Python equation parser

Question

I'm writing a program which needs a user input for an polynomial function of x. I'm using Tkinter and python 2.5.

I have a parser method which so far takes the inputted equation and splits it into terms without dropping the signs.

I want to take each term and parse it to get a tuple of the (coefficient, degree). For example, -2x^3 returns (-2,3). I can then add these to an array and manipulate them accordingly in the program.

Is there a way or standard module that can do this?

Here is the beginning of the parse method.

def parse(a):
    termnum=[]
    terms=[]
    hi=[]
    num1=0
    num=0
    f=list(a)

    count=0
    negative=False
    coef=0.0
    deg=0.0
    codeg=[]
    for item in f:
        if (item=='-' or item=='+') and count!=0:
            termnum.append(count)
        count+=1
    for item in termnum:
        num1=num
        num=item
        current=''
        while num1<num:
            current=current+f[num1]
            num1+=1
        terms.append(current)
    num1=num
    num=len(f)
    current=''
    while num1<num:
        current=current+f[num1]
        num1+=1
    terms.append(current)
    print terms
parse('-x^2+3x+2x^3-x')

Thanks! P.S I don't want to use external packages.

Answer 1

you can use regular expressions,

import re

test = '-x^2+3x+2x^3-x'

for m in re.finditer( r'(-{0,1}\d*)x\^{0,1}(-{0,1}\d*)', test ):
    coef, expn = list( map( lambda x: x if x != '' and x != '-' else x + '1' ,
                            m.groups( ) ))
    print ( 'coef:{}, exp:{}'.format( coef, expn ))

output:

coef:-1, exp:2
coef:3, exp:1
coef:2, exp:3
coef:-1, exp:1

Answer 2

Look for "recursive descent parser". It's the canonical method for analysis of strings where some operator precedence is involved.

Answer 3

It looks like you're implementing something that already exists, in python and other math languages. See for example:

http://www.gnu.org/software/octave/doc/interpreter/Solvers.html

http://stat.ethz.ch/R-manual/R-devel/library/base/html/solve.html