Count Number of Vowels using Sets in Given String - Python

Given a string, the goal is to count how many vowels it contains. Vowels include a, e, i, o, u (both uppercase and lowercase). For example:

Input: "Beautiful Day"
Output: 6

Explanation: Input has 6 vowels (e, a, u, i, u, a).

Let's explore different methods to count number of vowels in a given string in Python.

Using sum()

This method checks each character against a vowel set during a single pass through the string.

s = "Python is the best"
v = {'a','e','i','o','u','A','E','I','O','U'}
c = sum(1 for ch in s if ch in v)
print("Number of vowels:", c)

Number of vowels: 4

Explanation:

• v holds vowels for quick membership testing.
• The generator 1 for ch in s if ch in v contributes a count whenever a vowel appears.
• sum() aggregates the total without creating extra lists.

Using casefold()

This method converts the whole string to a single case to reduce the vowel set and simplify comparisons.

s = "Python Programming"
v = {'a','e','i','o','u'}
s = s.casefold()
c = sum(1 for ch in s if ch in v)
print("Number of vowels:", c)

Number of vowels: 4

Explanation:

• s.casefold() normalizes all characters for a uniform comparison.
• Using only lowercase vowels makes the set smaller.

Using count()

This method finds which vowels appear in the string, then counts their occurrences.

s = "Set operations in Python"
v = {'a','e','i','o','u','A','E','I','O','U'}
c = sum(s.count(ch) for ch in (set(s) & v))
print("Number of vowels:", c)

Number of vowels: 8

Explanation:

• set(s) & v extracts only the vowel characters present in the string.
• s.count(ch) gives the frequency of each vowel.
• sum() adds all frequencies to produce the final count.

Using for loop (brute force)

We can create a set of vowels and iterate over the string and if the current character is present in the dictionary then increment the count of vowels.

vow = {'a','e','i','o','u','A','E','I','O','U'}

s = "This example uses brute force"
res = 0

for c in s:
    if c in vow:
        res+=1
    
print(res)

10

Explanation:

• A set vow stores all vowels (both lowercase and uppercase).
• The string 's' is scanned character by character.
• For each character, if it's in the vowel set, the counter "res" is increased by 1.