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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
"""
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