[Rd] Reading 64-bit integers
Jon Clayden
jon.clayden at gmail.com
Tue Mar 29 19:33:49 CEST 2011
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. 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! This is what I meant by reconstructing the number manually...
All the best,
Jon
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