Unpack Regex List Comprehension

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]. The

Solution 1:

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.

Solution 2:

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 = 1if has_variable else0#   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)]