The function f(x) takes an input string and returns a count of the vowels (A, E, I, O, U) present in the string, ignoring the case of the characters.

[CODE]: 
```python
def count_vowels(input_string):
    vowels = 'AEIOU'
    count = 0
    for char in input_string.upper():
        if char in vowels:
            count += 1
    return count
```