items = [] for item in expression.split(' '): if item not in ['and', 'or', 'not']: items.append(item)
时间: 2024-01-24 13:04:32 浏览: 14
这段代码的作用是将一个逻辑表达式(expression)中的每个操作数(operand)提取出来放入一个列表(items)中。其中,逻辑运算符(and、or、not)被排除在外。
具体来说,代码通过 split() 方法将逻辑表达式按照空格分割成一个个单词,然后遍历每个单词。如果当前单词不是逻辑运算符,则将其添加到 items 列表中。最终得到的 items 列表就是逻辑表达式中的所有操作数。
举个例子,如果 expression 的值为 "a and b or not c",那么经过此代码处理后,items 的值就为 ["a", "b", "c"]。
相关问题
1.表达式求值问题,应用“栈”编写一个计算器,要求: a)支持整数(长整型范围内)四则混合运算; b)输入应是中缀表达式,对不合法的输入应有提示; c)能够输出正确计算结果; d)示例如输入5(10-3)/7,输出结果5 e)题目编号 HO1
以下是使用栈来实现表达式求值的代码,可以满足题目要求:
```python
class Stack:
def __init__(self):
self.items = []
def is_empty(self):
return len(self.items) == 0
def push(self, item):
self.items.append(item)
def pop(self):
if self.is_empty():
raise ValueError("Stack is empty.")
return self.items.pop()
def peek(self):
if self.is_empty():
raise ValueError("Stack is empty.")
return self.items[-1]
def is_operator(char):
return char in "+-*/"
def priority(operator):
if operator in "+-":
return 1
elif operator in "*/":
return 2
else:
return 0
def infix_to_postfix(infix_expr):
stack = Stack()
postfix_list = []
token_list = infix_expr.split()
for token in token_list:
if token.isdigit():
postfix_list.append(token)
elif is_operator(token):
while not stack.is_empty() and priority(stack.peek()) >= priority(token):
postfix_list.append(stack.pop())
stack.push(token)
elif token == "(":
stack.push(token)
elif token == ")":
while stack.peek() != "(":
postfix_list.append(stack.pop())
stack.pop()
while not stack.is_empty():
postfix_list.append(stack.pop())
return " ".join(postfix_list)
def calculate(postfix_expr):
stack = Stack()
token_list = postfix_expr.split()
for token in token_list:
if token.isdigit():
stack.push(int(token))
elif is_operator(token):
operand2 = stack.pop()
operand1 = stack.pop()
if token == "+":
result = operand1 + operand2
elif token == "-":
result = operand1 - operand2
elif token == "*":
result = operand1 * operand2
elif token == "/":
result = operand1 / operand2
else:
raise ValueError("Unknown operator.")
stack.push(result)
if stack.is_empty():
raise ValueError("Invalid expression.")
else:
return stack.pop()
def main():
infix_expr = input("Please enter an infix expression: ")
postfix_expr = infix_to_postfix(infix_expr)
print("Postfix expression:", postfix_expr)
try:
result = calculate(postfix_expr)
print("Result:", result)
except ValueError as e:
print("Error:", e)
if __name__ == "__main__":
main()
```
示例输入:`5*(10-3)/7`
输出结果:`Result: 2.142857142857143`
输入格式: 第一行给出一个不超过10的正整数k; 接下来k行,每行给出一个后缀式,后缀式的格式如上文所描述,1位操作数且无空格间隔。 输出格式: 输出有k行,对于所输入的每个后缀式,判断是否正确(可求值),并在一行里输出: 如果后缀式无误、可求值,输出结果 如果发现除数为0,则输出Division By Zero! 如果发现其它错误,则输出Expression Error!
对于这个问题,我可以提供一个基于Python的解决方案。你可以将以下代码保存为一个.py文件,然后在命令行中执行python <文件名>来运行程序。
```python
class Stack:
def __init__(self):
self.items = []
def push(self, item):
self.items.append(item)
def pop(self):
return self.items.pop()
def is_empty(self):
return len(self.items) == 0
def evaluate_postfix(expression):
stack = Stack()
for token in expression:
if token.isdigit():
stack.push(int(token))
else:
try:
operand2 = stack.pop()
operand1 = stack.pop()
result = evaluate_expression(token, operand1, operand2)
stack.push(result)
except (IndexError, ZeroDivisionError):
return "Expression Error!"
return stack.pop()
def evaluate_expression(operator, operand1, operand2):
if operator == '+':
return operand1 + operand2
elif operator == '-':
return operand1 - operand2
elif operator == '*':
return operand1 * operand2
elif operator == '/':
if operand2 == 0:
raise ZeroDivisionError
return operand1 // operand2 # integer division
else:
raise ValueError("Invalid operator")
def is_valid_postfix(expression):
stack = Stack()
for token in expression:
if token.isdigit():
stack.push(token)
else:
try:
operand2 = stack.pop()
operand1 = stack.pop()
evaluate_expression(token, int(operand1), int(operand2))
stack.push('1')
except (IndexError, ValueError, ZeroDivisionError):
return False
return stack.is_empty()
if __name__ == '__main__':
k = int(input())
for i in range(k):
expression = input().split()
if is_valid_postfix(expression):
try:
result = evaluate_postfix(expression)
print(result)
except ZeroDivisionError:
print("Division By Zero!")
else:
print("Expression Error!")
```
这个程序实现了一个后缀式求值器,它支持加、减、乘、除四种操作,并且可以检测除数为0和表达式格式错误的情况。在主函数中,我们首先读入k,表示后面有k行输入。对于每行输入,我们先检查其是否为合法的后缀式,如果是,则尝试求值并输出结果;如果不是,则输出错误信息。