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* prefix_evaluation: Add alternative recursive implementation * improve doc * better variable name calc->operators * Update prefix_evaluation.py * Update prefix_evaluation.py * Update prefix_evaluation.py * Update prefix_evaluation.py --------- Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>
93 lines
1.9 KiB
Python
93 lines
1.9 KiB
Python
"""
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Program to evaluate a prefix expression.
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https://en.wikipedia.org/wiki/Polish_notation
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"""
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operators = {
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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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>>> evaluate("2")
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2
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>>> evaluate("+ * 2 3 / 8 4")
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8.0
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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(operators[c](o1, o2))
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return stack.pop()
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def evaluate_recursive(expression: list[str]):
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"""
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Alternative recursive implementation
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>>> evaluate_recursive(['2'])
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2
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>>> expression = ['+', '*', '2', '3', '/', '8', '4']
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>>> evaluate_recursive(expression)
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8.0
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>>> expression
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[]
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>>> evaluate_recursive(['+', '9', '*', '2', '6'])
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21
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>>> evaluate_recursive(['/', '*', '10', '2', '+', '4', '1'])
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4.0
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"""
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op = expression.pop(0)
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if is_operand(op):
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return int(op)
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operation = operators[op]
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a = evaluate_recursive(expression)
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b = evaluate_recursive(expression)
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return operation(a, b)
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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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