Re: common warning id for double->object conversion

[Colin Macdonald]

On 22/06/16 18:07, Oliver Heimlich wrote:
> How do you decide whether to give a warning? If the parameter is no
> integer? There are some fractions which have no rounding errors, e. g.
> 0.5, 0.25, 0.125, … and sums thereof.

I do this:

https://github.com/cbm755/octsympy/blob/master/inst/%40sym/private/magic_double_str.m

Its very simple.  I don't care if 0.25 has an exact bit representation: 
"sym(0.25)" is bad code, one should use "sym(1)/4".  And generally I 
don't think symbolic users should be passing double values from other 
code into sym, so an aggressive warning is good.  (I'm sure there are 
valid use cases, but I've not seen one that outweighs the educational 
aspect for new users).

vpa is a little different: perhaps that's why I never added a warning 
there before.  One can imagine feeding double's from some other part of 
a code into vpa code (e.g., as an initial guess to vpasolve).  Instead 
there is a prominent warning in the docs.  But since a new user was 
confused by vpa(1/3)---*the same example in the docs*---I'm rethinking 
that choice...

> 4. yeah, pi might be problematic as well if the user expects to use the
> approximate floating point value returned by pi () instead of π.

An expert use; not too worried about that, e.g., could do "vpa(vpa(pi, 
16), 64)".

> We could stick to the official naming from IEEE 754, which is the
> “inexact” exception. I propose one of the following:
>     warning("Octave:inexact-argument", …)
>     warning("numeric:inexact-argument", …)
>     warning("double:inexact-argument", …)

Sounds reasonable.  I think I like either "numeric:" or "double:".  Is 
it appropriate for me to use such an IEEE754 convention even if I stick 
with my much-simplified tests (above)?  That is, may I raise such a 
thing on sym(0.25)?


Thanks for the detailed thoughts.  For syms, I might consider using 
something like your function *after* the warning...  Right now, sym has 
some heuristics using rat() that prefers "short" rationals or 
rational*pi.  Example:

>> a = pi/8 + 1000*eps
a =    0.392699081698946
>> sym(a)
warning: Using rat() heuristics for double-precision input (is this what 
you wanted?)
warning: called from
    sym at line 225 column 7
ans = (sym)

  π
  ─
  8

(The alternative is to give them back the exact real number 
corresponding to the double precision value they pass in...)

Colin