The function f(x) takes an input string and alternates between two operations for successive inputs. The first operation is converting the input string to uppercase letters. The second operation is applying a substitution cipher to the input string.

[CODE]:
```python
from itertools import cycle

def alternate_operations(input_strings):
    operations = [str.upper, substitution_cipher]
    result = []
    for input_string, operation in zip(input_strings, cycle(operations)):
        result.append(operation(input_string))
    return result

def substitution_cipher(s):
    # Replace this with the actual substitution cipher implementation
    return s.translate(str.maketrans('abcdefghijklmnopqrstuvwxyz', 'cipher_alphabet'))
```

The `substitution_cipher` function in the code above should be replaced with the actual implementation of the substitution cipher used by the original function.