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Updated postfix_evaluation.py to support Unary operators (#8787)
* Updated postfix_evaluation.py to support Unary operators and floating point numbers Fixes #8754 and #8724 Also merged evaluate_postfix_notations.py and postfix_evaluation.py into postfix_evaluation.py Signed-off-by: Arijit De <arijitde2050@gmail.com> * [pre-commit.ci] auto fixes from pre-commit.com hooks for more information, see https://pre-commit.ci * Updated postfix_evaluation.py to support Unary operators and floating point numbers. Fixes #8754 and formatted code to pass ruff and black test. Also merged evaluate_postfix_notations.py and postfix_evaluation.py into postfix_evaluation.py which fixes #8724 and made sure it passes doctest Signed-off-by: Arijit De <arijitde2050@gmail.com> * Fixed return type hinting required by pre commit for evaluate function Signed-off-by: Arijit De <arijitde2050@gmail.com> * Changed line 186 to return only top of stack instead of calling the get_number function as it was converting float values to int, resulting in data loss. Fixes #8754 and #8724 Signed-off-by: Arijit De <arijitde2050@gmail.com> * Made the requested changes Also changed the code to make the evaluate function first convert all the numbers and then process the valid expression. * Fixes #8754, #8724 Updated postfix_evaluation.py postfix_evaluation.py now supports Unary operators and floating point numbers. Also merged evaluate_postfix_notations.py and postfix_evaluation.py into postfix_evaluation.py which fixes #8724. Added a doctest example with unary operator. * Fixes #8754, #8724 Updated postfix_evaluation.py postfix_evaluation.py now supports Unary operators and floating point numbers. Also merged evaluate_postfix_notations.py and postfix_evaluation.py into postfix_evaluation.py which fixes #8724. Added a doctest example with unary operator. * Fixes #8754, #8724 Updated the parse_token function of postfix_evaluation.py ostfix_evaluation.py now supports Unary operators and floating point numbers. Also merged evaluate_postfix_notations.py and postfix_evaluation.py into postfix_evaluation.py which fixes #8724. Added a doctest example with unary operator and invalid expression. * Fixes #8754, #8724 Updated postfix_evaluation.py postfix_evaluation.py now supports Unary operators and floating point numbers. Also merged evaluate_postfix_notations.py and postfix_evaluation.py into postfix_evaluation.py which fixes #8724. Added a doctest example with unary operator and invalid expression. * Update postfix_evaluation.py * Update postfix_evaluation.py * Update postfix_evaluation.py * Update postfix_evaluation.py * Update postfix_evaluation.py --------- Signed-off-by: Arijit De <arijitde2050@gmail.com> Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com> Co-authored-by: Christian Clauss <cclauss@me.com>
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‎data_structures/stacks/evaluate_postfix_notations.py‎

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‎data_structures/stacks/postfix_evaluation.py‎

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Original file line numberDiff line numberDiff line change
@@ -1,4 +1,11 @@
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"""
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Reverse Polish Nation is also known as Polish postfix notation or simply postfix
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notation.
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https://en.wikipedia.org/wiki/Reverse_Polish_notation
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Classic examples of simple stack implementations.
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Valid operators are +, -, *, /.
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Each operand may be an integer or another expression.
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Output:
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Enter a Postfix Equation (space separated) = 5 6 9 * +
@@ -17,52 +24,177 @@
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Result = 59
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"""
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import operator as op
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# Defining valid unary operator symbols
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UNARY_OP_SYMBOLS = ("-", "+")
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# operators & their respective operation
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OPERATORS = {
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"^": lambda p, q: p**q,
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"*": lambda p, q: p * q,
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"/": lambda p, q: p / q,
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"+": lambda p, q: p + q,
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"-": lambda p, q: p - q,
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}
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def parse_token(token: str | float) -> float | str:
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"""
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Converts the given data to the appropriate number if it is indeed a number, else
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returns the data as it is with a False flag. This function also serves as a check
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of whether the input is a number or not.
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Parameters
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----------
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token: The data that needs to be converted to the appropriate operator or number.
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Returns
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-------
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float or str
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Returns a float if `token` is a number or a str if `token` is an operator
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"""
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if token in OPERATORS:
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return token
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try:
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return float(token)
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except ValueError:
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msg = f"{token} is neither a number nor a valid operator"
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raise ValueError(msg)
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def evaluate(post_fix: list[str], verbose: bool = False) -> float:
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"""
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Evaluate postfix expression using a stack.
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>>> evaluate(["0"])
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0.0
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>>> evaluate(["-0"])
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-0.0
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>>> evaluate(["1"])
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1.0
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>>> evaluate(["-1"])
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-1.0
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>>> evaluate(["-1.1"])
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-1.1
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>>> evaluate(["2", "1", "+", "3", "*"])
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9.0
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>>> evaluate(["2", "1.9", "+", "3", "*"])
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11.7
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>>> evaluate(["2", "-1.9", "+", "3", "*"])
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0.30000000000000027
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>>> evaluate(["4", "13", "5", "/", "+"])
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6.6
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>>> evaluate(["2", "-", "3", "+"])
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1.0
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>>> evaluate(["-4", "5", "*", "6", "-"])
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-26.0
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>>> evaluate([])
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0
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>>> evaluate(["4", "-", "6", "7", "/", "9", "8"])
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Traceback (most recent call last):
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...
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ArithmeticError: Input is not a valid postfix expression
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Parameters
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----------
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post_fix:
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The postfix expression is tokenized into operators and operands and stored
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as a Python list
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verbose:
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Display stack contents while evaluating the expression if verbose is True
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def solve(post_fix):
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Returns
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-------
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float
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The evaluated value
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"""
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if not post_fix:
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return 0
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# Checking the list to find out whether the postfix expression is valid
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valid_expression = [parse_token(token) for token in post_fix]
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if verbose:
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# print table header
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print("Symbol".center(8), "Action".center(12), "Stack", sep=" | ")
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print("-" * (30 + len(post_fix)))
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stack = []
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div = lambda x, y: int(x / y) # noqa: E731 integer division operation
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opr = {
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"^": op.pow,
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"*": op.mul,
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"/": div,
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"+": op.add,
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"-": op.sub,
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} # operators & their respective operation
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# print table header
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print("Symbol".center(8), "Action".center(12), "Stack", sep=" | ")
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print("-" * (30 + len(post_fix)))
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for x in post_fix:
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if x.isdigit(): # if x in digit
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for x in valid_expression:
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if x not in OPERATORS:
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stack.append(x) # append x to stack
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# output in tabular format
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print(x.rjust(8), ("push(" + x + ")").ljust(12), ",".join(stack), sep=" | ")
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else:
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if verbose:
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# output in tabular format
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print(
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f"{x}".rjust(8),
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f"push({x})".ljust(12),
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stack,
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sep=" | ",
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)
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continue
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# If x is operator
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# If only 1 value is inside the stack and + or - is encountered
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# then this is unary + or - case
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if x in UNARY_OP_SYMBOLS and len(stack) < 2:
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b = stack.pop() # pop stack
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if x == "-":
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b *= -1 # negate b
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stack.append(b)
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if verbose:
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# output in tabular format
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print(
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"".rjust(8),
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f"pop({b})".ljust(12),
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stack,
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sep=" | ",
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)
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print(
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str(x).rjust(8),
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f"push({x}{b})".ljust(12),
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stack,
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sep=" | ",
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)
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continue
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b = stack.pop() # pop stack
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if verbose:
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# output in tabular format
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print("".rjust(8), ("pop(" + b + ")").ljust(12), ",".join(stack), sep=" | ")
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print(
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"".rjust(8),
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f"pop({b})".ljust(12),
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stack,
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sep=" | ",
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)
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48-
a = stack.pop() # pop stack
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a = stack.pop() # pop stack
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if verbose:
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# output in tabular format
50-
print("".rjust(8), ("pop(" + a + ")").ljust(12), ",".join(stack), sep=" | ")
51-
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stack.append(
53-
str(opr[x](int(a), int(b)))
54-
) # evaluate the 2 values popped from stack & push result to stack
167+
print(
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"".rjust(8),
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f"pop({a})".ljust(12),
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stack,
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sep=" | ",
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)
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# evaluate the 2 values popped from stack & push result to stack
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stack.append(OPERATORS[x](a, b)) # type: ignore[index]
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if verbose:
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# output in tabular format
56177
print(
57-
x.rjust(8),
58-
("push(" + a + x + b + ")").ljust(12),
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",".join(stack),
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f"{x}".rjust(8),
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f"push({a}{x}{b})".ljust(12),
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stack,
60181
sep=" | ",
61182
)
62-
63-
return int(stack[0])
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# If everything is executed correctly, the stack will contain
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# only one element which is the result
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if len(stack) != 1:
186+
raise ArithmeticError("Input is not a valid postfix expression")
187+
return float(stack[0])
64188

65189

66190
if __name__ == "__main__":
67-
Postfix = input("\n\nEnter a Postfix Equation (space separated) = ").split(" ")
68-
print("\n\tResult = ", solve(Postfix))
191+
# Create a loop so that the user can evaluate postfix expressions multiple times
192+
while True:
193+
expression = input("Enter a Postfix Expression (space separated): ").split(" ")
194+
prompt = "Do you want to see stack contents while evaluating? [y/N]: "
195+
verbose = input(prompt).strip().lower() == "y"
196+
output = evaluate(expression, verbose)
197+
print("Result = ", output)
198+
prompt = "Do you want to enter another expression? [y/N]: "
199+
if input(prompt).strip().lower() != "y":
200+
break

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