本文實例講述了python實現逆波蘭計算表達式的方法。分享給大家供大家參考。具體分析如下：
逆波蘭表達式又叫做后綴表達式。在通常的表達式中，二元運算符總是置于與之相關的兩個運算對象之間，所以，這種表示法也稱為中綴表示。波蘭邏輯學家J.Lukasiewicz于1929年提出了另一種表示表達式的方法。按此方法，每一運算符都置于其運算對象之后，故稱為后綴表示。
# -*- coding: utf-8 -*-symbol_priority = {}symbol_priority[0] = ['#']symbol_priority[1] = ['(']symbol_priority[2] = ['+', '-']symbol_priority[3] = ['*', '/']symbol_priority[4] = [')']def comparePriority(symbol, RPN_stack, symbol_stack): '''Compare priority between two symbols''' global symbol_priority if len(symbol_stack) > 0: symbol_pop = symbol_stack.pop() else: return for list in symbol_priority.values(): if (symbol in list) and (symbol_pop in list): '''same priority''' symbol_stack.append(symbol_pop) symbol_stack.append(symbol) return elif symbol in list: '''symbol is smaller''' RPN_stack.append(symbol_pop) #recusion call comparePriority(symbol, RPN_stack, symbol_stack) return elif symbol_pop in list: '''symbol is bigger''' symbol_stack.append(symbol_pop) symbol_stack.append(symbol) return else: continue symbol_stack.append(symbol_pop) returndef scanEveryone(input_string, RPN_stack, symbol_stack): for ch in input_string: if ch.isdigit(): RPN_stack.append(ch) else: if len(symbol_stack) > 0: if ch == '(': symbol_stack.append(ch) elif ch == ')': while True: symbol_pop = symbol_stack.pop() if symbol_pop == '(': break else: RPN_stack.append(symbol_pop) else: comparePriority(ch, RPN_stack, symbol_stack) else: symbol_stack.append(ch)def scanInput(RPN_stack, symbol_stack): input_string = raw_input() input_string += '#' scanEveryone(input_string, RPN_stack, symbol_stack)def calRPN(RPN_stack): value_stack = [] RPN_stack.append('#') for value in RPN_stack: if value == '#': return value_stack.pop() break if value.isdigit(): value_stack.append(value) else: right_value = value_stack.pop() left_value = value_stack.pop() cal_string = left_value + value + right_value value_stack.append(str(eval(cal_string)))def main(): RPN_stack = [] symbol_stack = [] scanInput(RPN_stack, symbol_stack) print calRPN(RPN_stack)if __name__ == '__main__': main()calRPN.py
# -*- coding: utf-8 -*-symbol_priority = {}symbol_priority[0] = ['#']symbol_priority[1] = ['(']symbol_priority[2] = ['+', '-']symbol_priority[3] = ['*', '/']symbol_priority[4] = [')']def comparePriority(symbol, RPN_stack, symbol_stack): '''Compare priority between two symbols''' global symbol_priority if len(symbol_stack) > 0: symbol_pop = symbol_stack.pop() else: return for list in symbol_priority.values(): if (symbol in list) and (symbol_pop in list): '''same priority''' symbol_stack.append(symbol_pop) symbol_stack.append(symbol) return elif symbol in list: '''symbol is smaller''' RPN_stack.append(symbol_pop) #recusion call comparePriority(symbol, RPN_stack, symbol_stack) return elif symbol_pop in list: '''symbol is bigger''' symbol_stack.append(symbol_pop) symbol_stack.append(symbol) return else: continue symbol_stack.append(symbol_pop) returndef scanEveryone(input_string, RPN_stack, symbol_stack): for ch in input_string: if ch.isdigit(): RPN_stack.append(ch) else: if len(symbol_stack) > 0: if ch == '(': symbol_stack.append(ch) elif ch == ')': while True: symbol_pop = symbol_stack.pop() if symbol_pop == '(': break else: RPN_stack.append(symbol_pop) else: comparePriority(ch, RPN_stack, symbol_stack) else: symbol_stack.append(ch)def scanInput(RPN_stack, symbol_stack): input_string = raw_input() input_string += '#' scanEveryone(input_string, RPN_stack, symbol_stack)def calRPN(RPN_stack): value_stack = [] RPN_stack.append('#') for value in RPN_stack: if value == '#': return value_stack.pop() break if value.isdigit(): value_stack.append(value) else: right_value = value_stack.pop() left_value = value_stack.pop() cal_string = left_value + value + right_value value_stack.append(str(eval(cal_string)))def main(): RPN_stack = [] symbol_stack = [] scanInput(RPN_stack, symbol_stack) print calRPN(RPN_stack)if __name__ == '__main__': main()希望本文所述對大家的Python程序設計有所幫助。
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