map(text.find, ...) and tuple-if slicing clear solution for Between Markers by A5537 - python coding challenges

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map(text.find, ...) and tuple-if slicing solution in Clear category for Between Markers by A5537

def between_markers(text: str, begin: str, end: str) -> str:
    start, stop = map(text.find, (begin, end))
    return text[(start + len(begin), 0)[start < 0]:(stop, None)[stop < 0]]

March 11, 2019

Comments:

piter239 on April 21, 2020, 7:05 p.m.
if-slicing looks ... interesting! Thank you for this idea))

kiplimock on May 22, 2020, 8:40 a.m.
I like how your code is concise. But I have literally struggled to understand you return statement. I would appreciate if you or anyone else who can see this explain how you've sliced. Thanks!

A5537 on May 25, 2020, 7:13 a.m.
Sure, I'll do my best to explain it to you. Slicing start and stop positions depend on the presence of markers: if begin/end marker is there then we slice from/to it, otherwise we use the beginning/ending of the string. So in "python terms" for start position it's `start + len(begin)` if `start >= 0` (find() will return a non-negative value if the marker is present) or `0` if `start < 0`, for stop position it's `stop` if `stop >= 0` or `None` (slice() will pick the end of a sequence in this case) if `stop < 0`. In a numeric context boolean values turn into 0 (False) and 1 (True), so we can use this to pack if-statements into if-tuples with a boolean condition as an index: <pre class='brush: python'>return (start + len(begin), 0)[start < 0] </pre> is the same as <pre class='brush: python'>if start < 0: return 0 return start + len(begin) </pre> Unlike if-statements, these if-tuples can be used to pick the right value inline, so after putting our if-tuples into slice brackets we finally get: <pre class='brush: python'>return text[ (start + len(begin), 0)[start < 0] : (stop, None)[stop < 0] ] </pre> Hope this helps. Feel free to ask further questions if something's still not clear enough.

kiplimock on May 25, 2020, 7:51 a.m.
Thanks a lot. I now understand if-tuples. Didn't even know stuff like this existed. I'm excited about using them now.

A5537 on May 25, 2020, 11:24 a.m.
Well, I'm not sure that if-tuples exist in the real world - it's something I came up with while code golfing in python. The construct is not obvious or easy to understand at first glance, so I wouldn't recommend to use it in real code, unless you want to make the maintainers literally struggle.

2010min0ru on Oct. 18, 2020, 8:53 p.m.
Very clever but not so clear and readable. Awesome trick with True / False index though!

natter1 on April 4, 2021, 8:33 a.m.
Great idea! But maybe the wrong category ;)

olena.rudenko on Sept. 17, 2023, 3:48 p.m.
this looks complicated but very interesting

Strings Theory

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