I have a homework to make a calculator(infix-to postfix) and I need to do it asap. I get always an error when I run this code(Enter a postfix expression (or nothing to quit): +54 Traceback (most recent call last): File "C:\Python32\infix to postfix", line 69, in <module> main() File "C:\Python32\infix to postfix", line 62, in main print(" => {}".format(postfix_eval(expr))) File "C:\Python32\infix to postfix", line 42, in postfix_eval stack = Stack() NameError: global name 'Stack' is not defined – How do i fix this error?
By the way I am a new programmer,the question could be simple for experts.I will be very pleased, if you could just give the answer as soon as possible.
class Op:
def __init__(self, num_in, num_out, fn):
"""
A postfix operator
num_in: int
num_out: int
fn: accept num_in positional arguments,
perform operation,
return list containing num_out values
"""
assert num_in >= 0, "Operator cannot have negative number of arguments"
self.num_in = num_in
assert num_out >= 0, "Operator cannot return negative number of results"
self.num_out = num_out
self.fn = fn
def __call__(self, stack):
"""
Run operator against stack (in-place)
"""
args = stack.pop_n(self.num_in) # pop num_in arguments
res = self.fn(*args) # pass to function, get results
stack.push_n(self.num_out, res) # push num_out values back
ops = {
'*': Op(2, 1, lambda a,b: [a*b]), # multiplication
'/': Op(2, 1, lambda a,b: [a//b]), # integer division
'+': Op(2, 1, lambda a,b: [a+b]), # addition
'-': Op(2, 1, lambda a,b: [a-b]), # subtraction
'/%': Op(2, 2, lambda a,b: [a//b, a%b]) # divmod (example of 2-output op)
}
def postfix_eval(tokens):
"""
Evaluate a series of tokens as a postfix expression;
return the resulting stack
"""
if isinstance(tokens, str):
# if tokens is a string, treat it as a space-separated list of tokens
tokens = tokens.split()
stack = Stack()
for token in tokens:
try:
# Convert to int and push on stack
stack.append(int(token))
except ValueError:
try:
# Not an int - must be an operator
# Get the appropriate operator and run it against the stack
op = ops[token]
op(stack) # runs Op.__call__(op, stack)
except KeyError:
# Not a valid operator either
raise ValueError("unknown operator {}".format(token))
return stack
def main():
while True:
expr = input('\nEnter a postfix expression (or nothing to quit): ').strip()
if expr:
try:
print(" => {}".format(postfix_eval(expr)))
except ValueError as error:
print("Your expression caused an error: {}".format(error))
else:
break
if __name__=="__main__":
main()
