Unpack regex list comprehension

Question

Did some research, and asked someone on Stack if they could help me with a polynomial converter to take a polynomial and turn it into a list - Ex: 3x^2 - 8x + 5 --- [3, -8, 5]. They did an excellent job with it, however it is a bit over my head in terms of whats going on. Was wondering if anyone would be willing to give me a detailed breakdown on what's happening and help me rewrite it to make sure I understand what's going on when I come back to it. (Using regex btw) Here it is:

poly = ("4x^2 - 3x + 2")
s=re.sub('[xX^]','',poly)
print([int('-'+i[0]) if s[s.index(i)-2]=='-' else int(i[0]) for i in re.split(' [+|-] ',s)])

I tried putting it in terms that make sense to me order-wise but I don't really know what to do because it doesn't work.

if s[s.index(i) - 2]=="-":
    int("-" + i[0])
else:
    int(i[0])
    for i in re.split:
        (" [+|-] ", s)

Where am I going wrong with this?

That's certainly not what I would consider an "excellent job". It doesn't handle multi-digit coefficients, a minus sign on the leading term, or implicit "1" coefficients, and a lot of the regex parts are either superfluous or wrong - for example, `|` is not alternation inside a character class. — user2357112, Dec 12 '18 at 05:49
@user2357112 Maybe leave feedback on the actual answer: https://stackoverflow.com/questions/53716077/storing-specific-whole-numbers-python — tripleee, Dec 12 '18 at 05:53
Are you aware of [SymPy](https://www.sympy.org/en/index.html)? — o11c, Dec 12 '18 at 06:02
Also, even worse problem: `index` frequently returns a different index from the index you're looking for, leading to results like [`'3x^2 - 3x - 3'` getting parsed as `[3, 3, 3]`](https://ideone.com/QMdEP9). — user2357112, Dec 12 '18 at 06:06
@o11c There's another answer on the original question which suggests sympy — tripleee, Dec 12 '18 at 06:09

score 2 · Accepted Answer · answered Dec 12 '18 at 05:46

Try this:

for i in re.split(' [+|-] ',s):
    if s[s.index(i) - 2]=="-":
        x.append(int("-" + i[0]))
    else:
        x.append(int(i[0]))
print(x)

Output:

[4, -3, 2]

First, You need to understand how list comprehension works, in your code

[ `firstValue`  if `condition` else  `secondValue` while iterating over list]

So, the this small code, is iterating over a for loop (the for loop should be outside) evaluating condition on each element, and appending the output to main list.

That makes SO much more sense! I've never even heard of list comprehension so thank you :D — Jacob Smith, Dec 13 '18 at 04:31

score 1 · Answer 2 · answered Dec 12 '18 at 06:09

Interesting problem!

Another classic case of Python over-compression! List comprehensions can be a nightmare, and I hate to see them thrown at beginners. I also noticed a few potential problems with that approach, so I'd like to offer an alternative. For a value add, this will also keep track of the power of each term. I've commented the approach.

#   This is hopefully self-explanatory.
poly = "4x^2 - 3x^3 - 2x + 2"
#   We made this regex more complicated in order to capture an entire
#   term of a polynomial. Here's how that works:
#   ([+-]|)     - The first capture group, contain the sign or (|)
#                 nothing.
#   \s*         - Allow whitespace between the sign and the term.
#   ([0-9]+)    - The second capture group, contain the coefficient.
#   ([xX]|)     - Match the variable or nothing.
#   (\^[0-9]+|) - Match the exponent or nothing. Notice we escape the caret.
matches = re.finditer(r'([+-]|)\s*([0-9]+)([xX]|)(\^[0-9]+|)', poly)

#   Now we parse the results of the regex...
terms = list() # Create a list to store our processed terms.
for match in matches: # Iterate our matches.
    #   Step 1: Parse the groups' contents.
    negative = match.group(1) == '-' # Check if the sign is negative.
    coef = int(match.group(2)) # Retrieve the coefficient as an integer.
    has_variable = bool(match.group(3)) # Check whether there's a variable.
    exponent_str = match.group(4) # Get the exponent group.
    if exponent_str:
        #   There's an exponent, remove the caret and turn it into an
        #   integer.
        exponent = int(exponent_str[1:])
    else:
        #   There's no exponent, if there's a variable the power of this term
        #   is 1, otherwise it's 0.
        exponent = 1 if has_variable else 0

    #   Step 2: Create a (coefficient, power) tuple and add it to the list of
    #   terms.
    if negative:
        #   Make the coefficient negative if the sign was a -.
        coef *= -1
    #   Add a tuple containing the coefficient and exponent to our list of
    #   terms.
    terms.append((coef, exponent))

#   Optional: sort the terms in order of power (descending).
terms.sort(key=lambda tpl: -tpl[1])

#   Print the output.
print(terms)

This will output

[(-3, 3), (4, 2), (-2, 1), (2, 0)]

Unpack regex list comprehension

2 Answers2

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