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I am new to python and noticed something. My run time was significantly lower when i used ['a' for i in range(10)] compared to ['a']*10. Can someone please explain why this happens?

juanpa.arrivillaga
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user14531552
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    Well, `['a']*10` should be faster. In this case, there is no difference, but in general, it is very different. See: https://stackoverflow.com/questions/240178/list-of-lists-changes-reflected-across-sublists-unexpectedly – juanpa.arrivillaga Jul 21 '21 at 05:30
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    Show us your benchmark code. Several of us are dubious of your results. – Tim Roberts Jul 21 '21 at 05:35
  • it was used in a LC question. the run time was slower if i declared the array using for loop. – user14531552 Jul 21 '21 at 06:07

1 Answers1

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['a' for i in range(10)] executes the following in Python:

  • Create a range
  • Iterate the range, and for each iteration:
    • Assign the iterated value to a locally defined variable i
    • Produce 'a'
  • Collect the produced strings into a list

['a']*10 executes the following:

  • Produce 'a' (once)
  • Collect the given number of occurrences of 'a' into a list

This is simplified, but it is clear that the second version:

  • has no need of explicitly creating a (range) iterator;
  • has no need of defining a local variable i;
  • has no iteration that is explicitly running as Python code; the iteration only happens in lower-level, compiled code.
trincot
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