Python: Math expression parsing

Question

Python: So I am working on a program (which is a class assignment) that will take an expression such as 3/4/5 or 32432/23423/2354325 or 3425*343/254235 or 43252+34254-2435, etc(for all operators from +,-,/,*). and will solve the expression.

I CANT USE EVAL!!

I cant use higher level codes, I am limited to, at most, using string manipulators from the below website to split the string.

http://docs.python.org/2/library/stdtypes.html#typesseq

My method is to look at the expression the user enters and then use a find function to find the OPERATORS, and then use these operators and a slicing function (eg. s[0:x]). What I have is below and unfortunately it isnt working: *note that the print statements are in there for debugging purposes only. EDIT: why is x not defined when I run the program and enter an expression?

z= (input("expression:")).strip()

def finding(z):
    if "/" in z:
        x=z.find("/")
        print("hi1")
    elif "*" in z:
        x=z.find("*")
        print("hi2")
    elif "+" in z:
        x=z.find("+")
        print("hi3")
    elif "-" in z:
        x=z.find("-")
        print("hi4")
    else:
        print("error, not math expression")
    return x

def Parsing(z,x):

    x= finding(z)
    qw=z.s[0:x]
    print (qw)
# take the x-value from function finding(z) and use it to split 

finding(z)
Parsing(z,x)

Answer 1

If you're just having trouble splitting the input into its parts, here's something to help you. I left it as readable as I could so that you could at least understand what it does. I'll explain any part of it if you need me to:

def parse(text):
    chunks = ['']

    for character in text:
        if character.isdigit():
            if chunks[-1].isdigit():   # If the last chunk is already a number
                chunks[-1] += character  # Add onto that number
            else:
                chunks.append(character) # Start a new number chunk
        elif character in '+-/*':
            chunks.append(character)  # This doesn't account for `1 ++ 2`.

    return chunks[1:]

Example usage:

>>> parse('123 + 123')
['123', '+', '123']
>>> parse('123 + 123 / 123 + 123')
['123', '+', '123', '/', '123', '+', '123']

I'll leave the rest up to you. If you aren't allowed to use .isdigit(), you'll have to replace it with lower-level Python code.

Answer 2

The easiest way for doing this, I think, is implementing a Shunting-yard algorithm to convert your equation in postfix notation and then executing it from left to right.

But since this is a class assignment, you should do the actual implementation yourself, I already gave you more than I should have.

Answer 3

Why is x not defined when I run the program and enter an expression?

x is not in scope, you only define it in your methods, and you try to access it elsewhere.

z= (input("expression:")).strip()

def finding(z):
    # ... removed your code ...
    # in this method, you define x, which is local
    # to the method, nothing outside this method has
    # access to x
    return x

def Parsing(z,x):

    x= finding(z) # this is a different x that is assigned the 
                  # return value from the 'finding' method.
    qw=z.s[0:x] # I'm curious as to what is going on here.
    print (qw)
# take the x-value from function finding(z) and use it to split 

finding(z) # here, z is the value from the top of your code
Parsing(z,x) # here, x is not defined, which is where you get your error.

Since Parsing is already calling finding to get the value of x, you don't need to pass it into Parsing, you also don't need to call finding(z) outside Parsing, since you don't store the value anywhere.

def Parsing(z):

    x= finding(z) # this is a different x that is assigned the 
                  # return value from the 'finding' method.
    qw=z.s[0:x] # I'm curious as to what is going on here.
    print (qw)
# take the x-value from function finding(z) and use it to split 

# finding(z)  -- not needed 
Parsing(z)