This is really puzzling me. I'm parsing a string to do some simple math
operations and practice tossing functions around. My parser works on the
first run, then it continually fails on the same input.
"""
Takes the name of a binary math operation and two numbers from input,
repeatedly, and displays the results until done
"""
def add(a, b):
return a + b
def subtract(a, b):
return b - a
def minus(a, b):
return a - b
def multiply(a, b):
return a * b
def divide(a, b):
return a / b
operations = {'add': add, '+': add, 'plus': add, 'subtract': subtract,
'-': minus, 'minus': minus, 'multiply': multiply, '*':
multiply,
'times': multiply, 'divide': divide, '/': divide,
'divided': divide}
numbers = []
def test_number(astring):
"""
Input: A string that should represent a valid int or float. Output:
An int or float on success. None on failure.
"""
for make_type in (int, float):
try:
return make_type(astring)
except ValueError:
pass
return None
def parse_string(math_string):
"""Input: A math string with a verbal or mathematical operation
and two valid numbers to operate on. Extra numbers and operations
are ignored. Output: A tuple containing a function corresponding
to the operation and the two numbers. Returns None on failure.
"""
operation = None
tokens = math_string.split()
for token in tokens:
if token in operations:
operation = operations[token]
elif test_number(token) != None:
numbers.append(test_number(token))
if len(numbers) > 1:
break
if operation is None or len(numbers) < 2:
return None
else:
return operation, numbers[0], numbers[1]
REPL
>>> result = parse_string('1 minus 15')
>>> func, number1, number2 = result
>>> func(number1, number2)
-14
>>> result = parse_string('1 minus 15')
>>> print(result)
None
>>> result = parse_string('1 minus 15')
>>> print(result)
None
>>>
--
Jim
If you only had one hour left to live, would you spend it on Facebook,
Twitter, or Google Plus?
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