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* pre-commit: Upgrade psf/black for stable style 2023 Updating https://github.com/psf/black ... updating 22.12.0 -> 23.1.0 for their `2023 stable style`. * https://github.com/psf/black/blob/main/CHANGES.md#2310 > This is the first [psf/black] release of 2023, and following our stability policy, it comes with a number of improvements to our stable style… Also, add https://github.com/tox-dev/pyproject-fmt and https://github.com/abravalheri/validate-pyproject to pre-commit. I only modified `.pre-commit-config.yaml` and all other files were modified by pre-commit.ci and psf/black. * [pre-commit.ci] auto fixes from pre-commit.com hooks for more information, see https://pre-commit.ci --------- Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
60 lines
1.2 KiB
Python
60 lines
1.2 KiB
Python
"""
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Python3 program to evaluate a prefix expression.
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"""
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calc = {
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"+": lambda x, y: x + y,
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"-": lambda x, y: x - y,
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"*": lambda x, y: x * y,
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"/": lambda x, y: x / y,
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}
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def is_operand(c):
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"""
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Return True if the given char c is an operand, e.g. it is a number
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>>> is_operand("1")
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True
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>>> is_operand("+")
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False
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"""
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return c.isdigit()
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def evaluate(expression):
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"""
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Evaluate a given expression in prefix notation.
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Asserts that the given expression is valid.
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>>> evaluate("+ 9 * 2 6")
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21
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>>> evaluate("/ * 10 2 + 4 1 ")
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4.0
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"""
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stack = []
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# iterate over the string in reverse order
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for c in expression.split()[::-1]:
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# push operand to stack
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if is_operand(c):
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stack.append(int(c))
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else:
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# pop values from stack can calculate the result
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# push the result onto the stack again
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o1 = stack.pop()
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o2 = stack.pop()
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stack.append(calc[c](o1, o2))
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return stack.pop()
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# Driver code
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if __name__ == "__main__":
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test_expression = "+ 9 * 2 6"
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print(evaluate(test_expression))
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test_expression = "/ * 10 2 + 4 1 "
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print(evaluate(test_expression))
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