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The following 2 methods do the same thing. Which one is more efficient in terms of time/space complexity?

** Method A**
for student in group.students:
    for grade in student.grades:
        some_operation(grade)

** Method B**
for grade in [grade for student in group.students for grade in student.grades]
    some_operation(grade)
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    B: consumes more memory because it builds an intermediate list. Use a generator expression instead: for grade in (grade ...): some_operation(grade). Commented Sep 29, 2016 at 20:57
  • @Bakuriu, "()" indicate generator? So the list of 'grades' won't be created as we loop thru it? Commented Sep 29, 2016 at 21:03
  • Related threads stackoverflow.com/questions/47789/… stackoverflow.com/questions/19933753/… Commented Sep 29, 2016 at 21:14

2 Answers 2

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Method B looks weird and redundant. You could shorten it to:

[some_operation(grade) for student in group.students for grade in student.grades]

But method A is better either way because it doesn't create a list. Making a list simply to throw it away is confusing to the reader and wastes memory.

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11 Comments

Agreed. Just don't like multiple levels of loops, because of my OCD. Any idea how to write concise yet equally efficient code in this case?
I strongly dislike this. list-comprehensions are expressions and should not be used to just calla function on a sequence of values. Besides: it consumes memory. If you have a scenario where you have millions of items on which to perform the action you probably don't want to create a useless list with million of elements.
No, do not shorten method B to a single list comprehension. Method B does not create a new list; the list comprehension creates one that you immediately discard.
Agreed with Alex and @Bakuiu list comprehension is inefficient in space.
@cheng shorter code is not necessarily cleaner or clearer. Always choose to write code that is instantly obvious to a reader rather than trying hard to save lines.
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These have the same time complexity, O(nm), since it's a loop over another loop. So, the n is group.students, and m is students.grades. Functionally, these should be the same time complexity too since it's iterating over both lists either way.

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