Python parsing error message functions

Question

The code below was created by me with the help of many SO veterans:

The code takes an entered math expression and splits it into operators and operands for later use. I have created two functions, the parsing function that splits, and the error function. I am having problems with the error function because it won't display my error messages and I feel the function is being ignored when the code runs. An error should print if an expression such as this is entered: 3//3+4,etc. where there are two operators together, or there are more than two operators in the expression overall, but the error messages dont print. My code is below:

def errors():

    numExtrapolation,opExtrapolation=parse(expression)
    if (len(numExtrapolation) == 3) and (len(opExtrapolation) !=2):
        print("Bad1")
    if (len(numExtrapolation) ==2) and (len(opExtrapolation) !=1):
        print("Bad2")


def parse(expression):
    operators= set("*/+-")
    opExtrapolate= []
    numExtrapolate= []
    buff=[]
    for i in expression:
        if i in operators:
            numExtrapolate.append(''.join(buff))
            buff= []
            opExtrapolate.append(i)
            opExtrapolation=opExtrapolate
        else:
            buff.append(i)
    numExtrapolate.append(''.join(buff))
    numExtrapolation=numExtrapolate
    #just some debugging print statements
    print(numExtrapolation)
    print("z:", len(opExtrapolation))
    return numExtrapolation, opExtrapolation

    errors()

Any help would be appreciated. Please don't introduce new code that is any more advanced than the code already here. I am looking for a solution to my problem... not large new code segments. Thanks.

Himanshu · Accepted Answer · 2012-11-02 11:06:37Z

1

The errors() function is called after parse() returns because it appears inside the body of parse(). Hopefully that is a typo.

For this particular input, numExtrapolate is appended with an empty buffer because there is no operand between / and /. That makes its length 4 and your check for Bad1 fails. So put a check like this

 if buff:
     numExtrapolate.append(''.join(buff))

edited Nov 2, 2012 at 11:06

answered Nov 2, 2012 at 10:58

Himanshu

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6 Comments

Himanshu Over a year ago

Ok. How abt print(len(numExtrapolate)). What do u see there.

SeesSound Over a year ago

I do get 4 for numex, even though it should be 3

Himanshu Over a year ago

Yes. Because one of them is empty string that is betwn //. Sorry cant chat from phone

SeesSound Over a year ago

I dont understand how the code you entered is a check for errors? I am new to python so I have never seen an "if buff" statement that doesnt have a booleon after it. Also In the argument of the if statement. What does that line mean. I know I have that line in my original code but I am not sure what the .append part does exactly even though I searched it up.

Himanshu Over a year ago

if buff is equivalent to if len(buff) > 0. The check prevents addition of empty buffers that appear betwn consecutive ops

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