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From: Joshua Landau <joshua@landau.ws>
Date: Mon, 11 Nov 2013 20:50:35 +0000
Subject: Re: New user's initial thoughts / criticisms of Python
To: Chris Angelico <rosuav@gmail.com>
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On 11 November 2013 10:39, Chris Angelico <rosuav@gmail.com> wrote:
> On Mon, Nov 11, 2013 at 9:09 PM,  <lorenzo.gatti@gmail.com> wrote:
>> Regarding the "select" statement, I think the most "Pythonic" approach i=
s using dictionaries rather than nested ifs.
>> Supposing we want to decode abbreviated day names ("mon") to full names =
("Monday"):
>
> You can't [normally], for instance, build up a
> dictionary that handles inequalities, but you can do that with elif.
> [...] Consider the following logic:
>
> A 'minor weapon' is based on a roll of a 100-sided dice. If it's 01 to
> 70, "+1 weapon: 2,000gp [weapon]"; if it's 71 to 85, "+2 weapon:
> 8,000gp [weapon]"; if 86 to 90, "Specific weapon [minor specific
> weapon]"; and if 91 to 100, "Special ability [minor special weapon]
> and roll again".
>
> My code to handle that starts out with this array:
>
> "minor weapon":({
>     70,"+1 weapon: 2,000gp [weapon]",
>     85,"+2 weapon: 8,000gp [weapon]",
>     90,"Specific weapon [minor specific weapon]",
>     100,"Special ability [minor special weapon] and roll again",
> }),
>
> (that's Pike; in Python it'd be a list, or maybe a tuple of tuples),
> and denormalizes it into a lookup table by creating 70 entries quoting
> the first string, 15 quoting the second, 5, and 10, respectively. So,
> with a bit of preprocessing, a lookup table (which in this case is an
> array (list), but could just as easily be a dict) can be used to
> handle inequalities.

The obvious way to me is a binary search:

    from bisect import bisect_left

    class FloorMap:
        def __init__(self, dct):
            self.indexes =3D sorted(list(dct))
            self.dct =3D dct

        def __getitem__(self, itm):
            index =3D self.indexes[bisect_left(self.indexes, itm)]
            return self.dct[index]


    minor_weapon =3D FloorMap({
        70:  "+1 weapon: 2,000gp [weapon]",
        85:  "+2 weapon: 8,000gp [weapon]",
        90:  "Specific weapon [minor specific weapon]",
        100: "Special ability [minor special weapon] and roll again"
    })

    minor_weapon[63]
    #>>> '+1 weapon: 2,000gp [weapon]'

The precise details of the wrapper class here are just to make
initialisation pretty; it could be done straight from a pair of lists
too:

    from bisect import bisect_left

    minor_weapon_indexes =3D 70, 85, 90, 100
    minor_weapon_descriptions =3D (
        "+1 weapon: 2,000gp [weapon]",
        "+2 weapon: 8,000gp [weapon]",
        "Specific weapon [minor specific weapon]",
        "Special ability [minor special weapon] and roll again"
    )

    minor_weapon_descriptions[bisect_left(minor_weapon_indexes, 80)]
    #>>> '+2 weapon: 8,000gp [weapon]'

Compare to

    if 80 <=3D 70:
        res =3D "+1 weapon: 2,000gp [weapon]"

    elif 80 <=3D 85:
        res =3D "+2 weapon: 8,000gp [weapon]"

    elif 80 <=3D 90:
        res =3D "Specific weapon [minor specific weapon]"

    elif 80 <=3D 100:
        res =3D "Special ability [minor special weapon] and roll again"

which although shorter=C2=B9 is a lot less data-driven and much less reusab=
le.

=C2=B9 Longer if you ignore the import and class declaration.