def validation_2(formula, index, length):
'''(int) -> tuple of (FormulaTree, int)
Given a formula, a index and the length the function check whether
it is valid in parentheses.
Return the tuple of formula tree and the index if it is valid
otherwise return None
REQ: formula is not empty string
>>> formula = "(x*y)"
>>> index = 0
>>> length = 5
>>> validation_2(formula, index, length)
AndTree(Leaf('x'), Leaf('y'))
>>> formula = "(x+-----y)"
>>> index = 0
>>> length = 10
>>> validation_2(formula, index, length)
"OrTree(Leaf('x'), NotTree(NotTree(NotTree(NotTree(NotTree(Leaf('y')))))))"
>>> formula = "(+x*+-z)"
>>> index = 0
>>> length = 8
>>> validation_2(formula, index, length)
None
'''
# define the result which is None
result = None
# if it is not the end of formula at that index
if index != length:
# call teh validtion1 to check whether is valid
ret = validation_1(formula, index, length)
# if it is valid
if ret is not None:
# get the tuple of operand and index
operand_l, index = ret
# if it is not the end of formula
if index != length:
# get the operator at that index
operator = formula[index]
# if the operator is "+" or "*"
if operator in '+*':
# call validation1 function to check whether it is valid
ret = validation_1(formula, index + 1, length)
# if it is valid
if ret is not None:
# get the operand and the index
operator_r, index = ret
# if the operator is "+"
if operator == '+':
# build the OrTree for this and make tuple
# with current index
result = OrTree(operand_l, operator_r), index
# otherwise
else:
# build the AndTree for this and make tuple
# with current index
result = AndTree(operand_l, operator_r), index
return result
def build_tree(formula):
'''(str of formula) -> obj of FormulaTree
This function takes in a string of formula and returns the FormulaTree
that represents <formula> if it is a valid formula.
Return None if the formula is not valid
>>> build_tree('x')
Leaf('x')
>>> build_tree('-x')
NotTree(Leaf('x'))
>>> build_tree('(x+y)')
OrTree(Leaf('x'), Leaf('y'))
'''
# default the result to be None
# if it doesn't change the result (not go into any if statements),
# then formula is invalid
res = None
# check if the formula is an empty string, b/c I need to use formula[0]
if formula == '':
# formula is invalid
# do nothing since setted default of the result
pass
# base case when there is only one valid variable inside, like: 'x'
elif len(formula) == 1 and formula.islower():
# then it is a Leaf
res = Leaf(formula)
# check if the formula starts with '-'
elif formula[0] == '-':
# then it is a NotTree
# keep recursing the formula after '-'
res = NotTree(build_tree(formula[1:]))
# check if the formula starts with '('
elif formula[0] == '(' and formula[-1] == ')':
# then it is an OrTree or AndTree
# use get_root_index helper, find the index of the root in formula
index = get_root_index(formula)
# if we find the root (valid formula)
if index != len(formula) - 1:
# if the root is '+'
if formula[index] == '+':
# then it is an OrTree
# split the formula into left child and right child
# left child is on the left hand side of the '+' and vice versa
res = OrTree(build_tree(formula[1:index]),
build_tree(formula[index + 1:-1]))
# if the root is '*'
elif formula[index] == '*':
# then it is an AndTree
# split the formula into left child and right child
# left child is on the left hand side of the '*' and vice versa
res = AndTree(build_tree(formula[1:index]),
build_tree(formula[index + 1:-1]))
# if result have changed, but any of its child is None (formula invalid)
if res and None in res.get_children():
# adjust the result to be None again
res = None
# return the result
return res
def build_tree(formula):
if len(formula) is 1 and formula in "abcdefghijklmnopqrstuvwxyz":
node = Leaf(formula)
else:
if len(formula) > 1 and formula[0] == "(" and formula[-1] == ")":
formula = formula[1:-1]
connective_found = False
counter = 0
found_L_brac = False
while not connective_found:
if counter >= len(formula):
node = None
connective_found = True
elif found_L_brac and formula[counter] is ")":
found_L_brac = False
elif formula[counter] is "(":
found_L_brac = True
elif formula[counter] in "+" and not found_L_brac:
(node) = OrTree(build_tree(formula[:counter]), build_tree(
formula[counter + 1:]))
if (node.get_children())[0] is None or node.get_children()[1] is None:
node = None
connective_found = True
elif formula[counter] in "*" and not found_L_brac:
(node) = AndTree(build_tree(formula[:counter]), build_tree(
formula[counter + 1:]))
if (node.get_children())[0] is None or node.get_children()[1] is None:
node = None
connective_found = True
counter += 1
elif len(formula) >= 2 and formula[0] is "-":
if formula[1] not in "abcdefghijklmnopqrstuvwxyz-(":
connective_found = True
node = None
else:
node = NotTree(build_tree(formula[1:]))
if (node.get_children())[0] is None:
node = None
else:
node = None
return (node)
def build_tree_helper1(formula):
'''(list)-> list
Return the formula tree in the list
>>>build_tree_helper1(['(', 'x', '*', 'y', ')'])
[AndTree(Leaf('x'), Leaf('y'))]
>>>build_tree_helper1(['(', '(', 'x', '+', 'y', ')', '*',
'(', '-', 'x', '*', 'y', ')', ')'])
[AndTree(OrTree(Leaf('x'), Leaf('y')), AndTree(NotTree(Leaf('x')), Leaf('y')))]
'''
# return itself if just a variable inside list
if len(formula) == 1:
return formula
else:
# find the first ')'
for i in range(len(formula)):
if formula[i] == ')':
break
# find the first'(' in the list from the first ')'
for k in range(i, -1, -1):
if formula[k] == '(':
break
# find the AND or OR sign between the '()'
for m in range(k, i + 1):
if (formula[m] == '*') or (formula[m] == '+'):
break
# set the default of left and right child as Leaf
left_node = Leaf(formula[m - 1])
right_node = Leaf(formula[i - 1])
# if the right or left child is already a formulatree then
# itself is the child and it is not leaf
if type(formula[m - 1]) != str:
left_node = formula[m - 1]
if type(formula[i - 1]) != str:
right_node = formula[i - 1]
# find NOT sign if it exist and call Not_tree_helper() function
# to converte it to formatetree
if ((formula[m - 2]) == '-'):
left_node = NotTree(Not_tree_helper(formula[k + 2:m]))
if ((formula[m + 1]) == '-'):
right_node = NotTree(Not_tree_helper(formula[m + 2:i]))
# find the sign and use the right formatetree class to converte it
if (formula[m] == '*'):
formula[k:i + 1] = [AndTree(left_node, right_node)]
formula = build_tree_helper1(formula[:])
elif (formula[m] == '+'):
formula[k:i + 1] = [OrTree(left_node, right_node)]
formula = build_tree_helper1(formula[:])
return formula
def build_tree(formula):
'''
(string) -> FormulaTree
This function takes in a string representation of formula.
Returns the FormulaTree which represents the given formula if vaild.
Otherwise None is returned.
REQ: the formula given must be a string representation
'''
# idea: this functon use recursion to be efficient
# apart the formula into small parts and build each leaf
'''
this is a fail part for testing valid formula
symbol_list = "xyz+-*()"
for char in formula:
if symbol_list.find(char) != -1:
pass
else:
return None
'''
# the base case of a leaf on the tree is that only one lowercase letter
if len(formula) == 1 and formula[0].islower():
# build one leaf of the tree
return Leaf(formula)
# if the formula starts with "-", it means a not tree
elif formula[0] == "-":
# build a NotTree and send the rest of the formula into the recursion
return NotTree(build_tree(formula[1:]))
# for the symbol "+", "*", there must be a blanket before and after them
elif formula[0] == "(":
# here calls a helper function to find the place where the first symbol in the blanket is
index = _get_place(formula)
# build an AndTree, remove the blanket
# apart the formula to two parts: before the "*" and after
# send both parts into the recursion to get result for the AndTree
if formula[index] == "*":
return AndTree(build_tree(formula[1:index]),
build_tree(formula[index + 1:-1]))
# same step of "+" as the "*" above
elif formula[index] == "+":
return OrTree(build_tree(formula[1:index]),
build_tree(formula[index + 1:-1]))
# 'else' means the formula is not valid, therefore None is returned as required
else:
return None
def build_tree(formula):
'''(str) -> FormulaTree
If the str <formula> meets the criteria
of being a valid formula, create and return
the FormulaTree representation of the string.
Otherwise returns None.
REQ: formula is not empty
>>> build_tree('x')
Leaf('x')
'''
# Check if formula is valid
if is_valid(formula):
# Base case when length of list is one
if len(formula) == 1:
tree = Leaf(formula)
# Second base case is special case of the NotTree
elif formula[0] == '-':
# Recursively create the rest of the formula '--(..)'
if formula[1] == '-':
tree = NotTree(build_tree(formula[1:]))
# Create the NotTree for the rest of the bracket '-(..)'
elif formula[1] == '(':
tree = NotTree(build_tree(formula[1:formula.index(')') + 1]))
# Create the NotTree only for the variable
elif formula[1].islower():
tree = NotTree(build_tree(formula[1]))
else:
# Get the root and the index of the root
(r, index) = find_root(formula)
# Build the 'And' and/or 'Or' trees respectively
if r == '*':
tree = AndTree(build_tree(formula[1:index]),
build_tree(formula[index + 1:len(formula) - 1]))
elif r == '+':
tree = OrTree(build_tree(formula[1:index]),
build_tree(formula[index + 1:len(formula) - 1]))
else:
tree = None
return tree
def build_tree(formula):
'''(str) -> FormulaTree
Takes a string representing a formula. If the formula is valid, returns the
FormulaTree representation of the formula, else returns None.
There are some invalid formulas and reasons.
"X" variable not lower case letter
"x*y" missing parentheses
"-(x)" extraneous parentheses
"(x+(y)*z)" mismatched parentheses
REQ: formula is a string representing a boolean formula.
>>> build_tree('(x*y)') == AndTree(Leaf('x'), Leaf('y'))
True
>>> build_tree('(x+-y)') == OrTree(Leaf('x'), NotTree(Leaf('y')))
True
>>> build_tree('(((x*y)*z)*w)') == AndTree(AndTree(AndTree(Leaf('x'), Leaf\
('y')), Leaf('z')), Leaf('w'))
True
>>> build_tree('X') == None
True
>>> build_tree('x*y') == None
True
>>> build_tree('(x+(y)*z)') == None
True
'''
# set a condition for empty string
empty = len(formula) == 0
# if given an empty string, it is invalid
if (empty):
# result is None
result = None
# if given a string containing 1 lowercase letter, it is valid
elif (len(formula) == 1):
# set a condition for single variable
variable = formula.isalpha() and formula.islower()
if (variable):
# create a leaf of the letter
result = Leaf(formula)
# else it is invalid
else:
# result is None
result = None
else:
# set a condition for Not operation
no = formula[0] == "-"
# set a condition for formula closed by parentheses
# and the numbers of open parentheses and close parentheses are equal
paired = formula[0] == "(" and formula[-1] == ")" and\
formula.count("(") == formula.count(")")
# if - is the first character
if (no):
# build the subtree
sub = build_tree(formula[1:])
# if the subtree is valid
if (sub is not None):
# create a NotTree of the subtree
result = NotTree(sub)
# else the subtree is invalid
else:
# result is None
result = None
# if parentheses are to begin and end the string, and they are paired
elif (paired):
# find the root operator
i = build_help(formula)
# if the operator doesn'turns exist
if (i == -1):
# result is None
result = None
# else the operator exists
else:
# build a left subtree of the string before the operator
lc = build_tree(formula[1:i])
# build a right subtree of the string after the operator
rc = build_tree(formula[i + 1:-1])
# if one of the strings before & after the operator is invalid
if (lc is None) or (rc is None):
# result is None
result = None
# else the strings are both valid
else:
# if the operator is +
if (formula[i] == "+"):
# create an OrTree
result = OrTree(lc, rc)
# if the operator is *
elif (formula[i] == "*"):
# create a AndTree
result = AndTree(lc, rc)
# else the first character is not a lowercase letter, -, nor (
else:
# result is None
result = None
# return the result FormulaTree
return result
def build_tree(formula):
''' (str) -> FormulaTree or NoneType
Given a string representation of a formula, return a tree representation
of it with FormulaTree objects, if the formula is invalid then return
None instead
>>> build_tree("") == None
True
>>> build_tree("(-x)") == None
True
>>> build_tree("x+y") == None
True
>>> build_tree("x*y") == None
True
>>> build_tree("x-y") == None
True
>>> build_tree("(x+2)") == None
True
>>> build_tree("X") == None
True
>>> build_tree("(x+y+z)") == None
True
>>> build_tree("(x+(y)+z)") == None
True
>>> build_tree("((x)(x+y))") == None
True
>>> build_tree("(((x+)(x+y)))") == None
True
>>> build_tree("x") == Leaf("x")
True
>>> build_tree("(n+m)") == OrTree(Leaf("n"), Leaf("m"))
True
>>> build_tree("(c*v)") == AndTree(Leaf("c"), Leaf("v"))
True
>>> build_tree("-x") == NotTree(Leaf("x"))
True
>>> build_tree("-((x+y)*(a*-b))") == NotTree(AndTree(OrTree(Leaf("x"),\
Leaf("y")), AndTree(Leaf("a"), NotTree(Leaf("b")))))
True
'''
# If formula is 1 character, create a leaf from it if it's a valid
# variable name, otherwise return None
if (len(formula) == 1):
if (formula.isalpha() and formula.islower()):
result = Leaf(formula)
else:
result = None
# If the formula is an empty string, it is invalid therefore return None
elif (len(formula) == 0):
result = None
else:
# If the first character is -, then first build the smaller subtree
# and create a NotTree for the subtree if the subtree is valid
if (formula[0] == "-"):
child1 = build_tree(formula[1:])
if (child1 is None):
result = None
else:
result = NotTree(child1)
# if the first and last characters are brackets then
elif (formula[0] == "(" and formula[-1] == ")"
and formula.count("(") == formula.count(")")):
# find the operator that accompanies the bracket pair
index = find_operator_index(formula)
# if the operator is found, build the smaller subtree called
# child1 from the left, build the smaller subtree called child2
# from the right, and from those create an AndTree or
# OrTree based on the operator.
if (index != -1):
child1 = build_tree(formula[1:index])
child2 = build_tree(formula[index + 1:-1])
if (child1 is None or child2 is None):
result = None
else:
if (formula[index] == "*"):
result = AndTree(child1, child2)
else:
result = OrTree(child1, child2)
# if index is -1, it means there is no corresponding operator,
# which means the formula is invalid
else:
result = None
# If the leading character is neither a single variable, "-",
# nor "(", then the formula is invalid
else:
result = None
# Return the resulting root of the FormulaTree
return result
def build_tree_helper(formula, start, end):
'''
(str) -> FormulaTree
takes string <formula> and returns the FormulaTree representing the
formula.
returns None is there are invalid bracketing
end is inclusive
>>>
>>>
True
'''
# if no conditions are met
result = None
# base case/ charcter check:
if (start == end):
if (formula[start].isalpha() and formula[start].islower()):
# result is a leaf node with var
result = Leaf(formula[start])
# base case, formula starts with
elif (formula[start] == '-'):
sub_result = build_tree_helper(formula, start + 1, end)
if sub_result is None:
result = None
else:
result = NotTree(build_tree_helper(formula, start + 1, end))
# else:
else:
# vars to keep track of '(' and ')'
left_bracket_count = 0
right_bracket_count = 0
exit_loop = False
i = start
# find the most outer brackets and the connective associated with them
while (i <= end and not exit_loop):
bracket_difference = left_bracket_count - right_bracket_count
if (formula[i] == '('):
left_bracket_count += 1
elif (formula[i] == ')'):
right_bracket_count += 1
if bracket_difference < 0:
exit_loop = True
# connective count to throw exceptions****************
elif ((formula[i] == '*' or formula[i] == '+')
and bracket_difference <= 1): # might not need this one
connective = formula[i]
# use divide and conquer, and build the tree
left = build_tree_helper(formula, start + 1, i - 1)
right = build_tree_helper(formula, i + 1, end - 1)
if (left is None or right is None):
result = None
elif (connective == '*'):
result = AndTree(left, right)
elif (connective == '+'):
result = OrTree(left, right)
# exit the rest of the while loop
exit_loop = True
# increment the loop
i += 1
return result
def build_tree(formula):
'''(str) -> FormulaTree
Takes the infix expression represented by formula
and outputs the corresponding formula tree. build_tree works
with to_postfix in order to convert an equation into postfix notation
and then create the tree. (probably the cleanest function I've ever written)
build_tree also uses validate to make sure that a formula is valid before
attempting to construct the tree.
>>> build_tree('((a+b)*(x*m))'))
AndTree(OrTree(Leaf('a'), Leaf('b')), AndTree(Leaf('x'), Leaf('m')))
>>> build_tree('-a')
NotTree(Leaf('a'))
>>> build_tree('(a+X)')
None
'''
# first, we need to validate the formula using our helper
# validate function. If the function is valid, we can proceed
# constructing the tree
if validate(formula) == True:
# first, let's convert our equation into postfix notation
# using our to_postfix helper function
fixedFormula = to_postfix(formula)
# create an empty stack that we're gonna use to store
# sub-expressions and connect them later
stack = []
# looping through every character in the postfix expression
for character in fixedFormula:
# if we encounter an operand, make a new subtree
#we'll make the parent-child links as we go
if character in letters:
newTree = Leaf(character)
# if we encounter an and operator, we know that
# we have to have a left and right child, because and
# is binary, so pop two characters from the stack
# and set them to the left and right children of the
# AndTree we're constructing. We're guaranteed to have
# two or more characters in the stack because of the way
# postfix expression works.
elif character == '*':
right = stack.pop()
left = stack.pop()
newTree = AndTree(left,right)
# because not is a unary operator, all we need to do
# is set its one child to the subexpression that's
# highest up in the stack
elif character == '-':
newTree = NotTree(stack.pop())
# an or tree behaves the same way as an and tree, it just needs
# its own statement because we'll be using a different type of
# tree
elif character == '+':
right = stack.pop()
left = stack.pop()
newTree = OrTree(left,right)
# no matter what kind of sub-tree we've created, we need to re-add it back onto the stack
# so that we can later create the parent-child relationships of that tree
stack.append(newTree)
# in the case where the formula is invalid, we know that we can set the newTree to None and simply
# return that valud
else:
newTree = None
# return the tree represnted by the given equation
return newTree
def build_tree(formula):
'''(str) -> FormulaTree
given a string formula, return the FormulaTree representation of the
formula if it is a valid formula. Otherwise return None. *, +, - represent
AND OR NOT respectively. A node that represents * or + has 2 child nodes,
which in turn represent the two subformulas that are ANDed or ORed to
form the larger formula. A node that represents - has one child node,
which in turn represents the formula that is NOTed.
REQ:
- formula is not empty
- the variables in the formula must be lower case
- the representation of the formula must contain enough paranthesis(except
for '-'
>>> build_tree('X')
None
>>> build_tree('x*y')
None
>>> build_tree('-(x)')
None
>>> build_tree('(x+(y)*z)')
None
>>> build_tree('((-x+y)*-(-y+x))')
AndTree(OrTree(NotTree(Leaf('x')), Leaf('y')),
NotTree(OrTree(NotTree(Leaf('y')), Leaf('x'))))
>>> build_tree('(x*y)')
AndTree(Leaf('x'), Leaf('y'))
>>> build_tree('-y')
Leaf('y')
>>> build_tree('x')
Leaf('x')
>>> build_tree('v')
Leaf('v')
>>>build_tree('((x+y)*(z+-k))')
AndTree(OrTree(Leaf('x'), Leaf('y')), OrTree(Leaf('z'),
NotTree(Leaf('k'))))
'''
try:
# set the upper case checker as False
upper = False
# create lists
tokenlist = []
var = []
# loop through the formula do determine the symbols
for element in formula:
# if the formula does not contain a paranthesis:
if '(' not in formula:
return None
# if the element is '-'
elif element == "-":
# append the character to the list
tokenlist.append(element)
# if the element is '+'
elif element == "+":
# append the character to the list
tokenlist.append(element)
# if the element is '('
elif element == "(":
# append the character to the list
tokenlist.append(element)
# if the element is '*'
elif element == "*":
# append the character to the list
tokenlist.append(element)
# if the element is ')'
elif element == ")":
# remove and return the last object from the list and set it
# as a new variable
token = tokenlist.pop()
# loop through that variable
while token != '(':
# if the object is -
if token == "-":
# set the object as a variable, return the last object
# from the list to determine the character
var_formula = var.pop()
# append it to the list by using NotTree since the
# object has '-'
var.append(NotTree(var_formula))
token = tokenlist.pop()
# if the object is *
elif token == "*":
# set the left side of the *
var_formula = var.pop()
# now set the right side of the *
var_2 = var.pop()
# append it to the list by using And Tree class, since
# the symbol is '*'
var.append(AndTree(var_2, var_formula))
# remove and return the last item from the list
token = tokenlist.pop()
# if the object is '+'
elif token == "+":
# if the last item is '-'
if tokenlist[-1] == "-" and tokenlist[-1] is not None:
# set the left side of the '+'
var_formula = var.pop()
# set the right side of the '+'
var_2 = var.pop()
# since the object is '-', set it as NotTree
var_2 = NotTree(var_2)
# append it to the list by using OrTree class
var.append(OrTree(var_2, var_formula))
# pop the object twice/ remove and return the last
# item do it twice because of the '-' symbol
token = tokenlist.pop()
token = tokenlist.pop()
# otherwise
else:
# set the left side
var_formula = var.pop()
# set the right side
var_2 = var.pop()
# append it to the list by using OrTree class
var.append(OrTree(var_2, var_formula))
# remove and return the last object in the list
token = tokenlist.pop()
# if the element is a alphabetical character,
elif element.isalpha() is True:
# if the character is lower,
if element.islower() is True:
# then append to the list as 'Leaf'
var.append(Leaf(element))
# otherwise return None
else:
upper = True
return None
# if the character is ')'
elif element == ")":
# remove and return the last object from token list and set it
# as a variable
token = tokenlist.pop()
# loop through the list
while token != '(':
# if the object is '-'
if token == "-":
# remove and return the last object, that is next to
# the '-' sign.
var_formula = var.pop()
# append the charachter as a NotTree because of the
# '-' sign
var.append(NotTree(var_formula))
# set a new variable that is the last object
# from the list
token = tokenlist.pop()
# if the object is '*'
elif token == "*":
# remove and return the last object, the right side of
# the '*'
var_formula = var.pop()
# now the left side of the '*' sign.
var_2 = var.pop()
# append the left side and right side by using AndTree
# class
var.append(AndTree(var_2, var_formula))
token = tokenlist.pop()
# if the object is '+'
elif token == "+":
# if the last item is not none and '-' in the list,
if tokenlist[-1] is not None and tokenlist[-1] == "-":
# remove and return the last object, the right side
# of the '+'
var_formula = var.pop()
# now do the same process for the left side
var_2 = var.pop()
# since the item is '-', we need to assign it as
# NotTree
var_2 = NotTree(var_2)
# append the left side and right side by using
# OrTree class
var.append(OrTree(var_2, var_formula))
# pop the item twice/ remove and return the last
# item do it twice because of the '-' symbol
token = tokenlist.pop()
token = tokenlist.pop()
# otherwise
else:
# set the right side of the Or Tree
var_formula = var.pop()
# set the left side of the Or Tree
var_2 = var.pop()
# append the left side and right side by using
# OrTree
var.append(OrTree(var_2, var_formula))
# remove and return the last item in the list
token = tokenlist.pop()
# return the first item in the list
return var[0]
except IndexError:
return None
def binary_handler(formula):
''' (str) -> FormulaTree
REQ: formula[0] == '('
Returns the FormulaTree where AND or OR is the root of the tree
Returns None iff the BinaryFormula is invalid
>>> binary_handler('(x+y)')
OrTree(Leaf('x'), Leaf('y'))
>>> None == binary_handler('(x)')
True
'''
# define scan variable which index each character in formula we start off
# at 1 to skip the first parenthese
scan = 1
# First we evaluate the left child
# case 1 Left child is leaf
if formula[scan] in VARIABLES:
# then store left child as a leaf
left_c = Leaf(formula[scan])
# case 2 Left child is another parenthese
elif formula[scan] is LEFT_P:
# then call binary handler again
left_c = binary_handler(formula[scan:])
# case 3 Left child is Not tree
elif formula[scan] is NOT:
# then call Not Handler
left_c = not_handler(formula[scan:])
# case 4 anything else the Left Child is invalid
else:
# then set left child to None
left_c = None
# evaluate left child to see if it is valid we proceed only if
# it is valid otherwise tree is None
if left_c is None:
tree = None
else:
# check the length of the left child using tree2formula helper function
# and add it to scan
scan += len(tree2formula(left_c))
# the character immediately following the left tree must be AND or OR
# anything else the formula is invalid
if formula[scan] is not AND and formula[scan] is not OR:
tree = None
else:
# store the connective
connect = formula[scan]
# increment scan to evaluate whatever is Next
scan += 1
# Now we apply the same treatment with left child as with the right
if formula[scan] in VARIABLES:
right_c = Leaf(formula[scan])
elif formula[scan] is LEFT_P:
right_c = binary_handler(formula[scan:])
elif formula[scan] is NOT:
right_c = not_handler(formula[scan:])
else:
right_c = None
# check if right child is also invalid if it is then tree is
# invalid
if right_c is None:
tree = None
else:
# check the length of the left child using tree2formula helper
# function and add it to scan
scan += len(tree2formula(right_c))
# the last character must be a right parenthese otherwise the
# whole formula is invalid
if formula[scan] != RIGHT_P:
tree = None
else:
# now depending on the connective we evaluate the
# tree as an AND tree
if connect is AND:
tree = AndTree(left_c, right_c)
# if it is not an And tree it must be OR
else:
tree = OrTree(left_c, right_c)
# we return the result
return tree
def subtree_helper(formula):
''' (str, int) -> FormulaTree
Returns the FormulaTree built for the formula <formula>
REQ: length of the formula >= 1
>>> subtree = subtree_helper('(a+b)')
>>> print(subtree)
'OrTree(Leaf('a'), Leaf('b'))'
>>> subtree = subtree_helper('(-a)')
>>> print(subtree)
None
>>> subtree = subtree_helper('(((a+b)+c)*c)')
>>> print(subtree)
AndTree(OrTree(OrTree(Leaf('a'), Leaf('b')), Leaf('c')), Leaf('c'))
'''
# initialize your tree as none
tree = None
# 4 base cases:
# 1- if the formula length is 0
if (len(formula) == 0):
tree = None
# 2- check if its a just a variable (and lower case). If it is, then make
# that into a leaf
elif (len(formula) == 1) and (formula[0].isalpha()) and \
(formula[0].islower()):
tree = Leaf(formula[0])
# 3- if the formula doesn't start with a bracket or a
# not operator (and isn't alpha + lower case either -> checked that in the
# previous condition) then the tree is invalid -> return none
elif not (formula.startswith('(') or formula.startswith('-')):
tree = None
# 4- check if its a negator. If it is, make a not tree and send the rest of
# the string as a recursive call
elif (formula[0] == '-'):
# make the child of the Not Tree
child = subtree_helper(formula[1:])
# if the child is a valid formula, then make a Not tree, else make the
# tree None since the child is an invalid sub tree
if child:
tree = NotTree(child)
# else the child is invalid, therefore the whole tree becomes invalid
else:
tree = None
# else if it starts with an open bracket and ends with a closed bracket
# then we have an AND tree or an OR tree
elif (formula[0] == '(') and (formula[-1] == ')'):
# then find the root of the tree (find root checks if we have a valid
# root, otherwise would return none)
root_index = find_root(formula)
# if the root exists in the formula
if (root_index is not None):
# then make the left sub tree by making a recursive call on
# everything from the left +1 (not including the bracket) to the
# root
left_subtree = subtree_helper(formula[1:root_index])
# similarly, make the right sub tree by making a recursive call on
# everything from one character after the root till one index
# before the string ends (we don't want the bracket, because it
# will make the child invalid)
right_subtree = subtree_helper(formula[root_index + 1:-1])
# check if the sub trees are valid sub trees, if they are then
if (left_subtree and right_subtree):
# (finally!) make the tree. If the root is a + sign make an OR
# tree using the subtrees we have made
if (formula[root_index] == '+'):
tree = OrTree(left_subtree, right_subtree)
# else if its a * sign make an add tree using the sub trees we
# have made
elif (formula[root_index] == '*'):
tree = AndTree(left_subtree, right_subtree)
# if its neither (and we handled the case where we have -) then
# the tree is invalid because of an invalid symbol or placing
# an valid symbol some where wrong.
# therefore the whole tree becomes invalid/none
else:
tree = None
# else the left and right sub trees are not valid sub trees. hence
# making the entire tree invalid (aka none)
else:
tree = None
# if the root does not exist, then there is no tree to make. Hence the
# tree becomes none
else:
tree = None
# else, the string is too complicated! just leave it as it is, and hope the
# marker figures it out, lol
# jk
# else, the string is complicated.
else:
# so your find the root of the formula and split it from there.
root_index = find_root(formula)
# if there is a root in the formula
if (root_index is not None):
# make a recursive call on everything left from the root and
left_subtree = subtree_helper(formula[:root_index])
# make a recursive call on everything from the right of the root
right_subtree = subtree_helper(formula[root_index + 1:])
# if the left and right sub trees are valid
if (left_subtree and right_subtree):
# then make the tree based on the root
# if it is a +, then make an or tree
if (formula[root_index] == '+'):
tree = OrTree(left_subtree, right_subtree)
# else if its a *, then make an and tree
elif (formula[root_index] == '*'):
tree = AndTree(left_subtree, right_subtree)
# else the operator is invalid, hence making the entire tree
# invalid/None
else:
tree = None
# else the left or right subtrees are invalid -> the whole tree is
# invalid
else:
tree = None
# if the root does not exist, then there is no tree to make. Hence the
# tree becomes none
else:
tree = None
# return the tree
return tree
def build_tree(formula):
''' (str) -> FormulaTree
Returns True if the <formula> is valid. Else returns False.
How is a formula valid?
> all the variable names should be lower case
> there can be only be 26 unique varaibles, aka the total number of
alphabets in english language
> the only valid operators are the AND ('*'), the NOT ('-') and the
OR ('+') operator.
> when using the AND ('*') and OR ('+') operator, it is necessary to use
parentheses. e.g. '(x*y)' is a valid formula, but 'x*y' is not a valid
formula because it is missing parentheses
> Parentheses are not needed when using the NOT ('-') operator. e.g. '-x'
is a valid formula, but '-(x)' is not a valid formula because it has
extraneous brackets
> the simplest formula is of a string containing just one variable. e.g.
'x' is the simplest formula
Any formula that does not follow the above rules is an invalid formula
REQ: <formula> can not be an empty string
REQ: <formula> only has symbols '*', '-', '+' and all the lower case
letters from a - z
>>> build_tree('-x')
NotTree(Leaf('x'))
>>> build_tree('-(x)')
None
>>> build_tree('-(x*y)')
NotTree(AndTree(Leaf('x'), Leaf('y')))
>>> build_tree('x*y')
None
>>> build_tree('x')
Leaf('x')
'''
# initialize an empty tree
tree = None
# find the root using the root helper function
root_index = find_root(formula)
# if the root exists,
if root_index is not None:
# then see if the root is an OR operator
if (formula[root_index] == '+'):
# make the left sub tree (everything from the start to one index
# before the root
left_tree = subtree_helper(formula[1:root_index])
# make the right subtree (everything from one index after the root
# to the end)
right_tree = subtree_helper(formula[root_index + 1:-1])
# if the left sub tree or the right sub tree is not empty (meaning
# its not invalid), then make an OR tree
if (left_tree and right_tree):
tree = OrTree(left_tree, right_tree)
# else, left sub tree or right sub tree is invalid and therefore
# makes the entire tree invalid. Hence the tree is none
else:
tree = None
# else check if the root is an AND operator
elif (formula[root_index] == '*'):
# make the left sub tree (everything from the start to one index
# before the root
left_tree = subtree_helper(formula[1:root_index])
# make the right subtree (everything from one index after the root
# to the end)
right_tree = subtree_helper(formula[root_index + 1:-1])
# if the left sub tree or the right sub tree is not empty (meaning
# its not invalid), then make an OR tree
if (left_tree and right_tree):
tree = AndTree(left_tree, right_tree)
# else, left sub tree or right sub tree is invalid and therefore
# makes the entire tree invalid. Hence the tree is none
else:
tree = None
# else if the root is a NOT operator
elif (formula[root_index] == '-'):
# make the sub tree (everything from the root_index + 1 to the end
# since not only has one child/ excluding the not operator)
child = subtree_helper(formula[root_index + 1:])
# if the child is a valid tree
if child:
# then make a Not tree
tree = NotTree(child)
# else, the child is not valid, making the whole tree invalid/none
else:
tree = None
# if the operator is not from +, -, *, then the operator is invalid
# making the entire tree invalid
else:
tree = None
# else if the str a lower case alphabet of len 1, then make it into a leaf
# and return that as a tree
elif (len(formula) == 1) and (formula.isalpha()) and (formula.islower()):
tree = Leaf(formula)
# else if the root does not exist, then the tree is invalid
else:
tree = None
# return the tree that was built (or built but destroyed by the multiple
# checks, oof!)
return tree
def build_tree(formula):
'''
(string) -> formula_tree
This function takes a string and puts it
into a proper formula tree. If the string
is not in the correct format, then return
None.
>>> build_tree('-x')
NotTree(Leaf('x'))
>>> build_tree('((-x+y)*-(-y+x))')
AndTree(OrTree(NotTree(Leaf('x')), Leaf('y')), \
NotTree(OrTree(NotTree(Leaf('y')), Leaf('x'))))
>>> x = build_tree('Pancer Square')
>>> x == None
True
'''
# find length of formula
length = len(formula)
# base case, if only one character
if (length == 1):
# return a leaf
return Leaf(formula)
# if a not symbol occurs
if (formula[0] == "-"):
# create NotTree and continue building
return NotTree(build_tree(formula[1:]))
# this means that the root will either be
# a AndTree or OrTree
if (formula[0] == '('):
# create counters and initialize variables
counter = 0
root = ''
# to use in while loop
x = 0
# this loop finds the root
while x < len(formula) and root == '':
# subtract 1 if rigth bracket is found
if formula[x] == ")":
counter = counter - 1
# if a not symbol is used
if formula[x] == '-':
x = x + 1
# add 1 if left bracket is found
if formula[x] == "(":
counter = counter + 1
# if the counter reaches 1
elif counter == 1:
root = formula[x + 1]
x = x + 1
# if root is a +
if root == '+':
return OrTree(build_tree(formula[1:x]),
build_tree(formula[x + 1:-1]))
# if the root is a *
if root == '*':
return AndTree(build_tree(formula[1:x]),
build_tree(formula[x + 1:-1]))
# in case of incorrect format
if formula[0].isalpha() is False:
return None
def build_tree(formula):
'''(str) -> FormulaTree
Takes in a string representation of a boolean formula, returns the
corresponding FormulaTree representation of the formula, given that it is
valid. If the formula is not valid, None is returned.
Valid formulas are of one of the following types:
F1
(F1 * F2)
(F1 + F2)
- F1
where F1 and F2 are also valid boolean formulas.
REQ: all characters in formula are in VARIABLES and SYMBOLS
>>> build_tree("x-y") is None
True
>>> build_tree("(-x+y)")
OrTree(NotTree(Leaf('x')), Leaf('y'))
>>> build_tree("((x*y)+z)")
OrTree(AndTree(Leaf('x'), Leaf('y')), Leaf('z'))
'''
# Defaulted the variable for the FormulaTree to an invalid input
tree = None
# Base Case, formula is empty: invalidate the formula
if not formula:
tree = None
# Base Case, formula is a boolean variable
elif formula in VARIABLES:
tree = Leaf(formula)
# Recursive Case 1: Formula is a negation of a boolean formula (NotTree)
# Set that formula's children to be everything after the symbol
elif formula[0] == '-':
tree = NotTree(build_tree(formula[1:]))
# Recursive Case 2: Formula is a boolean formula
elif formula[0] == '(' and formula[-1] == ')':
# Loop through the formula string
ind = 1
while ind < len(formula) - 1:
# Check if formula contains nested boolean formulas
if formula[ind] == '(':
brackets = 1
ind += 1
while brackets != 0 and ind < len(formula) - 1:
# For each open bracket, add a new level of nesting
if formula[ind] == '(':
brackets += 1
# For each closing bracket, remove a level of nesting
elif formula[ind] == ')':
brackets -= 1
# Invalidate the tree if an invalid character is found
elif not (formula[ind] in VARIABLES
or formula[ind] in SYMBOLS):
tree = None
ind = len(formula) - 2
ind += 1
# Check if formula is an AndTree/OrTree formula
# Set the formula's children to be everything before the symbol,
# and everything after the symbol, then stop the while loop
if formula[ind] == '*':
tree = AndTree(build_tree(formula[1:ind]),
build_tree(formula[ind + 1:-1]))
ind = len(formula) - 1
elif formula[ind] == '+':
tree = OrTree(build_tree(formula[1:ind]),
build_tree(formula[ind + 1:-1]))
ind = len(formula) - 1
ind += 1
# Check if there is an invalid formula nested in the boolean formula
# The whole tree is then made invalid
if 'None' in str(tree):
tree = None
return tree
def build_tree(formula):
'''(str) -> FormulaTree or None
take a string which represents the formula and return the FormulaTree when
the formula is valid or None when the formula is invalid
REQ: formula should be the right combinations of lower case
latter, '-', '+', '*', '(' and ')'
>>> build_tree("(x*y+z*x)")
>>> build_tree('(x*y)')
AndTree(Leaf('x'), Leaf('y'))
'''
# create two list:
# one is the list which stores the operators
# the other is the list which stores the lower case letters
operator = list()
letter = list()
# go through the formula and the formula need to be valid
index = 0
is_valid = True
while (index < len(formula) and is_valid is True):
# the first element in formula[index:] is '(', '-', '+' or '*'
if (formula[index] in ('(', '+', '-', '*')):
# put this element into the operator list
operator.append(formula[index])
# the first element in formula[index:] is lower case letter
elif (formula[index].islower() is True):
# the previous str is also a letter
if (index > 0 and formula[index - 1].islower() is True):
is_valid = False
# the last element in operator list is '-'
# that means '-' is the parent of the current leaf
elif (len(operator) > 0 and operator[-1] == '-'):
# pop the last element in the operator list
# build a NotTree whose child is the current leaf
# push this subtree to letter list
operator.pop()
letter.append(NotTree(Leaf(formula[index])))
# there is still symbol '-' exists in operator list
while (len(operator) > 0 and operator[-1] == '-'):
operator.pop()
current_formula = letter.pop()
letter.append(NotTree(current_formula))
# the last element in operator is not '-'
else:
# push this letter to letter list
letter.append(Leaf(formula[index]))
# the first element in formula[index:] is ')'
elif (formula[index] == ')'):
# the operator list is empty
# or the letter list does not have two element
if (len(operator) <= 1 or len(letter) < 2):
is_valid = False
else:
# pop the last element in the operator list
symbol = operator.pop()
# pop two element from letter list
# the last one is right child
# and the second last one is left child
right_child = letter.pop()
left_child = letter.pop()
# the symbol is '+'
if (symbol == '+'):
# build a OrTree for this subtree
subtree = OrTree(left_child, right_child)
# the symbol is '+'
elif (symbol == '*'):
# build a AndTree for this subtree
subtree = AndTree(left_child, right_child)
# the other symbol should be a invalid formula
else:
is_valid = False
if (is_valid is True):
# pop the last element in operator, it should be '('
# otherwise, it should be invalid
previous_symbol = operator.pop()
if (previous_symbol != '('):
is_valid = False
else:
# the last element in operator is '-'
# pop '-' and it is the parent of the subtree
if (len(operator) > 0 and operator[-1] == '-'):
operator.pop()
# push this subtree to letter list
letter.append(NotTree(subtree))
while (len(operator) > 0 and operator[-1] == '-'):
operator.pop()
current_formula = letter.pop()
letter.append(NotTree(current_formula))
# the last element in operator is not '-'
# push subtree to letter list straightly
else:
letter.append(subtree)
# there are other symbol in the formula
else:
is_valid = False
# increase the index
index += 1
# the is_valid is True, the operator list is empty
# and there is only one element in the letter list
# the last element in the letter list is the FormulaTree
if (is_valid is True and len(operator) == 0 and len(letter) == 1):
result = letter.pop()
else:
result = None
return result
def build_tree(formula):
''' (str) -> FormulaTree
The function will create a FormulaTree based on
the input formula str.
The input str has to be in the right form otherwise,
return None.
The function will return a root of FormulaTree if input
formula is in the valid form.
The input str has to be in a valid form, which is:
1. "+" and "*" connect to leaf and arounded by "(" and ")"
2. for "-", there is no need for parenthesis.
The function will start at an empty root,
when we have a "(", we will create a left child, if
we have a leaf, we will create that leaf and then go back to
its parent and continune. When we have a ")" we will go
back to parent. It has used concept of stack, which is
a list to store parents.
The function will change input formula into a good form, which is
for each "-" sign, we have a matching ")" after its leaf.
For example "-x" will change to "-x)"; "-(-x+y)" will change to
"-(-x)+y))"
>>> build_tree('a+b') == None
True
>>> build_tree('(a)') == None
True
>>> build_tree('-(a)') == None
True
>>> build_tree('(a+b+c)') == None
True
>>> tree = build_tree('(b+(c+d))')
>>> tree == OrTree(Leaf('b'), OrTree(Leaf('c'), Leaf('d')))
True
>>> tree = build_tree('(x+y)')
>>> tree == OrTree(Leaf('x'), Leaf('y'))
True
>>> tree = build_tree('-(--x+y)')
>>> tree == NotTree(OrTree(NotTree(NotTree(Leaf('x'))), Leaf('y')))
True
>>> tree = build_tree('------x')
>> tree == NotTree(NotTree(NotTree(NotTree(NotTree(NotTree(Leaf('x')))))))
True
'''
# create a list to store all parents
stack = []
# create an empty tree, add it into stack and let curr be it
tree = FormulaTree(None, [])
stack.append(tree)
curr = tree
# try to do the following
try:
# change formula into a good form for code
formula = build_tree_helper(formula)
# loop through each symbol in formula
for s in formula:
# if we have "("
if s == '(':
# create a left subtree, and store parent in stack
left = FormulaTree(None, [])
curr.children.append(left)
stack.append(curr)
curr = curr.children[0]
# if we have a laef
elif s not in ['+', '*', '-', ')']:
# if s is not alphabet or it is upper case, return None
if not s.isalpha() or s.isupper():
return None
# create a leaf of its symbol
leaf = Leaf(s)
# get the parent from stack
parent = stack.pop()
# if curr node has parent
if curr in parent.children:
# let parent pointer to this leaf
index = parent.children.index(curr)
parent.children[index] = leaf
# let curr be parent (go back)
curr = parent
# if curr node has no parent
else:
# let tree be curr leaf
tree = leaf
# if after any leaf, stack is empty, then
# it must be in invalid form, raise error
if len(formula) != 1 and stack == []:
raise SyntaxError
# if we have "+", "*", or "-"
elif s in ['+', '*', '-']:
# get the parent from stack
parent = stack[-1]
# create a new node for its child
new_node = FormulaTree(None, [])
# if symbol is * then create AndTree
if s == '*':
replace = AndTree(curr.children[0], new_node)
# if symbol is + then create OrTree
elif s == '+':
replace = OrTree(curr.children[0], new_node)
# if symbol is - then create NotTree
else:
replace = NotTree(new_node)
# if curr node has parent
if curr in parent.children:
# get the index and let parent pointer to curr node
index = parent.children.index(curr)
parent.children[index] = replace
# get the new curr node
curr = replace
# if curr node has no parent
else:
# let tree be replace node
tree = replace
# get new curr node and reset stack
curr = tree
stack = [curr]
# put curr into stack as parent and get next curr
stack.append(curr)
curr = curr.children[0] if s == '-' else curr.children[1]
# if we have ")", then let curr go back to parent
elif s == ')':
curr = stack.pop()
# if there is other situation, it must be error
else:
raise SyntaxError
# if any error, then return None since it is an invalid form
except:
return None
# if stack has anything which means invalid form, or
# there are still some FormulaTree in result tree, return None
if stack != [] or 'FormulaTree' in str(tree):
return None
return tree
def build_tree(formula):
'''(str) -> FormulaTree
Builds a tree out of the formula given to the function an returns an obj
representation of it. A proper tree does not have any capitalized
variables, any extraneous parenthesses, unmatches parenthesses, or missing
parenthesses.
>>> build_tree('X')
None
>>> build_tree('(x*y)')
AndTree(Leaf('x'), Leaf('y'))
>>> build_tree("(x+y*(z))")
None
>>> build_tree("((-x+y)*-(-y+x))")
AndTree(OrTree(NotTree(Leaf('x')), Leaf('y')),
NotTree(OrTree(NotTree(Leaf('y')), Leaf('x'))))
'''
# Set an arbitrary tree solely for error detection
tree = NotTree(None)
# Base case when the formula is empty
if (len(formula) == 0):
tree = None
# Base case when the formula is just a variable/leaf
elif (len(formula) == 1 and formula.islower()):
tree = Leaf(formula)
# Base case when the formula has a Not connective
elif (len(formula) >= 2 and formula.startswith('-')):
tree = NotTree(build_tree(formula[1:]))
else:
# Set the variables required for the while loop
# i is the index, o_p_c stands for Open Parenthesses Count and
# root found confirms when the outermost root is found, this variable
# reduces the amount of iteration increasing overall efficency
i, o_p_c, root_found = 0, 0, False
# Loop through the entire string. The outermost root is found when the
# o_p_c is 1; once its found, loop cancels. If the outermost root isn't
# found by some unrecognizable form, the loop cancels and the tree
# stays as the same arbitrary tree assigned earlier.
while (root_found is False and i < len(formula)):
# Increment o_p_c by 1 if an open parenthesses is found
if (formula[i] == '('):
o_p_c += 1
# Decrement o_p_c by 1 if a closed parenthesses is found
elif (formula[i] == ')'):
o_p_c -= 1
# Recur over the outermost Or connective when found; root is found
elif (formula[i] == '+' and o_p_c == 1):
tree, root_found = OrTree(build_tree(formula[1:i]),
build_tree(formula[i + 1:-1])), True
# Recur over the outermost And connective when found; root is found
elif (formula[i] == '*' and o_p_c == 1):
tree, root_found = AndTree(build_tree(formula[1:i]),
build_tree(formula[i + 1:-1])), True
# Increment index by 1
i += 1
# If statement to know when the tree is in an unrecognizable form
if (tree is None):
tree = None
elif (None in tree.get_children()):
tree = None
# Return the built tree
return tree
def build_tree_helper(formula, start, end):
'''
(str, int, int) -> FormulaTree/None
takes string <formula> and returns the root of the FormulaTree that
represents <formula>. Returns None if formula is not valid
<start> is starting index of <formula> to start building tree
<end> is ending index of <formula> to start building tree, is inclusive
this function is recursive
a valid formula contains the following:
- boolean variables: lowercase alphabetical letters
- connectives: '*' represents AND, '+' represent OR, '-' represent NOT
- parentheses: '(' and ')'
rules for formula:
- the simpliest formula is just a boolean variable
- more complex formulas uses connectives and variables
- arguments for '*' and '+' are always enclosed by parentheses
- arugments for connectives are formulas
any string that cannot be constructed with rules above are not valid
(i.e empty, invalid characters, using connectives without using brackets)
>>> a = build_tree_helper('(x+y)', 0 ,len('(x+y)') - 1)
>>> repr(a) == "OrTree(Leaf('x'), Leaf('y'))"
True
>>> a = build_tree_helper('(-x*(y+z))', 1, 2)
>>> repr(a) == "NotTree(Leaf('x'))"
True
'''
# initialize result as None, so iff no valid conditions are met, ret None
result = None
# base case: 1 character
if (start == end):
# if the charcter is lowercase alphabetical letter
if (formula[start].isalpha() and formula[start].islower()):
# result is a Leaf node with the given letter
result = Leaf(formula[start])
# base case: formula starts with '-'
elif (formula[start] == '-'):
# check whether rest of the formula is valid
sub_result = build_tree_helper(formula, start + 1, end)
# if the rest of the formula is not valid
if sub_result is None:
# the whole formula is not valid
result = None
else:
# else attach the FormulaTree representing the rest of the formula
# to NotTree and ret
result = NotTree(sub_result)
# else recursive step
else:
# vars to keep track of '(' and ')'
left_bracket_count = 0
right_bracket_count = 0
# vars for efficient while loop
exit_loop = False
i = start
# find the most outer brackets and the connective associated with them
# iterate through forumula until they are found
while (i <= end and not exit_loop):
bracket_difference = left_bracket_count - right_bracket_count
# keep track of how many of each bracket is in the formula
if (formula[i] == '('):
left_bracket_count += 1
elif (formula[i] == ')'):
right_bracket_count += 1
# if there are more ')' than '(' then formula is invalid
# exit loop and return None
if bracket_difference < 0:
exit_loop = True
# connective for outer brackets is when <bracket_difference> <= 1
# and the current character is '*' or '+'
elif ((formula[i] == '*' or formula[i] == '+')
and bracket_difference <= 1):
connective = formula[i]
# divide and conquer, and build the tree to the left and right
# of the connective
left = build_tree_helper(formula, start + 1, i - 1)
right = build_tree_helper(formula, i + 1, end - 1)
# if any part of that is invalid, then whole forulma is invalid
if (left is None or right is None):
result = None
# set the left and right tree as the children of the found
# connective
elif (connective == '*'):
result = AndTree(left, right)
elif (connective == '+'):
result = OrTree(left, right)
# exit the rest of the while loop, if connective is found
exit_loop = True
# increment the loop
i += 1
# return result
return result
def build_tree_helper(formula):
''' (str) -> FormulaTree
Takes in a string (formula) and returns the FormulaTree that represents
formula if it is a valid formula. Otherwise None is returned
REQ: formula must be a valid string formula
>>> asdf = build_tree("((-x+y)*-(-y+x))")
>>> print(asdf)
AndTree(OrTree(NotTree(Leaf('x')), Leaf('y')),
NotTree(OrTree(NotTree(Leaf('y')), Leaf('x'))))
>>> asdf = build_tree("((-x+y)*-(-y+x)")
>>> print(asdf)
None
>>> asdf = build_tree("(x+y")
>>> print(asdf)
None
>>> asdf = build_tree("x+y")
>>> print(asdf)
None
>>> asdf = build_tree("x+y)")
>>> print(asdf)
None
>>> asdf = build_tree("x+y*x")
>>> print(asdf)
None
>>> asdf = build_tree("x")
>>> print(asdf)
Leaf('x')
>>> asdf = build_tree("-y")
>>> print(asdf)
NotTree(Leaf('y'))
>>> asdf = build_tree("(x*y)")
>>> print(asdf)
AndTree(Leaf('x'), Leaf('y'))
>>> asdf = build_tree("X")
>>> print(asdf)
None
>>> asdf = build_tree("x*y")
>>> print(asdf)
None
>>> asdf = build_tree("-(x)")
>>> print(asdf)
None
>>> asdf = build_tree("(x+(y)*z)")
>>> print(asdf)
None
'''
# Checks if current character is a NOT operation
if (formula[0] == "-"):
# Adds a NotTree to the FormulaTree
return NotTree(build_tree_helper(formula[1:]))
# Checks if current character is a valid variable
if (len(formula) == 1 and formula[0].islower()):
# Adds a Leaf to the FormulaTree
return Leaf(formula[0])
# Counter for keeping track of brackets
counter = 0
# Keeps track of the formula(s) within the current bracket
lastIndex = len(formula) - 1
# Goes through each character of the current string
for i in range(1, lastIndex):
# Keeps track of brackets
if (formula[i] == "("):
counter += 1
elif (formula[i] == ")"):
counter -= 1
# If it finds the current operation
elif (counter == 0):
# If current operation is an OR
if (formula[i] == "+"):
# Adds an OrTree to the FormulaTree
return OrTree(build_tree_helper(formula[1:i]),
build_tree_helper(formula[i + 1:lastIndex]))
# If current operation is an AND
if (formula[i] == "*"):
# Adds an AndTree to the FormulaTree
return AndTree(build_tree_helper(formula[1:i]),
build_tree_helper(formula[i + 1:lastIndex]))
# For invalid formulas so a NoneType can be returned
raise Exception()
