Storing and retrieving
mrgrynch at comcast.net
Sat Jul 2 17:33:22 CEST 2005
Thanks for the information! I am currently already using the C++ wrappers that come with GMP. So far, I've wirtten the code to stream these text values to a file. The question arises when I read them back...
Can I just read the value in and set an mpf_class to that string value, and be comfortable that the conversion will work? The documentation for this is a bit confusing for me, as it says...
int mpf_set_str (mpf t rop, char *str, int base) [Function]
Set the value of rop from the string in str. The string is of the form `M at N' or...., if the base is 10 or less, alternatively `MeN'.
Assuming I am using base 10, then I believe what this means is I can just call this method with a number like "-3.17335744356735673e-14", and it will properly set the destination object to that value, assuming a suitable precision level of the destination structure.
Am I correct in this? If I am, then I think I've all I need to do what I need.
-------------- Original message --------------
> At 03:53 +0000 2005/07/02, mrgrynch at comcast.net wrote:
> >I have a need to preserve a bunch of GMP floating point values (from
> >an array of mpf_class objects) for retrieval at a later time. The
> >experiement I am running will take about a year, and I'd hate to
> >lose it all if the power goes out, etc...
> > I was thinking the easiest way to do this is with strings. Has
> >anyone done this before?
> One can write binary data to a file and read it back, but I gather if
> the core parts that the GMP relies on would somehow change, it would
> make the file binary data unreadable. So the safest way, though
> slower, is to write it out as text, and read it back again. You are
> then embarking, in its extension, on the path of making two programs,
> a parser that can read this kind of data, and an expresser, or
> "pretty-printer", that can write out it. Ultimately, if you develop a
> computer language, you may need the use of tools such as the lexer
> generator Flex and the parser generator Bison.
> But start simply, expand at need.
> >Reading through the documentation, I'm not entirely sure about the
> >argument list to be passed to the functions which handle this, nor
> >how they would work through the C++ class interface, which I am
> >using. Would I have to extract a reference to the underlying C
> >structure before converting to and from strings?
> GMP has C read and write functions, and further, some C++ support. I
> haven't used the latter, as I wrote my own C++ wrap. I can send it to
> you by private mail, if you so like.
> The major drawback of my C++ wrap is that I had my own version of the
> GMP read function. So it means that if GMP changes that, I will have
> to do some rewriting. But this part is there only to make a C++ nice
> istream interface; you could use something more version-stable, if
> you like. Otherwise, it is just a C++ interface on top of C, using
> reference counts to avoid unneeded recopying.
> Hans Aberg
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