__lt__ clear solution for Bigger Together by veky - python coding challenges

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__lt__ solution in Clear category for Bigger Together by veky

def bigger_together(ints):
    class NumStr(str): __lt__ = lambda x, y: x+y < y+x
    order = sorted(map(NumStr, ints))
    return int(''.join(reversed(order))) - int(''.join(order))

July 30, 2017

Comments:

Tinus_Trotyl on Aug. 3, 2017, 4:55 p.m.
Again . . . yours = in_shorthand(mine) ;-)

veky on Aug. 5, 2017, 7:18 a.m.
I thought about cmptokey, but it seems awfully roundabout (as it always does)... Guido's great insight is that almost all cmp functions are of the form `lambda a, b: cmp(f(a), f(b))` and factored out f... however, not all of them are. Sometimes just that outer cmp is given by something else (or in this case, f(a) and f(b) both depend on both a and b), but you still can make a total order out of it. Converting into a cmp format just to convert it back to key format is just busywork. :-)

piter239 on April 23, 2020, 11:12 a.m.
Wow! So the notation <pre class='brush: python'>class NumStr(str): __lt__ = lambda x, y: x+y < y+x </pre> is somehow equivalent to <pre class='brush: python'>class NumStr(str): def __lt__(self, other): return x+y < y+x </pre> and it silently calls the <pre class='brush: python'>str.__init__() </pre> Really nice way to write it!

veky on April 23, 2020, 6:21 p.m.
"somehow" is the same lambda-def equivalence I always point out. <pre class='brush: python'>name = lambda args: expression <~~> def name(args): return expression </pre> It really doesn't matter that it's within a class. And of course, it doesn't matter what the args are called here. `init` is almost always called silently, that's about its whole point. :-D And `NumStr.init is str.init`, of course.

piter239 on April 23, 2020, 6:35 p.m.
Many thanks for explanation!

veky on April 26, 2020, 5:54 a.m.
Ah, now that I read it the second time, I think I understood what you find so fascinating. `str.init` is empty anyway, you cannot meaningfully initialize an immutable object once it's already made. It is `str.new` that is the real hero here, since Python usually has polymorphic constructors. That is, constructors are class methods, but they as a rule return an instance of a class they are called on, not a class they are implemented on. Look: <pre class='brush: python'>class Pair(tuple): @classmethod def construct(cls, first, second): return cls([first, second]) Pair.construct(3, 4) (3, 4) type(_) <class '__main__.Pair'> class AnglePair(Pair): angles = str.maketrans('()', '<>') def __repr__(self): return super().__repr__().translate(self.angles) AnglePair.construct(2, 3) <2, 3> </pre> That last object is an AnglePair, but it is also a Pair, so it has a classmethod construct. But Pair.construct doesn't necessarily construct raw Pairs, it can construct whatever class called it. So it can construct AnglePairs too. Same thing happens here: whoever wrote the constructor for str (taking an int), was very careful ensuring it doesn't just return a str, but it returns a class it was called on. So NumStr(5) is "like" a str(5), but with the class NumStr. Just like a Pair.construct(3, 4) is "like" a tuple([3, 4]) (it even prints like one, since `Pair.repr is tuple.repr`), but with a class Pair. TLDR: So the notation ... is somehow equivalent to: <ul> <li>NumStr is like a str, except it is ordered differently. </li> </ul>

piter239 on April 26, 2020, 6:10 a.m.
You are precisely right again! My problem with Python at the moment is - it still seems too "magical" to me. Coming from C++, where I got formal training "deciding which constructors and destructors will be called in the lifecycle of complex object structures", I lack this kind of deep understanding in Python. For example, I still wonder why `init` doesn't have to return "a pointer to the constructed object" (probably really silly of me), and the information about some `str.new` function is totally new (excuse the pun) There is probably no king road to deep understanding, but at times like this I wish there would be some kind of mental model compiler.

veky on April 26, 2020, 6:25 a.m.
Well, my problem is opposite. I'm so used to Python's model, that I find it hard to see what someone might find problematic with it. :-) You might call it magical, but I find it deeply intuitive. (On the other hand, I taught C++'s model for five years, but it still feels extremely clunky.) The only problem is people trying to map Python's concepts to those of other languages, but it always felt like literally translating Japanese to English. You miss much in the process, and people look at you in a weird way. :-D For example, many clueless people call `init` a "constructor", simply because in C++, a method which initializes the object is called "a constructor" (and a method which constructs the object is called "operator new"; yes, I know why, but it doesn't make it easier;). :-D In Python, you construct objects using `new`, and you initialize them using `init`. Of course there is nothing to initialize if the object is immutable. :-o Python's object model couldn't exist without `new`, while it could exist without `init` perfectly fine. `init` is just a hack for people coming from Java and expecting to write a method that's automatically called after the object is already constructed but before it is used, because "it's the way we do it in my kingdom". It doesn't return self (or `*this` or whatever) because it doesn't do so in Java and C++. It is `new` that does return "self", though it's weird to call it that name since it's not an object that `new` was called on.

piter239 on April 29, 2020, 4:50 p.m.
I somehow missed this comment of yours the first time. And today tried hard to remember "what was the problem veky solved so efficient with `lt`?", came here - and here it is! Btw, here on CheckIO seems to be no kind of direct messages, or I haven't found it jet. What would be your suggestion for the best way of getting used to Python model of thinking about functions, iterators, objects etc? Studying your solutions for various missions here does help, of course, but perhaps there is some book with excersises or something))

veky on April 29, 2020, 5:44 p.m.
<blockquote> I somehow missed this comment of yours the first time. </blockquote> That's very weird, since you liked it three days ago. :-P :-D <blockquote> Btw, here on CheckIO seems to be no kind of direct messages, or I haven’t found it jet. </blockquote> By design. I think they have relented in the meantime, but for a long time I tried to persuade CiO not to introduce any 1on1 messaging on the platform. Why? I think it's obvious. How many of my posts have you seen, liked and learned a lot of them? Despite they being intended for someone else? And how many would you see if they were sent as private messages? If by direct you didn't mean "private" but just "target gets notification", you may mention people with `@`. You can ask @oduvan to explain how that works. :-D <blockquote> What would be your suggestion for the best way of getting used to Python model of thinking about functions, iterators, objects etc? </blockquote> How about simply reading the official Python tutorial? :-) It's funny how many people just skip this step by default. :-> <blockquote> Studying your solutions for various missions here does help, of course, but perhaps there is some book with excersises or something)) </blockquote> Then I'd have to write it, and currently there are much better uses of my time. :-P Why don't you write it? I'll provide solutions. :-D

piter239 on April 29, 2020, 6:26 p.m.
<blockquote> That’s very weird, since you liked it three days ago. :-P :-D </blockquote> It is weird indeed - today it had 0 likes)) and I was able to like it 1 hour ago. <blockquote> <blockquote> Btw, here on CheckIO seems to be no kind of direct messages, or I haven’t found it jet. </blockquote> By design. I think they have relented in the meantime, but for a long time I tried to persuade CiO not to introduce any 1on1 messaging on the platform. Why? I think it’s obvious. How many of my posts have you seen, liked and learned a lot of them? Despite they being intended for someone else? And how many would you see if they were sent as private messages? </blockquote> Well, this idea did cross my mind - many times)) <blockquote> <blockquote> What would be your suggestion for the best way of getting used to Python model of thinking about functions, iterators, objects etc? </blockquote> How about simply reading the official Python tutorial? :-) It’s funny how many people just skip this step by default. :-> </blockquote> I started it several times. Now I will have to do it thorougly end to end. Thanks for the hint! <blockquote> <blockquote> Studying your solutions for various missions here does help, of course, but perhaps there is some book with excersises or something)) </blockquote> Then I’d have to write it, and currently there are much better uses of my time. :-P </blockquote> Of course! I thought, perhaps there is such a book already)) <blockquote> Why don’t you write it? I’ll provide solutions. :-D </blockquote> If there is no such book jet - why not? )) But first I've got to work through the <a href="https://docs.python.org/3/tutorial/">https://docs.python.org/3/tutorial/</a>

piter239 on May 2, 2020, 9:51 a.m.
<blockquote> How about simply reading the official Python tutorial? :-) It’s funny how many people just skip this step by default. :-> </blockquote> Thank you very much for your advice about official Python tutorial!!! I started to read it - and remembered that I did indeed done so at the very start. But this time I understood it much more deeply, and some things did stick. Today I was able to use some of the knolegde found <a href="https://docs.python.org/3/tutorial/controlflow.html#unpacking-argument-lists">here </a>- and in very short time got promoted to "golden league" on a site-competitor of checkio.org )) I for sure will proceed with reading - presently I am at <a href="https://docs.python.org/3/tutorial/inputoutput.html">https://docs.python.org/3/tutorial/inputoutput.html</a>

tea1mer on April 25, 2020, 10:13 p.m.
I have spend my time on this solution around of two hours for understood, now, I thing this solution is a very good thing.

veky on April 26, 2020, 5:29 a.m.
The whole nontrivial idea in this solution is the alternative ordering on `NumStr`s. Everything else is just ordinary Python. :-)

paradox81ru on Dec. 22, 2023, 4:16 a.m.
an excellent solution

Master of the Time Stone

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