|
|
| def rpn_eval(tokens): |
| def op(symbol, a, b): |
| return { |
| '+': lambda a, b: a + b, |
| '-': lambda a, b: a - b, |
| '*': lambda a, b: a * b, |
| '/': lambda a, b: a / b |
| }[symbol](a, b) |
|
|
| stack = [] |
|
|
| for token in tokens: |
| if isinstance(token, float): |
| stack.append(token) |
| else: |
| a = stack.pop() |
| b = stack.pop() |
| stack.append( |
| op(token, b, a) |
| ) |
|
|
| return stack.pop() |
|
|
|
|
| """ |
| Reverse Polish Notation |
| |
| Four-function calculator with input given in Reverse Polish Notation (RPN). |
| |
| Input: |
| A list of values and operators encoded as floats and strings |
| |
| Precondition: |
| all( |
| isinstance(token, float) or token in ('+', '-', '*', '/') for token in tokens |
| ) |
| |
| Example: |
| >>> rpn_eval([3.0, 5.0, '+', 2.0, '/']) |
| 4.0 |
| """ |
|
|
