python實現逆波蘭計算表達式的方法

 　　這篇文章主要介紹了python實現逆波蘭計算表達式的方法,較為詳細的分析了逆波蘭表達式的概念及實現技巧,具有一定參考借鑒價值,需要的朋友可以參考下

　　逆波蘭表達式又叫做後綴表達式。在通常的表達式中，二元運算符總是置於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於1929年提出了另一種表示表達式的方法。按此方法，每一運算符都置於其運算對象之後，故稱為後綴表示。

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 # -*- 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) return def 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

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 # -*- 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) return def 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, R