Two sums creative solution for Even the Last by nickie - python coding challenges

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Two sums solution in Creative category for Even the Last by nickie

checkio=lambda a:sum(a[0::2])*sum(a[-1:])

Feb. 20, 2014

Comments:

veky on Feb. 20, 2014, 6:57 p.m.
LOL. Ok, this surprised me (but not puzzled:). :-D

A_bak_cfff596eda7840afb01132b2 on April 16, 2014, 6:19 p.m.
'0' is excessive. checkio=lambda a:sum(a[::2])*sum(a[-1:])

nickie on April 16, 2014, 8:02 p.m.
Right.

EthyBOX on Jan. 27, 2015, 9:09 a.m.
Nice way to deal with the empty array !

p4r4d0x42 on Feb. 16, 2016, 11:43 p.m.
Nice one liner. :)

byronboulton on April 7, 2019, 2:27 p.m.
I really like you’re solution to getting the last element of the array without separately checking if the array is empty. I personally find it really wired when people assign lambdas. I feel like lambdas are supposed to be anonymous and that if your assigning them a name then you should define a regular function.

veky on April 8, 2019, 4:22 a.m.
Since I don't think @nickie is on CiO anymore, let me try to reply to this from my perspective. First, yes, lambdas are anonymous, but I don't think that it's their defining characteristic (in Python -- of course, in some other languages it might be). Almost no one reaches for lambda when they think "Oh, I really can't bother to pick a name for this function." :-D People reach for lambda for entirely different reason: it's that they want an expression, instead of a statement. Since Python enforces the language hierarchy in which expressions are below statements, when you're deep inside <pre class='brush: python'>sorted({some.stuff for some in various[other, stuff]}, key=...)</pre>, you can't simply start writing statements to calculate the key. But in most cases it doesn't matter, since all you want for a key to be is an expression anyway. So you write a lambda. Note that it's not really anonymous, almost by definition: inside the namespace of sorted, it's named <pre class='brush: python'>key</pre>. It has to be, since that namespace expects that name to refer to a function it can call to extract a key from each element of whatever it is to be sorted. I'd argue CiO is the same, only on a slightly higher level. There is a checking framework, into which you have to insert your code, by naming it <pre class='brush: python'>checkio</pre> (or whatever). It's kinda callback hell, but it's only one level deep, so people don't really notice it much. But calling it <pre class='brush: python'>checkio</pre> is serving the same purpose as calling the above lambda <pre class='brush: python'>key</pre>---just a protocol to follow, so upstream will know what to do with it. See also <a href="https://py.checkio.org/mission/binary-count/publications/veky/python-3/hinted/share/7bcd07614f4c5183a12b675c05cceb5d/#comment-42974">these</a> <a href="https://py.checkio.org/mission/binary-count/publications/veky/python-3/hinted/share/7bcd07614f4c5183a12b675c05cceb5d/#comment-40299">two</a> posts for a slightly different articulation of the same argument.

nickie on April 8, 2019, 6:51 a.m.
Thanks, @veky, I definitely agree with your answer. (And it's true, I'm not really active in CiO nowadays; I'm unfortunately busy otherwise, but I miss the old days...) I'll add one more argument which, however stupid, was from my perspective the key argument for writing this solution with an assigned <pre class='brush: python'>lambda</pre> instead of a <pre class='brush: python'>def</pre>. Golfing! At some point, CiO became a contest about writing the shortest (and, occasionally, the weirdest) solution to the given task. With an assigned <pre class='brush: python'>lambda</pre> instead of a <pre class='brush: python'>def</pre> you save 5 characters: <pre class='brush: python'>$ echo "f=lambda a: x" | wc -c 14 $ echo "def f(a): return x" | wc -c 19 </pre> So, the truth is, I agree with @byronboulton here. I would rather use a <pre class='brush: python'>def</pre> anytime in my regular code, instead of an assigned <pre class='brush: python'>lambda</pre>. Also, I would probably not use <pre class='brush: python'>sum(a[::2])*sum(a[-1:])</pre> in my regular code as a solution for this task, because the second part takes a while (even for me who wrote it, five years later) to understand why and how exactly it works. I generally prefer simpler code that speaks for itself --- not excessively verbose without reason, but not cryptically short either. I'd use something like <pre class='brush: python'>sum(a[::2])*a[-1] if a else 0</pre>. For novice programmers, this solution is bad and (allow me) dangerous. Don't try to write stuff like this! For programmers who are starting to build a good understanding of Python, it has some merit. Solutions like this can show you some corners of the language where you wouldn't normally go.

veky on April 8, 2019, 2:31 p.m.
Since I am probably the guiltiest for introducing golfing to CiO, let me say something in my defense in that area as well. As incredible as it may seem to people here, golfing was not my original idea. The problem is, two things going on in tandem, which resulted in such a situation. Firstly, Python is a beautifully designed language (or at least it was while Guido was BDFL, but that's another story). Practically every language feature is examined from all angles. As a consequence, many times Python explicitly rewards doing the right thing in the context. Not always, but surprisingly often, the reward comes in the number of keystrokes. for is shorter than while, nonlocal is longer and uglier than global, with is longer than try but shorter than try+finally, continue is longer than not plus one extra indentation, and so on. str and int are shortened, but frozenset and bytearray are not (and set and bytes are thrice/twice shorter:). And yes, lambda is longer than def, but it is shorter than def+return. It's as if BDFL said "oh well, people will try to save keystrokes anyway. Let's make it so that when they do that, they automatically use the right abstraction level". :-D Too many times, my code was shorter than many others, not because I invested any particular effort to shave the characters, but simply because I wrote Python naturally, without any boilerplate that people religiously put in. Secondly, the missions here are often ridiculously simple. When you actually know all the standard library, most missions reduce to simply <a href="https://py.checkio.org/mission/bigger-price/publications/veky/python-3/connecting-the-dots/">putting the right pieces together</a>. When people saw me writing in such succint way, they naturally supposed I was after minimizing keystrokes---while I was just trying to minimize extraneous ideas, sometimes brutally. The rest, as they say, is history. :-Q

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