Simple exercise to introduce the concept of FSM.

surrbyplus0.py

"""
Write a function that determines if all alpha characters in a string 
  are surrounded (the characters immediately before and after) by a plus sign.
Function should return false if any alpha character present in the string isn't
  surrounded by a plus sign. Otherwise the function should return true.
"""


def symbols(input_str: str) -> bool:
    """Fill your code bellow to make all tests pass."""


def test_main():
    assert symbols("") is True
    assert symbols("0") is True
    assert symbols("123") is True
    assert symbols("01%2-@") is True
    assert symbols("+1+") is True
    assert symbols("+a+") is True
    assert symbols("+ab+") is True
    assert symbols("+ab++") is True
    assert symbols("+Z+Y+") is True
    assert symbols("+ab+a+") is True
    assert symbols("+a+b+7") is True
    assert symbols("+a+=5=+d+") is True
    assert symbols("12+ab+a+12") is True
    assert symbols("a") is False
    assert symbols("a+") is False
    assert symbols("+a") is False
    assert symbols("-a+") is False
    assert symbols("+a-") is False
    assert symbols("-a-") is False
    assert symbols("+ab1+") is False
    assert symbols("+a1b+") is False
    assert symbols("+1ab+") is False
    assert symbols("+ab+a") is False
    assert symbols("+a+b=") is False


if __name__ == "__main__":
    import pytest
    pytest.main(["-s", __file__])

surrbyplus1.py

"""
Write a function that determines if all alpha characters in a string 
  are surrounded (the characters immediately before and after) by a plus sign.
Function should return false if any alpha character present in the string isn't
  surrounded by a plus sign. Otherwise the function should return true.
"""


def symbols(input_str: str) -> bool:
    """Almost done version 1 submitted by student."""
    plus = False
    foundalpha = False
    validInput = True
    for i, l in enumerate(input_str):
        if l == "+":
            if plus and foundalpha:
                foundalpha = False
                validInput = True
            plus = True
        elif l.isalpha():
            validInput = False
            foundalpha = True
        elif l != "+" and foundalpha:
            return False

    return validInput


def test_main():
    assert symbols("") is True
    assert symbols("0") is True
    assert symbols("123") is True
    assert symbols("01%2-@") is True
    assert symbols("+1+") is True
    assert symbols("+a+") is True
    assert symbols("+ab+") is True
    assert symbols("+ab++") is True
    assert symbols("+Z+Y+") is True
    assert symbols("+ab+a+") is True
    assert symbols("+a+b+7") is True
    assert symbols("+a+=5=+d+") is True
    assert symbols("12+ab+a+12") is True
    assert symbols("a") is False
    assert symbols("a+") is False
    assert symbols("+a") is False
    assert symbols("-a+") is False
    assert symbols("+a-") is False
    assert symbols("-a-") is False
    assert symbols("+ab1+") is False
    assert symbols("+a1b+") is False
    assert symbols("+1ab+") is False
    assert symbols("+ab+a") is False
    assert symbols("+a+b=") is False


if __name__ == "__main__":
    import pytest
    pytest.main(["-s", __file__])

surrbyplus2.py

"""
Write a function that determines if all alpha characters in a string 
  are surrounded (the characters immediately before and after) by a plus sign.
Function should return false if any alpha character present in the string isn't
  surrounded by a plus sign. Otherwise the function should return true.
"""


def symbols(input_str: str) -> bool:
    """Working version 2 submitted by student."""
    onValidation = False

    if len(input_str) < 3:
        for l in input_str:
            if l.isalpha():
                return False
        return True

    for i, l in enumerate(input_str):
        if onValidation:
            if l.isalpha():
                continue
            elif l == "+":
                onValidation = False
                continue

            return False

        if l.isalpha():
            if i == len(input_str) - 1:
                return False
            elif input_str[i - 1] == "+":
                onValidation = True
            else:
                return False

    return True


def test_main():
    assert symbols("") is True
    assert symbols("0") is True
    assert symbols("123") is True
    assert symbols("01%2-@") is True
    assert symbols("+1+") is True
    assert symbols("+a+") is True
    assert symbols("+ab+") is True
    assert symbols("+ab++") is True
    assert symbols("+Z+Y+") is True
    assert symbols("+ab+a+") is True
    assert symbols("+a+b+7") is True
    assert symbols("+a+=5=+d+") is True
    assert symbols("12+ab+a+12") is True
    assert symbols("a") is False
    assert symbols("a+") is False
    assert symbols("+a") is False
    assert symbols("-a+") is False
    assert symbols("+a-") is False
    assert symbols("-a-") is False
    assert symbols("+ab1+") is False
    assert symbols("+a1b+") is False
    assert symbols("+1ab+") is False
    assert symbols("+ab+a") is False
    assert symbols("+a+b=") is False


if __name__ == "__main__":
    import pytest
    pytest.main(["-s", __file__])

surrbyplus3.py

"""
Write a function that determines if all alpha input_str[idx]acters in a string are surrounded 
(the input_str[idx]acters immediately before and after) by a plus sign.
Function should return false if any alpha input_str[idx]acter present in the string isn't 
surrounded by a plus sign. Otherwise the function should return true.
"""


def symbols(input_str: str) -> bool:
    """Working version 3 submitted by student."""
    idx = 0

    while idx < len(input_str):
        if input_str[idx].isalpha():
            if len(input_str) < 3:
                return False

            # Check the set of alphas start with plus
            if not input_str[idx - 1] == "+":
                return False

            idx += 1

            if idx >= len(input_str):
                return False

            # Verify with a set of alphas are surround by plus
            while idx < len(input_str):
                # Check the set of alphas end with plus
                if input_str[idx] == "+":
                    break

                if input_str[idx].isalpha():
                    idx += 1
                    continue

                return False

        idx += 1

    return True


def test_main():
    assert symbols("") is True
    assert symbols("0") is True
    assert symbols("123") is True
    assert symbols("01%2-@") is True
    assert symbols("+1+") is True
    assert symbols("+a+") is True
    assert symbols("+ab+") is True
    assert symbols("+ab++") is True
    assert symbols("+Z+Y+") is True
    assert symbols("+ab+a+") is True
    assert symbols("+a+b+7") is True
    assert symbols("+a+=5=+d+") is True
    assert symbols("12+ab+a+12") is True
    assert symbols("a") is False
    assert symbols("a+") is False
    assert symbols("+a") is False
    assert symbols("-a+") is False
    assert symbols("+a-") is False
    assert symbols("-a-") is False
    assert symbols("+ab1+") is False
    assert symbols("+a1b+") is False
    assert symbols("+1ab+") is False
    assert symbols("+ab+a") is False
    assert symbols("+a+b=") is False


if __name__ == "__main__":
    import pytest
    pytest.main(["-s", __file__])

surrbyplus4.py

"""
Write a function that determines if all alpha characters in a string 
  are surrounded (the characters immediately before and after) by a plus sign.
Function should return false if any alpha character present in the string isn't
  surrounded by a plus sign. Otherwise the function should return true.
"""

ZERO, ONE, TWO = 0, 1, 2


def symbols(s: str) -> bool:
    """Version 4 introducing the concept of states with explicit exits."""
    state = ZERO

    for char in s:
        if state == ZERO:
            if char == "+":
                state = ONE
            elif not char.isalpha():
                continue
            else:
                return False
        elif state == ONE:
            if char == "+":
                continue
            elif char.isalpha():
                state = TWO
            else:
                state = ZERO
        elif state == TWO:
            if char == "+":
                state = ONE
            elif char.isalpha():
                continue
            else:
                return False

    if state == TWO:
        return False

    return True


def test_main():
    assert symbols("") is True
    assert symbols("0") is True
    assert symbols("123") is True
    assert symbols("01%2-@") is True
    assert symbols("+1+") is True
    assert symbols("+a+") is True
    assert symbols("+ab+") is True
    assert symbols("+ab++") is True
    assert symbols("+Z+Y+") is True
    assert symbols("+ab+a+") is True
    assert symbols("+a+b+7") is True
    assert symbols("+a+=5=+d+") is True
    assert symbols("12+ab+a+12") is True
    assert symbols("a") is False
    assert symbols("a+") is False
    assert symbols("+a") is False
    assert symbols("-a+") is False
    assert symbols("+a-") is False
    assert symbols("-a-") is False
    assert symbols("+ab1+") is False
    assert symbols("+a1b+") is False
    assert symbols("+1ab+") is False
    assert symbols("+ab+a") is False
    assert symbols("+a+b=") is False


if __name__ == "__main__":
    import pytest
    pytest.main(["-s", __file__])

surrbyplus5.py

"""
Write a function that determines if all alpha characters in a string
  are surrounded (the characters immediately before and after) by a plus sign.
Function should return false if any alpha character present in the string isn't
  surrounded by a plus sign. Otherwise the function should return true.
"""

NORMAL, PLUS, ALPHA, ERROR = 0, 1, 2, 3

# Truth table:
# NORMAL, TESTE, PLUS
# NORMAL, TESTE, NORMAL
# NORMAL, TESTE, RAISE
# PLUS, TESTE, PLUS
# PLUS, TESTE, ALPHA
# PLUS, TESTE, NORMAL
# ALPHA, TESTE, PLUS
# ALPHA, TESTE, ALPHA
# ALPHA, TESTE, RAISE

def symbols(s: str) -> bool:
    """Version 5 introducing the concept of error state."""
    state = NORMAL

    for char in s:
        if state == NORMAL:
            if char == "+":
                state = PLUS
            elif not char.isalpha():
                continue
            else:
                state = ERROR
        elif state == PLUS:
            if char == "+":
                continue
            elif char.isalpha():
                state = ALPHA
            else:
                state = NORMAL
        elif state == ALPHA:
            if char == "+":
                state = PLUS
            elif char.isalpha():
                continue
            else:
                state = ERROR
        elif state == "ERROR":
            break

    return state not in (ALPHA, ERROR)


def test_main():
    assert symbols("") is True
    assert symbols("0") is True
    assert symbols("123") is True
    assert symbols("01%2-@") is True
    assert symbols("+1+") is True
    assert symbols("+a+") is True
    assert symbols("+ab+") is True
    assert symbols("+ab++") is True
    assert symbols("+Z+Y+") is True
    assert symbols("+ab+a+") is True
    assert symbols("+a+b+7") is True
    assert symbols("+a+=5=+d+") is True
    assert symbols("12+ab+a+12") is True
    assert symbols("a") is False
    assert symbols("a+") is False
    assert symbols("+a") is False
    assert symbols("-a+") is False
    assert symbols("+a-") is False
    assert symbols("-a-") is False
    assert symbols("+ab1+") is False
    assert symbols("+a1b+") is False
    assert symbols("+1ab+") is False
    assert symbols("+ab+a") is False
    assert symbols("+a+b=") is False


if __name__ == "__main__":
    import pytest
    pytest.main(["-s", __file__])

surrbyplus6.py

"""
Write a function that determines if all alpha characters in a string 
  are surrounded (the characters immediately before and after) by a plus sign.
Function should return false if any alpha character present in the string isn't
  surrounded by a plus sign. Otherwise the function should return true.
"""

NORMAL, PLUS, ALPHA, ERROR = 0, 1, 2, 3


def symbols(s: str) -> bool:
    """Version 5 removing redundant exits to enforce the transition to error
    state."""
    state = NORMAL

    for char in s:
        if state == NORMAL:
            if char == "+":
                state = PLUS
            elif char.isalpha():
                state = ERROR
        elif state == PLUS:
            if char.isalpha():
                state = ALPHA
            elif char != "+":
                state = NORMAL
        elif state == ALPHA:
            if char == "+":
                state = PLUS
            elif not char.isalpha():
                state = ERROR
        elif state == "ERROR":
            break

    return state not in (ALPHA, ERROR)


def test_main():
    assert symbols("") is True
    assert symbols("0") is True
    assert symbols("123") is True
    assert symbols("01%2-@") is True
    assert symbols("+1+") is True
    assert symbols("+a+") is True
    assert symbols("+ab+") is True
    assert symbols("+ab++") is True
    assert symbols("+Z+Y+") is True
    assert symbols("+ab+a+") is True
    assert symbols("+a+b+7") is True
    assert symbols("+a+=5=+d+") is True
    assert symbols("12+ab+a+12") is True
    assert symbols("a") is False
    assert symbols("a+") is False
    assert symbols("+a") is False
    assert symbols("-a+") is False
    assert symbols("+a-") is False
    assert symbols("-a-") is False
    assert symbols("+ab1+") is False
    assert symbols("+a1b+") is False
    assert symbols("+1ab+") is False
    assert symbols("+ab+a") is False
    assert symbols("+a+b=") is False


if __name__ == "__main__":
    import pytest
    pytest.main(["-s", __file__])

surrbyplus7.py

"""
Write a function that determines if all alpha characters in a string 
  are surrounded (the characters immediately before and after) by a plus sign.
Function should return false if any alpha character present in the string isn't
  surrounded by a plus sign. Otherwise the function should return true.
"""

NORMAL, PLUS, ALPHA, ERROR = 0, 1, 2, 3

is_plus = lambda char: char == "+"
not_plus = lambda char: char != "+"
not_alpha = lambda char: not char.isalpha()
is_alpha = lambda char: char.isalpha()

FSM = {
    NORMAL: (
        (is_plus, PLUS),
        (is_alpha, ERROR),
    ),
    PLUS: (
        (is_alpha, ALPHA),
        (not_plus, NORMAL),
    ),
    ALPHA: (
        (is_plus, PLUS),
        (not_alpha, ERROR),
    ),
    ERROR: (),
}


def symbols(s: str) -> bool:
    """Version 6 refactoring to a declarative approach."""
    state = NORMAL

    for char in s:
        for condition, new_state in FSM.get(state):
            if condition(char):
                state = new_state
                break

    return state not in (ALPHA, ERROR)


def test_main():
    assert symbols("") is True
    assert symbols("0") is True
    assert symbols("123") is True
    assert symbols("01%2-@") is True
    assert symbols("+1+") is True
    assert symbols("+a+") is True
    assert symbols("+ab+") is True
    assert symbols("+ab++") is True
    assert symbols("+Z+Y+") is True
    assert symbols("+ab+a+") is True
    assert symbols("+a+b+7") is True
    assert symbols("+a+=5=+d+") is True
    assert symbols("12+ab+a+12") is True
    assert symbols("a") is False
    assert symbols("a+") is False
    assert symbols("+a") is False
    assert symbols("-a+") is False
    assert symbols("+a-") is False
    assert symbols("-a-") is False
    assert symbols("+ab1+") is False
    assert symbols("+a1b+") is False
    assert symbols("+1ab+") is False
    assert symbols("+ab+a") is False
    assert symbols("+a+b=") is False


if __name__ == "__main__":
    import pytest
    pytest.main(["-s", __file__])