解压正则表达式列表

Question

做了一些研究，并问Stack上的某个人是否可以帮助我使用多项式转换器将多项式转换为列表-例如： 3x^2 - 8x + 5 --- [3, -8, 5] 。 他们在此方面做得非常出色，但是就发生的事情而言，这让我有些头疼。 想知道是否有人愿意给我详细的事件细目，并帮助我重新编写它，以确保当我重新了解它时正在发生什么。 （使用regex btw）这是：

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)])

我尝试将其放在对我来说有意义的术语上，但是我真的不知道该怎么办，因为它不起作用。

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

我在哪里错呢？

Answer 1

尝试这个：

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)

输出：

[4, -3, 2]

首先，您需要在代码中了解列表理解的工作原理

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

因此，这个小代码遍历了for循环（for循环应在外部），以评估每个元素上的条件，并将输出附加到主列表中。

Answer 2

有趣的问题！

Python过度压缩的另一个经典案例！ 列表理解可能是一场噩梦，我不希望看到它们被初学者抛弃。 我还注意到该方法的一些潜在问题，因此我想提供一种替代方法。 对于增值，这还将跟踪每个术语的功效。 我已经评论了这种方法。

#   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)

这将输出

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