Re^7: Using exists to check element in array (existence-by-reference for hashes too)

In hindsight, this existence-by-reference trick works for hashes too. Hence it's at least consistent.

The big question is now, if and what rationale there this.

Probably not really a problem at all.

  DB<1> %x=(a=>1)

  DB<2> $a= \$x{b}

  DB<3> p exists $x{b}
1
  DB<4> p exists $x{a}
1
  DB<5> p exists $x{c}

  DB<6>
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Cheers Rolf
_{(addicted to the Perl Programming Language :)

see Wikisyntax for the Monastery}

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Re^8: Using exists to check element in array (existence-by-reference for hashes too) by ikegami (Patriarch) on Feb 01, 2024 at 17:03 UTC
As with all lvalue context operations, the rational is that it must exist so you can assign to it (e.g. `$$a = 123;`) Except not quite. Cause the element is evaluated in lvalue context in both `for (@a)` and `f(@a)` as well. So why don't they replace the NULL with a scalar? That's cause they create and return a proxy scalar containing a reference to the array and the index. Using magic, accesses to this scalar access the original array element. `$ perl -MDevel::Peek -e'sub { Dump($_[0]) }->( $a[0] )' SV = PVLV(0x558b3a14b8a0) at 0x558b3a0f4538 REFCNT = 1 FLAGS = (GMG,SMG) <- Get and set magic IV = 0 NV = 0 PV = 0 MAGIC = 0x558b3a119460 MG_VIRTUAL = &PL_vtbl_defelem MG_TYPE = PERL_MAGIC_defelem(y) TYPE = y TARGOFF = 0 <- index TARGLEN = 1 TARG = 0x558b3a1241d8 <- array FLAGS = 0 SV = PVAV(0x558b3a0f6868) at 0x558b3a1241d8 REFCNT = 2 FLAGS = () ARRAY = 0x0 FILL = -1 MAX = -1 FLAGS = (REAL)` [download] Creating this proxy for `\` would cause issues.	[reply] [d/l] [select]

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Re^8: Using exists to check element in array (existence-by-reference for hashes too)
by ikegami (Patriarch) on Feb 01, 2024 at 17:03 UTC

As with all lvalue context operations, the rational is that it must exist so you can assign to it (e.g. $$a = 123;)

Except not quite. Cause the element is evaluated in lvalue context in both for (@a) and f(@a) as well. So why don't they replace the NULL with a scalar?

That's cause they create and return a proxy scalar containing a reference to the array and the index. Using magic, accesses to this scalar access the original array element.

$ perl -MDevel::Peek -e'sub { Dump($_[0]) }->( $a[0] )'
SV = PVLV(0x558b3a14b8a0) at 0x558b3a0f4538
  REFCNT = 1
  FLAGS = (GMG,SMG)                       <- Get and set magic
  IV = 0
  NV = 0
  PV = 0
  MAGIC = 0x558b3a119460
    MG_VIRTUAL = &PL_vtbl_defelem
    MG_TYPE = PERL_MAGIC_defelem(y)
  TYPE = y
  TARGOFF = 0                             <- index
  TARGLEN = 1
  TARG = 0x558b3a1241d8                   <- array
  FLAGS = 0
  SV = PVAV(0x558b3a0f6868) at 0x558b3a1241d8
    REFCNT = 2
    FLAGS = ()
    ARRAY = 0x0
    FILL = -1
    MAX = -1
    FLAGS = (REAL)
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Creating this proxy for \ would cause issues.

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