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What is reverse Polish expression?
The reverse Polish expression, also known as a postfix expression, is a method of expressing arithmetic expressions that does not require parentheses to distinguish operator precedence. Its characteristic is that the operator is behind the operand. For example, converting the infix expression "3 4 5" into a reverse Polish expression is "3 4 5 ".
What are Python regular expressions?
Python regular expressions are tools for matching and processing text data. Regular expressions can be used to search for patterns in text. Python provides the "re" module to use regular expression functionality.
How to use Python regular expressions for reverse Polish expression evaluation?
Implementing the reverse Polish expression evaluation algorithm requires following the following steps:
We can use Python regular expressions to easily implement the function of converting reverse Polish expressions into lists. The sample code is as follows:
import re expression = "3 4 5 * +" tokens = re.findall("d+|S", expression) print(tokens) # ['3', '4', '5', '*', '+']
Then, we can follow the above steps to implement the evaluation algorithm of the reverse Polish expression. The sample code is as follows:
stack = [] for token in tokens: if re.match("d+", token): stack.append(int(token)) else: operand2 = stack.pop() operand1 = stack.pop() if token == "+": stack.append(operand1 + operand2) elif token == "-": stack.append(operand1 - operand2) elif token == "*": stack.append(operand1 * operand2) elif token == "/": stack.append(int(operand1 / operand2)) result = stack.pop() print(result) # 23
This code creates an empty stack, iterates through the reverse Polish expression list, checks each operator and operand, performs the corresponding operation on the stack, and finally returns the element at the top of the stack as a result.
Conclusion
Using Python regular expressions you can easily convert reverse Polish expressions into lists and perform arithmetic calculations on the stack. Python's regular expressions are very powerful and can help us achieve fast, flexible and reliable text matching and processing.
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