[Python-ideas] Float range class

Chris Barker chris.barker at noaa.gov
Thu Jan 8 20:05:46 CET 2015

> I would think that a floating range class would necessarily use
> multiplication rather than repeated addition (to allow indexing at
> arbitrary point), which would avoid cumulative floating-point errors
> (although it would still have a smaller floating point error at the end),
> and for the same reason the final value would have to be pre-computed
> rather than using a naive ">=" which would allow it to be a bit smarter.

That's the trick -- range() (and arange) does not specify a final value, it
specifies a final value not to include. This is well defined and easy to
understand for integers, but not so for floating point. But you are right
about the multiplication and pre-computing of final value -- that's a good
reason to provide this as a built-in -- it's very easy to implement, but
even easier to implement badly.

 But, at least in my own experience, I use arange when I want an
>> interval-based range, and linspace when I want a count-based range.
I would argue (and do!) that you should not do this -- if you know what you
are doing with FP, then fine, but it really is tricky. You would be better
off computing the count you want then then using linspace anyway. I suppose
an interval-based API to something like linspace would be a nice
convenience, though.

I haven't managed to come up with a quick an easy example where this
matters, but they DO happen.

I guess I'm arguing that a range-like object for FP should be a closed
rather than open interval -- specifying the starting and end points. That
is because defining an open interval where numbers are of finite, but hard
to know know in advance the interval, is just too ugly and complex.

I see your point that sometimes you want a specific delta, and sometimes
you want a specific end point, but I supect that most of the time you want
a specific delta you ALSO want a specific and point and/or want to know how
many values you are going to get.

In fact, in the most common use of integer range, you really are defining
the number of values you want.

And note that the range convention of starting at zero and not including
the stop value was designed to match python indexing convention, i.e.:

for i in range( len(sequence) ):

is natural an easy to write, and does what's expected. And also:

for i in range(n):

will loop n times.

but neither of these apply to floating point ranges.



Christopher Barker, Ph.D.

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Chris.Barker at noaa.gov
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