[Rd] Reading 64-bit integers

Simon Urbanek simon.urbanek at r-project.org
Wed Mar 30 20:22:53 CEST 2011 

Bill,

thanks. I like that idea of the output parameter better, especially if we ever add different scalar vector types. Admittedly, what=integer() is the most useful case. What I was worried about is things like what=double(), output=integer() which could be legal, but are more conveniently dealt with via as.integer(readBin()) instead.
I won't have more time today, but I'll have a look tomorrow.

Thanks,
Simon


On Mar 30, 2011, at 1:38 PM, William Dunlap wrote:

> 
>> -----Original Message-----
>> From: r-devel-bounces at r-project.org 
>> [mailto:r-devel-bounces at r-project.org] On Behalf Of Simon Urbanek
>> Sent: Tuesday, March 29, 2011 6:49 PM
>> To: Duncan Murdoch
>> Cc: r-devel at r-project.org
>> Subject: Re: [Rd] Reading 64-bit integers
>> 
>> 
>> On Mar 29, 2011, at 8:47 PM, Duncan Murdoch wrote:
>> 
>>> On 29/03/2011 7:01 PM, Jon Clayden wrote:
>>>> Dear Simon,
>>>> 
>>>> On 29 March 2011 22:40, Simon 
>> Urbanek<simon.urbanek at r-project.org>  wrote:
>>>>> Jon,
>>>>> 
>>>>> On Mar 29, 2011, at 1:33 PM, Jon Clayden wrote:
>>>>> 
>>>>>> Dear Simon,
>>>>>> 
>>>>>> Thank you for the response.
>>>>>> 
>>>>>> On 29 March 2011 15:06, Simon 
>> Urbanek<simon.urbanek at r-project.org>  wrote:
>>>>>>> 
>>>>>>> On Mar 29, 2011, at 8:46 AM, Jon Clayden wrote:
>>>>>>> 
>>>>>>>> Dear all,
>>>>>>>> 
>>>>>>>> I see from some previous threads that support for 
>> 64-bit integers in R
>>>>>>>> may be an aim for future versions, but in the meantime 
>> I'm wondering
>>>>>>>> whether it is possible to read in integers of greater 
>> than 32 bits at
>>>>>>>> all. Judging from ?readBin, it should be possible to 
>> read 8-byte
>>>>>>>> integers to some degree, but it is clearly limited in 
>> practice by R's
>>>>>>>> internally 32-bit integer type:
>>>>>>>> 
>>>>>>>>> x<- as.raw(c(0,0,0,0,1,0,0,0))
>>>>>>>>> (readBin(x,"integer",n=1,size=8,signed=F,endian="big"))
>>>>>>>> [1] 16777216
>>>>>>>>> x<- as.raw(c(0,0,0,1,0,0,0,0))
>>>>>>>>> (readBin(x,"integer",n=1,size=8,signed=F,endian="big"))
>>>>>>>> [1] 0
>>>>>>>> 
>>>>>>>> For values that fit into 32 bits it works fine, but 
>> for larger values
>>>>>>>> it fails. (I'm a bit surprised by the zero - should 
>> the value not be
>>>>>>>> NA if it is out of range?
>>>>>>> 
>>>>>>> No, it's not out of range - int is only 4 bytes so only 
>> 4 first bytes (respecting endianness order, hence LSB) are used.
>>>>>> 
>>>>>> The fact remains that I ask for the value of an 8-byte 
>> integer and
>>>>>> don't get it.
>>>>> 
>>>>> I think you're misinterpreting the documentation:
>>>>> 
>>>>>    If 'size' is specified and not the natural size of the object,
>>>>>    each element of the vector is coerced to an appropriate type
>>>>>    before being written or as it is read.
>>>>> 
>>>>> The "integer" object type is defined as signed 32-bit in 
>> R, so if you ask for "8 bytes into object type integer", you 
>> get a coercion into that object type -- 32-bit signed integer 
>> -- as documented. I think the issue may come from the 
>> confusion of the object type "integer" with general "integer 
>> number" in mathematical sense that has no representation 
>> restrictions. (FWIW in C the "integer" type is "int" and it 
>> is 32-bit on all modern OSes regardless of platform - that's 
>> where the limitation comes from, it's not something R has made up).
>>>> 
>>>> OK, but it still seems like there is a case for raising a 
>> warning. As
>>>> it is there is no way to tell when reading an 8-byte integer from a
>>>> file whether its value is really 0, or if it merely has 0 in its
>>>> least-significant 4 bytes. If 99% of such stored numbers are below
>>>> 2^31, one is going to need some extra logic to catch the other 1%
>>>> where you (silently) get the wrong value. In essence, unless you're
>>>> certain that you will never come across a number that actually uses
>>>> the upper 4 bytes, you will always have to read it as two 4-byte
>>>> numbers and check that the high-order one (which is endianness
>>>> dependent, of course) is zero. A C-level sanity check seems more
>>>> efficient and more helpful to me.
>>> 
>>> Seems to me that the S-PLUS solution (output="double") 
>> would be a lot more useful.  I'd commit that if you write it; 
>> I don't think I'd commit the warning.
>>> 
>> 
>> I was going to write some thing similar (idea = good, patch 
>> welcome ;)). My only worry is that the "output" argument is a 
>> bit misleading in that one could expect to use any 
>> combination of "input"/"output" which may be a maintenance 
>> nightmare. If I understand it correctly it's only a special 
>> case for integer input. I don't have S+ so can't say how they 
>> deal with that.
> 
> In S+'s readBin the output argument can be
> only double() or single() when what is double()
> or single() (S+ still  has a real single
> precision storage mode) and can be any
> numeric type or logical when what is integer().
> 
> The output=double() seemed like the only useful case.
> 
> It does not warn when precision is lost in the 8-byte
> integer to double conversion.  Perhaps it should.
> 
> Bill Dunlap
> Spotfire, TIBCO Software
> wdunlap tibco.com  
> 
>> 
>> Cheers,
>> Simon
>> 
>> 
>>> 
>>>> 
>>>>>> Pretending that it's really only four bytes because of
>>>>>> the limits of R's integer type isn't all that helpful. Perhaps a
>>>>>> warning should be put out if the cast will affect the 
>> value of the
>>>>>> result? It looks like the relevant lines in 
>> src/main/connections.c are
>>>>>> 3689-3697 in the current alpha:
>>>>>> 
>>>>>> #if SIZEOF_LONG == 8
>>>>>>                  case sizeof(long):
>>>>>>                      INTEGER(ans)[i] = (int)*((long *)buf);
>>>>>>                      break;
>>>>>> #elif SIZEOF_LONG_LONG == 8
>>>>>>                  case sizeof(_lli_t):
>>>>>>                      INTEGER(ans)[i] = (int)*((_lli_t *)buf);
>>>>>>                      break;
>>>>>> #endif
>>>>>> 
>>>>>>>> ) The value can be represented as a double,
>>>>>>>> though:
>>>>>>>> 
>>>>>>>>> 4294967296
>>>>>>>> [1] 4294967296
>>>>>>>> 
>>>>>>>> I wouldn't expect readBin() to return a double if an 
>> integer was
>>>>>>>> requested, but is there any way to get the correct 
>> value out of it?
>>>>>>> 
>>>>>>> Trivially (for your unsigned big-endian case):
>>>>>>> 
>>>>>>> y<- readBin(x, "integer", n=length(x)/4L, endian="big")
>>>>>>> y<- ifelse(y<  0, 2^32 + y, y)
>>>>>>> i<- seq(1,length(y),2)
>>>>>>> y<- y[i] * 2^32 + y[i + 1L]
>>>>>> 
>>>>>> Thanks for the code, but I'm not sure I would call that trivial,
>>>>>> especially if one needs to cater for little endian and 
>> signed cases as
>>>>>> well!
>>>>> 
>>>>> I was saying for your case and it's trivial as in read as 
>> integers, convert to double precision and add.
>>>>> 
>>>>> 
>>>>>> This is what I meant by reconstructing the number manually...
>>>>>> 
>>>>> 
>>>>> You didn't say so - you were talking about reconstructing 
>> it from a raw vector which seems a lot more painful since you 
>> can't compute with enough precision on raw vectors.
>>>> 
>>>> True - I should have been more specific. Sorry.
>>>> 
>>>> Jon
>>>> 
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>>> 
>>> 
>> 
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