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Re^3: Converting to number doesn't always work... (updated)

by haukex (Bishop)
on Nov 22, 2019 at 09:15 UTC ( #11109054=note: print w/replies, xml ) Need Help??


in reply to Re^2: Converting to number doesn't always work... (updated)
in thread Converting to number doesn't always work...

$x = \$r;

I am confused why you're taking this step? This means that $x is a reference, and a reference is like a dual-valued variable*: As a string, it's "SCALAR(0xabc)", which I would guess is what looks_like_number is looking at², and in numeric context, it's the memory address, which is why $x += 1 doesn't warn.

* Update: Triple-valued? It's a reference, string, and number ;-) (Or rather: It's a reference, that gets converted to different values depending on context.)

² Update 2: Hmmm, nope, looks like it's checking the flags in this case.

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Re^4: Converting to number doesn't always work... (updated)
by syphilis (Archbishop) on Nov 22, 2019 at 12:18 UTC
    I am confused why you're taking this step?

    Nothing insidious or profound.
    I read "this gives you the exact internal function that Perl uses to check strings and generate that warning in the first place" as implying both that:

    1) if looks_like_number($x) returns true, then there will be no warning given when $x is used in numeric context;
    &&
    2) if looks_like_number($x) returns false, then a "non-numeric" warning will be given when $x is used in numeric context.

    I find it interesting that the latter of the two is not necessarily true - furthermore, I found it so interesting that I decided to provide an example where looks_like_number($x) returns false, yet no warning is issued when $x is used in numeric context.

    Your second update references the use of looks_like_number() in sv.c.
    Note that Scalar::Util::looks_like_number() is a different function. (That is, it doesn't simply wrap the perl API function of the same name ... it does some other stuff as well.)

    Cheers,
    Rob

      Yes, I guess I was a little too unspecific in what I wrote: Given a string $x, looks_like_number($x) should accurately report whether the warning will occur.

      Note that Scalar::Util::looks_like_number() is a different function. (That is, it doesn't simply wrap the perl API function of the same name ... it does some other stuff as well.)

      True, it's a different function - AFAICT, Scalar::Util::looks_like_number just adds get-magic to Perl_looks_like_number in sv.c, which in turn calls Perl_grok_number in numeric.c, if the scalar has a valid string component (SvPOK). And my understanding is that grok_number is the same internal function that is used to inspect strings for their numeric content - see e.g. Perl_sv_2nv_flags in sv.c, which is called by the SvNV macro, and which calls not_a_number to generate the "Argument isn't numeric" warning.

      I had done all this research in the past, and perhaps remembered it a little too simplified, thanks for poking me on this :-)

      Update: Well, to nitpick: I did write "the exact internal function that Perl uses to check strings", and you didn't give it a string ;-P But "exact" is indeed inaccurate.

        Well, to nitpick: I did write "the exact internal function that Perl uses to check strings"

        Actually, I did pick that up on a subsequent (but not first) reading of your post.
        But I decided to ignore that.... as to do otherwise was of absolutely no advantage to me ;-)

        Cheers,
        Rob

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