Python Program to Count characters surrounding vowels

Given a String, the task is to write a Python program to count those characters which have vowels as their neighbors.

Examples:

Input : test_str = ‘geeksforgeeksforgeeks’ 
Output : 10 
Explanation : g, k, f, r, g, k, f, r, g, k have surrounding vowels.

Input : test_str = ‘geeks’ 
Output : 2 
Explanation : g, k have surrounding vowels. 

Method 1 : Using loop

In this, we increment counter while checking previous and successive element for vowels using loop.

Python3

# initializing string
test_str = 'geeksforgeeksforgeeks'

# printing original string
print("The original string is : " + str(test_str))

res = 0
vow_list = ['a', 'e', 'i', 'o', 'u']
for idx in range(1, len(test_str) - 1):

    # checking for preceding and succeeding element to be vowel
    if test_str[idx] not in vow_list and (test_str[idx - 1] in vow_list or test_str[idx + 1] in vow_list):
        res += 1

# solving for 1st and last element
if test_str[0] not in vow_list and test_str[1] in vow_list:
    res += 1

if test_str[-1] not in vow_list and test_str[-2] in vow_list:
    res += 1

# printing result
print("Characters around vowels count : " + str(res))

Output:

The original string is : geeksforgeeksforgeeks

Characters around vowels count : 10

Method 2 : Using sum() and list comprehension

In this, we perform the task of getting count using sum() and iteration and filtering is done using list comprehension.

Python3

# initializing string
test_str = 'geeksforgeeksforgeeks'

# printing original string
print("The original string is : " + str(test_str))

vow_list = ['a', 'e', 'i', 'o', 'u']

# sum() accumulates all vowels surround elements
res = sum([1 for idx in range(1, len(test_str) - 1) if test_str[idx]
           not in vow_list and (test_str[idx - 1] in vow_list or test_str[idx + 1] in vow_list)])

# solving for 1st and last element
if test_str[0] not in vow_list and test_str[1] in vow_list:
    res += 1

if test_str[-1] not in vow_list and test_str[-2] in vow_list:
    res += 1

# printing result
print("Characters around vowels count : " + str(res))

 Output:

The original string is : geeksforgeeksforgeeks

Characters around vowels count : 10

The time and space complexity for all the methods are the same:

Time Complexity: O(n)

Space Complexity: O(n)

Method 3: Using enumerate () method.

Python3

# initializing string
test_str = 'geeksforgeeksforgeeks'

# printing original string
print("The original string is : " + str(test_str))

count=0

#counting vowel surrounding characters using enumerate() method

for idx, val in enumerate(test_str):
  if val in 'aeiouAEIOU':
    if idx > 0 and test_str[idx-1] not in 'aeiouAEIOU':
      count += 1
    if idx < len(test_str) - 1 and test_str[idx+1] not in 'aeiouAEIOU':
       count += 1

# printing result
print("Characters around vowels count : " + str(count))

Output

The original string is : geeksforgeeksforgeeks
Characters around vowels count : 10

Time Complexity:  O(n)

Auxiliary Space: O(1)

Method 4: Using Regex

Explanation
In this approach, we use the re.findall() method to find the characters that are surrounded by vowels.
The regular expression r'[^aeiouAEIOU]([aeiouAEIOU])[^aeiouAEIOU]’ matches a character that is not a vowel, followed by a vowel, followed by a character that is not a vowel.
The len() function is used to count the number of matches found.

Python3

#Importing the regular expression module
import re

#initializing string
test_str = 'geeksforgeeksforgeeks'

#printing original string
print("The original string is : " + str(test_str))

#Using re.findall() to find the number of characters surrounded by vowels
res = len(re.findall(r'[^aeiouAEIOU]([aeiouAEIOU])', test_str)) + len(re.findall(r'([aeiouAEIOU])[^aeiouAEIOU]', test_str))

#printing result
print("Characters around vowels count : " + str(res))

Output

The original string is : geeksforgeeksforgeeks
Characters around vowels count : 10

Time Complexity: O(n)

Auxiliary Space: O(n)