I can use if
and for
in list comprehensions/generator expressions as
list(i for i in range(100) if i*i < 30)
I know this is not the most efficient but bear with me as the condition could be much more complicated and this is just an example. However, this still goes through hundred iterations and only yields a value in the first 6. Is there a way to tell the generator expression where to stop with something like this:
list(i for i in range(100) while i*i < 30)
However, while
is not understood in generator expressions. So, my question is, how do I write a generator expression with a stopping condition so it does not continue computation, even if it doesn't yield new values.
No, you cannot use while in a list comprehension.
The only difference between Generator Comprehension and List Comprehension is that the former uses parentheses.
List comprehensions and generators are not different at all; they are just different ways of writing the same thing. A list comprehension produces a list as output, a generator produces a generator object.
Thus, generator expressions are faster than list comprehension and hence time efficient.
Because the syntax of takewhile()
and dropwhile()
is not the clearest, here are the actual examples of your question:
>>> [i for i in itertools.takewhile(lambda x: x*x<30, range(10))]
[0, 1, 2, 3, 4, 5]
>>> [i for i in itertools.dropwhile(lambda x: x*x<30, range(10))]
[6, 7, 8, 9]
Know that the author of itertools has questioned whether to deprecate these functions.
The various functions in itertools
(takewhile()
comes to mind) can help.
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