range(len(list)) or enumerate(list)?
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Only index needed: enumerate or (x)range?
Which of these would be considered better/clearer/faster/more 'Pythonic'? I don't care about the content of the list L , just how long it is.
a = [f(n) for n, _ in enumerate(L)]
or
a = [f(n) for n in range(len(L))]
If it makes any difference, the function f makes use of len(list) as well.
Some quick timing runs seem to give the 2nd option using range() a slight edge over enumerate() :
timeit a = [f(n) for n, _ in enumerate(mlist)]
10000 loops, best of 3: 118 us per loop
timeit a = [f(n) for n in range(len(mlist))]
10000 loops, best of 3: 102 us per loop
and just for fun using xrange() (Python v2.7.2)
timeit a = [f(n) for n in xrange(len(mlist))]
10000 loops, best of 3: 99 us per loop
I would favor readable code first, then using xrange() if available (ie, Pre-Python v 3.x), followed by range() and enumerate() .
The (x)range solution is faster, because it has less overhead, so I'd use that.
In Python 2.x, use xrange instead of range , because xrange uses less memory, because it doesn't create a temporary list. In Python 3.x, there is only range , which is the less-memory version.
Assuming you're using Python 2.x, if you use len() , you should use xrange() as it will avoid creating a list of the numbers in the range.
And in this case, I'd go with len() because you are using the indices, not the items in the list.
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