How to use reversed() function and index() In Python

Step-by-step approach:

Reverse the test_string and tar_word
Use the index() method to get the first occurrence of the reversed tar_word in the reversed test_string.
Calculate the index of the last occurrence of the tar_word by subtracting the index from the length of the test_string and the length of the tar_word.

Python3

#initializing string
test_string = "GfG is best for CS and also best for Learning"
 
#initializing target word
tar_word = "best"
 
#using index() and slicing
#Find last occurrence of substring
res = len(test_string) - test_string[::-1].index(tar_word[::-1]) - len(tar_word)
 
#print result
print("Index of last occurrence of substring is : " + str(res))

Output

Index of last occurrence of substring is : 28

Time complexity: O(n) where n is the length of the string
Space complexity: O(1)

Using the numpy:

Algorithm:

Initialize the test string and target word.
Create an empty numpy array to store the indices of all occurrences of the target word.
Iterate over all possible substrings of length equal to the length of the target word in the test string. For each substring, check if it matches the target word or not. If the substring matches the target word, append the index of the first character of the substring to the numpy array.
Check if any occurrences of the target word were found or not. If the numpy array is not empty, set the result to be the last element of the array (i.e., the index of the last occurrence of the target word in the test string). Otherwise, set the result to -1 (indicating that the target word was not found in the test string).
Print the original string and the index of the last occurrence of the target word (or -1 if the target word was not found).

Below is the implementation of the above approach:

Python3

# Python program for the above approach
 
import numpy as np
 
# Driver Code
test_string = "GfG is best for CS and also best for Learning"
tar_word = "best"
 
# find the indices of all occurrences
# of the substring
indices = np.array([i for i in range(len(test_string)-len(tar_word)+1)
                    if test_string[i:i+len(tar_word)] == tar_word])
 
# check if any occurrences were found
# and get the last index
if indices.size > 0:
    res = indices[-1]
else:
    res = -1
 
print("The original string : " + str(test_string))
print("Index of last occurrence of substring is : " + str(res))
 
# This code is contributed by Jyothi pinjala

Output:

The original string : GfG is best for CS and also best for Learning
Index of last occurrence of substring is : 28

Time Complexity: O(n * m), where n is the length of the test string and m is the length of the target word. This is because the list comprehension iterates over all possible substrings of length m in the test string, which takes O(n * m) time in the worst case.

Space Complexity: O(k), where k is the number of occurrences of the target word in the test string. This is because the indices of all occurrences of the target word are stored in a numpy array, which takes O(k) space. The rest of the variables used in the code take constant space.

Using rpartition() method

Define the string and target word to find last occurrence of substring.
Use the rpartition() method to split the string at the last occurrence of the target word.
If the length of split list is 1, print that the substring was not found in the string.
Otherwise, calculate the index of the last occurrence of the target word using the length of the original string, the length of the third element in the split list (which contains the characters after the last occurrence of the target word), and the length of the target word.
Print the index of the last occurrence of the target word.

Python3

# initializing a string
test_string = "GfG is best for CS and also best for Learning"
tar_word = "best"
 
# printing original string
print("The original string : " + str(test_string))
 
# split the string into 3 parts, the last part being the target word
split_list = test_string.rpartition(tar_word)
 
# if the target word is not found in the string, print 
if len(split_list) == 1:
   print("Substring not found in string")
else:
   # calculate the index of the last occurrence of the target word
   res = len(test_string) - len(split_list[2]) - len(tar_word)
   print("Index of last occurrence of substring is : " + str(res))

Output

The original string : GfG is best for CS and also best for Learning
Index of last occurrence of substring is : 28

Time Complexity: O(n), where n is the length of the input string.

Auxiliary Space: O(1), as no extra is used.

Python | Find last occurrence of substring

Sometimes, while working with strings, we need to find if a substring exists in the string. This problem is quite common and its solution has been discussed many times before. The variation of getting the last occurrence of the string is discussed here. Let’s discuss certain ways in which we can find the last occurrence of substring in string in Python.

Tags:

#Python string-programs #Python #Python Programs #python

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