ZenD'oh clear solution for Mind Switcher by veky - python coding challenges

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ZenD'oh solution in Clear category for Mind Switcher by veky

def mind_switcher(journal):  # I'm afraid we need to use... PYTHON
    fix, swap, todo, N, S = [], False, set.union(*journal), "nikola", "sophia"
    def brain(x):
        for a, b in reversed(journal):
            x = {a: b, b: a}.get(x, x)
        return x
    while todo:
        step, first = [], todo.pop()
        second = current = brain(first)
        while current != first:
            step.append({S, current})
            todo.remove(current)
            current = brain(current)
        if step:
            fix.extend([{N, first}] + step + [{N, second}, {S, first}])
            swap = not swap
    return fix + [{N, S}]*swap

July 25, 2014

Comments:

bryukh on July 25, 2014, 9:33 a.m.
I like 5th line.

veky on Aug. 25, 2014, 5:39 a.m.
In first iteration it was <pre class='brush: python'>if x == a: x = b elif x == b: x = a </pre> But hey, 1 line is better than 2 lines. And since dict is usual Python way to express a switch, it was natural. :-)

StefanPochmann on April 28, 2015, 1:31 a.m.
Another alternative: <pre class='brush: python'>x = {x: x, a: b, b: a}[x] </pre>

veky on April 28, 2015, 5:59 a.m.
:-) Yes. But since get for dicts serves the same purpose as slices for list (mapping the monad), I thought you'd like it more. :-)

ignalion on Dec. 19, 2014, 10:34 p.m.
Nice. You are backtracking the swaps however the solution is very similar to mine. Except I do the dict of actual brains and then iterates through it.

hrvoje on Aug. 3, 2015, 12:31 a.m.
Waaaaaaaaat? What did you do on line 5? :)

veky on Aug. 3, 2015, 5:38 a.m.
Read other comments above. ;-P dict is Python's data switch.

hrvoje on Aug. 3, 2015, 6:44 a.m.
I read them, of course, and I'm still slightly confused. :)

veky on Aug. 3, 2015, 7 a.m.
Hm. Let's say you want to do one of three things, depending on whether x is a, b or c (and it surely is one of these three values). You can write <pre class='brush: python'>if x == a: f() elif x == b: g() elif x == c: h() </pre> But it has a lot of duplication, and it even isn't the right usage of Python's data model (sequential instead of associative). You can best see it if you try to eliminate duplication: <pre class='brush: python'>for y in a, b, c: if x == y: ???() </pre> For ??? you have to somehow either calculate f, g or h depending on x, or you can look it up. Look up is of course best done using a dict: <pre class='brush: python'>d = {a:f, b:g, c:h} for y in d: if x == y: d[y]() </pre> But of course, this is <a href="http://blog.plover.com/prog/loaded-uzi.html">beating someone to death with a loaded uzi</a>. It's much simpler and direct to just say <pre class='brush: python'>d = {a:f, b:g, c:h} d[x]() </pre> Or simply <pre class='brush: python'>{a:f, b:g, c:h}[x]() </pre> Clearer?

hrvoje on Aug. 3, 2015, 8:22 a.m.
Slightly. That's not the part that was confusing but the .get(x,x). Why two x-es? :)

veky on Aug. 3, 2015, 9:48 a.m.
Ah, that's the "else" case, in switch-parlance also known as "default". Imagine x can be more things besides a and b, and you want h to execute in all these cases. <pre class='brush: python'>if x == a: f() elif x == b: g() else: h() </pre> Then you don't have the luxury of enumerating all possibilities for x, and dict will just raise KeyError... if you attack it with `[]`. But you can attack it with other methods: since <pre class='brush: python'>try: val = d[key] except KeyError: val = default </pre> is often needed, and has duplicated "val =", people invented dict.get: <pre class='brush: python'>val = d.get(key, default) </pre> This, combined with the above post, says the code can be written as <pre class='brush: python'>{a:f, b:g}.get(x, h)() </pre> Now?

hrvoje on Aug. 3, 2015, 10 a.m.
Yep, didn't get why there were two x-es. :) <a href="https://media0.giphy.com/media/J8U6jQs5JYXnO/200.gif">https://media0.giphy.com/media/J8U6jQs5JYXnO/200.gif</a>

veky on Aug. 3, 2015, 10:38 a.m.
<a href="http://cdn.meme.am/instances/56195161.jpg">http://cdn.meme.am/instances/56195161.jpg</a> In fact there are three: one is to the left of =. It cancels the third one. ;-D

JimmyCarlos on July 27, 2018, 4:24 p.m.
Like everyone else, I am mind-blown by Line 5, it's such a perfect implementation of that. The little pun on Line 1 makes this code very fun to read - nice job!

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