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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 ( #11109061=note: print w/replies, xml ) Need Help??


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

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
  • Comment on Re^4: Converting to number doesn't always work... (updated)

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Re^5: Converting to number doesn't always work...
by haukex (Bishop) on Nov 22, 2019 at 13:04 UTC

    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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