What the heck is an xrange?

#1

Post by dcrosta » Tue, Jun 19, 2012, 1:05 AM UTC

Re: What the heck is an xrange?

#2

Post by sltkr » Tue, Jun 19, 2012, 3:05 AM UTC

Nitpicking, but this statement doesn't work with large numbers (which xrange() is supposed to handle correctly):

    self._len = int(ceil(float(stop - start) / step))

Python supports arbitrary-length integers; you can't just cast those to (fixed-length) floating point numbers without losing precision. It's better to use integer division here, for example:

    self._len = (stop - start)//step + bool((stop - start)%step)

(A variant like (stop - start + step - 1)//step works only for positive numbers; I guess it could work if you put it in the earlier if-clause and put a corresponding assignment with +1 at the end in the other branch.)

Re: What the heck is an xrange?

#3

Post by dcrosta » Tue, Jun 19, 2012, 3:11 AM UTC

sltkr wrote: post #2

Nitpicking, but this statement doesn't work with large numbers (which xrange() is supposed to handle correctly): self._len = int(ceil(float(stop - start) / step)) Python supports arbitrary-length integers; you can't just cast those to (fixed-length) floating point numbers without losing precision. It's better to use integer division here, for example: self._len = (stop - start)//step + bool((stop - start)%step) (A va…

Great catch. If you want to submit a pull request on github you can get the credit -- github.com/dcrosta/xrange

Re: What the heck is an xrange?

#4

Post by gus_massa » Tue, Jun 19, 2012, 3:12 AM UTC

Does anyone know how this is implemented in Pypy?

Re: What the heck is an xrange?

#5

Post by dcrosta » Tue, Jun 19, 2012, 3:15 AM UTC

sltkr wrote: post #2

Nitpicking, but this statement doesn't work with large numbers (which xrange() is supposed to handle correctly): self._len = int(ceil(float(stop - start) / step)) Python supports arbitrary-length integers; you can't just cast those to (fixed-length) floating point numbers without losing precision. It's better to use integer division here, for example: self._len = (stop - start)//step + bool((stop - start)%step) (A va…

Huh -- CPython 2.x doesn't let you create an xrange with values past 263-1. CPython 3.x does.

Re: What the heck is an xrange?

#6

Post by dbecker » Tue, Jun 19, 2012, 3:16 AM UTC

I didn't expect to learn much when I started reading this, but I was wrong. Excellent post. Thanks.

Re: What the heck is an xrange?

#7

Post by brian_cloutier » Tue, Jun 19, 2012, 3:18 AM UTC

gus_massa wrote: post #4

Does anyone know how this is implemented in Pypy?

https://bitbucket.org/pypy/pypy/src/default/pypy/annotation/...

Re: What the heck is an xrange?

#8

Post by mercuryrising » Tue, Jun 19, 2012, 3:31 AM UTC

This is cool. I know this isn't how they actually do it, but seeing it in a language I can understand (Python) and not in C is really pleasing.

You can do tons of examples, but until you figure out how the machine works inside, you'll be completely lost (you could get it with examples, but you won't necessarily know WHY you got it, so you'll be useless in helping other people learn).

Re: What the heck is an xrange?

#9

Post by sltkr » Tue, Jun 19, 2012, 3:31 AM UTC

dcrosta wrote: post #5

sltkr wrote: post #2

Nitpicking, but this statement doesn't work with large numbers (which xrange() is supposed to handle correctly): self._len = int(ceil(float(stop - start) / step)) Python supports arbitrary-length integers; you can't just cast those to (fixed-length) floating point numbers without losing precision. It's better to use integer division here, for example: self._len = (stop - start)//step + bool((stop - start)%step) (A va…

Huh -- CPython 2.x doesn't let you create an xrange with values past 2 63-1. CPython 3.x does.

Even 63-bit integers aren't (all) representable in IEEE double floating point values (that Python uses) which have a 53 bits mantissa.

For example, int(float(10¹⁸ - 1)) != 10¹⁸ - 1, but xrange(1, 10¹⁸) is perfectly valid (even in Python 2).

edit: how do I type two consecutive asterisks on Hacker News? Backslash doesn't seem to work as an escape character.

Re: What the heck is an xrange?

#10

Post by sltkr » Tue, Jun 19, 2012, 3:33 AM UTC

dcrosta wrote: post #3

sltkr wrote: post #2

Nitpicking, but this statement doesn't work with large numbers (which xrange() is supposed to handle correctly): self._len = int(ceil(float(stop - start) / step)) Python supports arbitrary-length integers; you can't just cast those to (fixed-length) floating point numbers without losing precision. It's better to use integer division here, for example: self._len = (stop - start)//step + bool((stop - start)%step) (A va…

Great catch. If you want to submit a pull request on github you can get the credit -- github.com/dcrosta/xrange

Thanks for the consideration, but I don't really use github, so you go ahead and take the credit yourself.