On Fri, Nov 6, 2015 at 7:18 AM, Stepan Kasal <kasal@ucw.cz> wrote:

Hello,

I was learning Cython from this tutorial when I noticed a glitch:

Near the end, after boundscheck False, some variables are changed to unsigned int. But this has unexpected consequences: variables t_from and such are not determined correctly, as one of the arguments is negative, but gets cast to unsigned for comaprison. (Yes, this is a weird feature of C.)

Actually, the use of unsigned does not seem to have any impact on performance, so I suggest to scratch that part of tutorial. Just explain that with boundscheck off, negaitive indices no longer work.

(But there might perhaps be one small improvent in cython itself: with boundscheck off, the index might be cast to unsigned before it is used: this way we probably crash on negative values, instead of silently accessing the negative offset of the array data.)

There's two directives: wraparound and boundscheck. The wraparound directive can be used to disable the check for negative values. I heartily agree that using unsigned types is the source of much surprising pain, we shouldn't be recommending it: https://github.com/cython/cython/commit/06f75b803d91805f3068bbad2b9241efbd5c...