Python – Remove all consonants from string

Sometimes, while working with Python, we can have a problem in which we wish to remove all the non vowels from strings. This is quite popular question and solution to it is useful in competitive programming and day-day programming. Lets discuss certain ways in which this task can be performed.
Method #1 : Using loop 
This is one of the ways in which this task can be performed. In this, we iterate through the list and then check for non presence of vowels and filter.
 

The original string is : Gfg is best for Lazyroar
String after consonants removal : ieoee

 
Method #2 : Using list comprehension 
This is one of the ways in which this task can be performed. In this, we iterate through the list and then filter out vowels in similar manner but in one-liner.
 

The original string is : Gfg is best for Lazyroar
String after consonants removal : ieoee

The Time and Space complexity for all methods are the same:

Time Complexity: O(n)

Space Complexity: O(n)

Method #3 : Using replace() method
There is one more way, while iterating through the string if consonant occur the we will replace that char with empty char.

The original string is : Gfg is best for Lazyroar
String after consonants removal : ieoee

Method #4 : Using operator.countOf() method

The original string is : Gfg is best for Lazyroar
String after consonants removal : ieoee

Time Complexity: O(n)
Auxiliary Space: O(n)

Method #5 : Using re module (regular expressions)

The original string is : Gfg is best for Lazyroar
String after sentants removal : ieoee

Time Complexity: O(n)
Auxiliary Space: O(n)

Method #6 : Using ord()+for loop

The original string is : Gfg is best for Lazyroar
String after consonants removal : ieoee

Time Complexity: O(n)
Auxiliary Space: O(n)

Method #7: Using filter() and lambda function

In this method, we can use the built-in filter() function with a lambda function to filter out all the consonants in the string.

Step-by-step approach:

1. Initialize the string to be processed.
2. Define a lambda function that will filter out all the consonants from the string. The lambda function should take one argument (a character), and it should return True if the character is a vowel and False otherwise.
3. Use the filter() function with the lambda function and the string to create a filter object that contains only the vowels.
4. Convert the filter object to a string using the join() method to get the final output string.

The original string is : Gfg is best for Lazyroar
String after consonants removal : ieoee

Time complexity: O(n)
Auxiliary space: O(n)

Method #8: Using numpy:

Algorithm:

1. Initialize the string to be processed.
2. Use numpy to convert the string to a numpy array of integers, using the ‘fromstring’ method.
3. Define a boolean mask to identify vowels, using the numpy ‘isin’ method.
4. Use the boolean mask to filter the array to retain only vowels.
5. Use numpy ‘chararray’ method to convert the numpy array back to a string.

Output:
The original string is : Gfg is best for Lazyroar
String after consonants removal : ieoee

Time Complexity:

The code involves creating two NumPy arrays, which takes O(n) time, where n is the length of the input string.
The operation of selecting only the vowels involves a comparison between two arrays, which takes O(n) time as well.
The final step of converting the array back to a string takes O(n) time too.
Therefore, the total time complexity of the code is O(n).
Space Complexity:

The code creates two NumPy arrays, which take O(n) space, where n is the length of the input string.
There are no other significant memory allocations in the code.
Therefore, the total space complexity of the code is also O(n).