x: test_str[x], indx_list)))
print ( "Substring of selective characters : " + res)
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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:
- 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
test_str = "gfgisbest"
indx_list = [ 1 , 3 , 4 , 5 , 7 ]
f = lambda x, i: x + test_str[i]
res = reduce (f, indx_list, "")
print ( "Substring of selective characters : " + res)
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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.
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