PARI/GP Bug report logs - #2227
gp2c fails to create local variables

Package: gp2c; Maintainer for gp2c is Bill Allombert <Bill.Allombert@math.u-bordeaux.fr>; Source for gp2c is src:gp2c.

Reported by: "Perez Bustamante, Adrian" <adrian.pb@gatech.edu>

Date: Sat, 2 May 2020 01:48:01 UTC

Severity: normal

Merged with 2226

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Subject: Bug#2227: gp2c fails to create local variables
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Date: Sat, 2 May 2020 11:22:55 +0200
From: Bill Allombert <Bill.Allombert@math.u-bordeaux.fr>
To: "Perez Bustamante, Adrian" <adrian.pb@gatech.edu>,
	2227@pari.math.u-bordeaux.fr
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merge 2227 2226
thanks
On Sat, May 02, 2020 at 01:35:08AM +0000, Perez Bustamante, Adrian wrote:
> Package: gp2c
> 
> Dear Pari/GP team.
> 
> Thank you very much about the excellent and generous
> work you do for all the scientific community.
> 
> This concerns gp2c
> 
> The specification of the vector() command indicates
> that you can put an command with local variables and
> that this does not affect the index.
> 
> This seems to work in GP as explained. On the other hand,
> the code produced by gp2c fails to create local variables.

Dear Adrian,

I agree with your bug report and diagnostic.
Indeed GP2C does not handle this construction.
However GP2C does create a local variable, as you can see in the 
C code.

  {
->  long i;
    p3 = cgetg(6, t_VEC);
    for (i = 1; i <= 5; ++i)
    {
      i = 2*i; /* Here is the problem !!! */
      gel(p3, i) = gcopy(gel(a, i));
    }
  }
However you are changing its value. So GP2C would need to add
an extra local variable:

  {
    long l1;
    p3 = cgetg(6, t_VEC);
    for (l1 = 1; l1 <= 5; ++l1)
    {
      long i = l1;
      i = 2*i; /* OK now */
      gel(p3, i) = gcopy(gel(a, i));
    }
  }

This is possible, but this leads to less readable C code.

Cheers,
Bill.

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Bill Allombert <allomber@math.u-bordeaux.fr>. Last modified: Sat Aug 26 16:37:10 2023; Machine Name: pari

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