[Rd] [R] Integer bit size and the modulus operator (PR#8541)

[ripley@stats.ox.ac.uk]

On Mon, 30 Jan 2006 murdoch at stats.uwo.ca wrote:

> On 1/30/2006 1:39 PM, Ionut Florescu wrote:
>> Thank you for the quick reply, I will look into the R packages.
>> For crashing R try this:
>>
>> generator.zp=function(x,p)
>> {a=1:(p-1); b=x^a%%p;
>> if(all(b[1:(p-2)]!=1)&&(b[p-1]==1)){return(x, " Good ")}
>> else{return(x, " No Good, try another integer ")}
>> }
>
> Thanks, I can reproduce the crash using
>
> for (x in 10:100) generator.zp(x, 41)
>
> I'll see if I can track down what's going wrong.  By the way, you're not
> supposed to use two arguments to return():  that's not supposed to be
> allowed any more.  I'm somewhat surprised you don't get an error from
> it.  But that's not the cause of the crash.

You do get a warning, though.  It *is* allowed, see ?return.

One error is the following in real_binary:

     if (n1 > n2)
 	copyMostAttrib(s1, ans);
     else if (n1 == n2) {
 	copyMostAttrib(s2, ans);
 	copyMostAttrib(s1, ans);
     }
     else
 	copyMostAttrib(s2, ans);

Here ans is not PROTECTED.

The second is in the warning, which causes allocation when ans is not 
protected.

Fixed in R-devel and 2.2.1 patched.


> Duncan Murdoch
>
>
>>
>> This checks if element x is a generator of the group Z_p. If you try
>> this function for p = 41 and x various increasing values eventually it
>> will crash R. That is what I meant by random, at first I started x=2,3
>> so on, when I got to 8, R crashed. Now apparently I can get to 15. When
>> I tried again I got to 20.
>>
>> Ionut Florescu
>>
>>
>> Duncan Murdoch wrote:
>>> On 1/30/2006 11:32 AM, Ionut Florescu wrote:
>>>> I am a statistician and I come up to an interesting problem in
>>>> cryptography. I would like to use R since there are some statistical
>>>> procedures that I need to use.
>>>> However, I run into a problem when using the modulus operator %%.
>>>>
>>>> I am using R 2.2.1 and when I calculate modulus for large numbers
>>>> (that I need with my problem) R gives me warnings. For instance if
>>>> one does:
>>>> a=1:40;
>>>> 8^a %% 41
>>>> one obtains zeros which is not possible since 8 to any power is not a
>>>> multiple of 41.
>>>> In addition when working with numbers larger that this and with the
>>>> mod operator R crashes randomly.
>>>
>>> Could you keep a record of the random crashes, and see if you can make
>>> any of them repeatable?  R shouldn't crash.  If you can find a
>>> repeatable way to make it crash, then that's a bug that needs to be
>>> fixed.  (If it crashes at random it should still be fixed, but it's so
>>> much harder to fix that it's unlikely to happen unless the cases are
>>> ones that look likely to come up in normal situations.)
>>>
>>>
>>>>
>>>> I believe this is because R stores large integers as real numbers
>>>> thus there may be lack of accuracy when applying the modulus operator
>>>> and converting back to integers.
>>>>
>>>> So my question is this: Is it possible to increase the size of memory
>>>> used for storing integers? Say from 32 bits to 512 bits (Typical size
>>>> of integers in cryptography).
>>>
>>> No, but there is at least one contributed package that does multiple
>>> precision integer arithmetic.  I can't remember the name of it right
>>> now, but Google should be able to find it for you...
>>>
>>> Duncan Murdoch
>>>>
>>>> Thank you, any help would be greatly appreciated.
>>>> Ionut Florescu
>>>>
>>>> ______________________________________________
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>>>> PLEASE do read the posting guide!
>>>> http://www.R-project.org/posting-guide.html
>>>
>>>
>
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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