def rpn_calc(postfix_expr):
"""Evaluate a postfix expression"""
operandStack = Stack()
tokenList = postfix_expr.split()
try:
for token in tokenList:
if token in "0123456789":
operandStack.push(int(token))
elif token in "+-*/":
if operandStack.size() < 2:
raise StackError("Stack is empty")
else:
operand2 = operandStack.pop()
operand1 = operandStack.pop()
result = doMath(token, operand1, operand2)
operandStack.push(result)
else:
raise TokenError(f"Unknown token: {token}")
if operandStack.size() > 1:
raise StackError("Stack is not empty")
except TokenError as err:
raise TokenError(f"{err}")
except SyntaxError:
raise SyntaxError("Invalid syntax")
except StackError as err:
raise StackError(f"{err}")
else:
return operandStack.pop()
def postfix_eval(postfix_expr: str) -> int:
""" evaluate postfix expressions in a string """
operandStack = Stack()
tokenList = postfix_expr.split()
try:
for token in tokenList:
if token.isnumeric():
operandStack.push(int(token))
elif token in "* ** / // % + -":
## this if statement is not working - why not?
if operandStack.size() < 2:
raise SyntaxError("invalid syntax")
else:
operand2 = operandStack.pop()
operand1 = operandStack.pop()
result = do_math(token, operand1, operand2)
operandStack.push(result)
elif token == "=":
if operandStack.size() > 1:
raise StackError("Stack is not empty")
elif operandStack.size() == 0:
raise StackError("Stack is empty")
else:
return operandStack.pop()
else:
raise TokenError(f"Unknown token: {token}")
except ZeroDivisionError as err:
raise ZeroDivisionError(f"{err}")
except SyntaxError:
raise SyntaxError(f"Invalid syntax")
except TokenError as err:
raise TokenError(f"{err}")
def rev_string(my_str):
"""Reverse characters in a string using a stack"""
char_stack = Stack()
for char in my_str:
char_stack.push(char)
new_chars = []
while char_stack.size() > 0:
new_chars.append(char_stack.pop())
new_str = "".join(str(char) for char in new_chars)
return new_str
def base_converter(dec_num, base):
"""Convert a decimal number to any base"""
digits = "0123456789ABCDEF"
remstack = Stack()
try:
if base in (2, 8, 16):
while dec_num > 0:
rem = dec_num % base
remstack.push(rem)
dec_num = dec_num // base
new_string = ""
ctr = remstack.size()
while ctr > 0:
new_string = new_string + digits[remstack.pop()]
ctr -= 1
return new_string
else:
raise BaseError
except BaseError:
print("Invalid base: only 2, 8, or 16 may be used")
print(my_list.size())
print(my_list.search(93))
print(my_list.search(100))
# Alternative implementation from the pythonds3 library
# -> sudo pip3 install pythonds3
from pythonds3.basic import Stack
s = Stack()
print(s.is_empty())
s.push(4)
s.push("dog")
print(s.peek())
s.push(True)
print(s.size())
print(s.is_empty())
s.push(8.4)
print(s.pop())
print(s.pop())
print(s.size())
m = Stack()
m.push("x")
m.push("y")
m.push("z")
while not m.is_empty():
print(m.peek())
m.pop()