why i can't reverse a list of list in python

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不思量自难忘°
不思量自难忘° 2021-02-08 04:22

i wanted to do something like this but this code return list of None (i think it\'s because list.reverse() is reversing the list in place):

map(lambda row: row.r         


        
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  • 2021-02-08 04:26
    map(lambda row: list(reversed(row)), figure)
    
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  • 2021-02-08 04:35

    You can also simply do

    for row in figure:
        row.reverse()
    

    to change each row in place.

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  • 2021-02-08 04:38
    reversed_lists = [list(reversed(x)) for x in figure]
    
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  • 2021-02-08 04:47

    The mutator methods of Python's mutable containers (such as the .reverse method of lists) almost invariably return None -- a few return one useful value, e.g. the .pop method returns the popped element, but the key concept to retain is that none of those mutators returns the mutated container: rather, the container mutates in-place and the return value of the mutator method is not that container. (This is an application of the CQS principle of design -- not quite as fanatical as, say, in Eiffel, the language devised by Bertrand Meyer, who also invented CQS, but that's just because in Python "practicality beats purity, cfr import this;-).

    Building a list is often costlier than just building an iterator, for the overwhelmingly common case where all you want to do is loop on the result; therefore, built-ins such as reversed (and all the wonderful building blocks in the itertools module) return iterators, not lists.

    But what if you therefore have an iterator x but really truly need the equivalent list y? Piece of cake -- just do y = list(x). To make a new instance of type list, you call type list -- this is such a general Python idea that it's even more crucial to retain than the pretty-important stuff I pointed out in the first two paragraphs!-)

    So, the code for your specific problem is really very easy to put together based on the crucial notions in the previous paragraphs:

    [list(reversed(row)) for row in figure]
    

    Note that I'm using a list comprehension, not map: as a rule of thumb, map should only be used as a last-ditch optimization when there is no need for a lambda to build it (if a lambda is involved then a listcomp, as well as being clearer as usual, also tends to be faster anyway!-).

    Once you're a "past master of Python", if your profiling tells you that this code is a bottleneck, you can then know to try alternatives such as

    [row[::-1] for row in figure]
    

    applying a negative-step slicing (aka "Martian Smiley") to make reversed copies of the rows, knowing it's usually faster than the list(reversed(row)) approach. But -- unless your code is meant to be maintained only by yourself or somebody at least as skilled at Python -- it's a defensible position to use the simplest "code from first principles" approach except where profiling tells you to push down on the pedal. (Personally I think the "Martian Smiley" is important enough to avoid applying this good general philosophy to this specific use case, but, hey, reasonable people could differ on this very specific point!-).

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  • 2021-02-08 04:52

    You can also use a slice to get the reversal of a single list (not in place):

    >>> a = [1,2,3,4]
    >>> a[::-1]
    [4, 3, 2, 1]
    

    So something like:

    all_reversed = [lst[::-1] for lst in figure]
    

    ...or...

    all_reversed = map(lambda x: x[::-1], figure)
    

    ...will do what you want.

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