IndexError: string index out of range on equation cleaning function

问题

I have code to append ^0 to all the constants in my code so that if you had the string "3x^2+14+2" it would become "3x^2+14^0+2^0" however I am getting a IndexError and I have no idea what I am doing wrong. This is my code:

def cleanEquation(equation):
    equation = ''.join(equation.split())
    for i in range(len(equation)):
        if equation[i].isdigit():
            if equation[i-1] != "^":
                if i == len(equation)-1:
                    equation = equation[:i+1] + '^0'
                if equation[i+1] == "+" or equation[i+1] == "-":
                    equation = equation[:i+1] + '^0' + equation[i+1]

cleanEquation("x+14+y+14")

Whenever I try to run this I get:

IndexError: string index out of range

This is only a snippet of the function the whole function adds 1 to the beggining of every coefficient and also adds ^1 to every variable with no coefficient and those 2 parts work fine for some reason even though they have the same format is this part of the function. I can post the full function if needed.

回答1:

for i in range(len(equation)):

uses the original length of equation as the limit of i. But the line:

equation = equation[:i+1] + '^0' + equation[i+1]

removes characters from equation. When i gets to the new length of equation, you get an error.

You need to use a while loop so you compare with the current length rather than the original length.

i = 0
while i < len(equation):
    if equation[i].isdigit():
        if equation[i-1] != "^":
            if i == len(equation)-1:
                equation = equation[:i+1] + '^0'
            if equation[i+1] == "+" or equation[i+1] == "-":
                equation = equation[:i+1] + '^0' + equation[i+1]
    i += 1

You could do the whole thing with a regular expression substitution:

import re

def cleanEquation(equation):
    equation = ''.join(equation.split())
    equation = re.sub(r'(?<=\d\b)(?!\^)', '^0', equation)
    return equation

(?<=\d\b) is a lookbehind that matches a digit followed by a word boundary, i.e. the last digit of a numb. (?!\^) is a negative lookahead that prevents matching if the number is already followed by ^.

来源：https://stackoverflow.com/questions/58383273/indexerror-string-index-out-of-range-on-equation-cleaning-function